The Internet's Premier Geography Knowledge Base Wed, 22 Nov 2017 21:57:58 +0000 en-US hourly 1 32 32 Gentrification Wed, 22 Nov 2017 19:45:48 +0000   What is gentrification? Gentrification is a socio-spatial process in which higher income groups move into lower income neighborhoods. The higher income groups typically renovate

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A gentrifying neighborhood in Pittsburgh, PA. New businesses and middle-class housing bring in much-needed tax dollars for the city, but will likely result in the displacement of current lower-income residents. Credit: Author.

What is gentrification?

Gentrification is a socio-spatial process in which higher income groups move into lower income neighborhoods. The higher income groups typically renovate and increase the land value of properties, resulting in the displacement of lower incomes groups who can no longer afford the rent or taxes on property they own.

Gentrification also often results in the closure of certain types of businesses and the opening of others. Retail stores in gentrified or gentrifying neighborhoods after cater to middle and upper class clientele, and may include wine stores and bars, bicycle shops, coffee shops, gourmet bistros, cafes, and bakeries. These stores often replace those that serve (and sometimes prey upon) lower income groups such as neighborhood bars, liquor shops, payday loan and rent-to-own businesses, convenience stores, and “mom-and-pop” restaurants. The goods and services that the invading businesses offer are also frequently too expensive for lower income groups, further decreasing the affordability of the neighborhood.

While some gentrifiers may be moving “back to the city” from outer suburban areas, many are simply moving from other urban neighborhoods within the same city or from out of town. So-called “first wave” gentrifiers — those that initially move into a lower class neighborhood where rent is cheap — are often bohemians (those involved in the arts), students, “hipsters”, and young professionals looking for an affordable place to live just beyond the fringe of already-gentrified areas of the city.

What causes gentrification?

Gentrification is often said to be the product of two primary forces: economic and personal preferences. On the economic side, land owners and developers have a financial incentive to renovate older buildings when the gap between what they are earning now from their property (generally in terms of rent) is substantially less than it would be if it were improved. The difference between current and potential earnings from real property is called the “rent gap.”

When the rent gap reaches some critical threshold it becomes likely that the property owner will upgrade/renovate their buildings or other property as to reap greater profit. Even for individual residential units, it is common for people to buy, renovate, and then sell properties in a process known as “flipping.” The rent gap generally increases as the surrounding area of the city becomes more desirable and more money is being invested in nearby properties. One can see how gentrification then can spread from one or multiple locations over time like a wave.

Even a single investment in a distressed neighborhood can kick-start the process of gentrification. Today, large mixed-use infill developments, with a blend of retail and (often) up-scale housing, are often used as a catalyst for neighborhood revitalization. As soon as these large developments are built and start attracting middle-income residents and/or clientele, other developers move in to take advantage of the rising land values and widening rent gaps on adjacent properties.

In addition to economic drivers like rent-gap, there are also lifestyle preferences that play a role in gentrification. Professionals without children, members of the “creative class” (those with “creative” or high-skill occupations), and young people in particular have, over the last 30 years, shown a growing preference  for urban lifestyles. Renovated historical buildings, including old warehouses, have been repurposed as chic loft-style condos in many U.S. cities. As the residential population of central cities grows, so too does the density of social, cultural, and entertainment amenities, which further attracts middle- and upper-income groups. The construction of transit corridors, particularly light rail lines, can also increase the desirability of a neighborhood by improving accessibility to other parts of the city including employment centers.

A gentrified neighborhood in Portland, OR. Coffee shops and cafes are commonly seen in gentrified areas. Credit: Author.

This growing demand for urban spaces and lifestyles coincides with larger socio-economic and demographic shifts. In many developed countries, men and women are getting married later and having fewer children, potentially reducing the desire for suburban housing with large yards. Rising educational attainment has also likely played a role in declining birth rates and growing demand for arts/cultural/entertainment amenities. Violent crime rates have also declined since 1990 in most large U.S. cities, as their economies have begun to recover from the deindustrialization of the 1960s and 70s. The increasing sense of safety and security in urban centers has been reflected in the overall positive view of cities seen on most TV shows from 1990s onwards, such as Friends and Seinfeld. 

Because gentrification often results in the displacement of lower income groups, it is generally viewed cautiously and even negatively by many scholars and community organizations. Developers and municipal leaders, however, are more likely to be pro-gentrification as it frequently results in more profit opportunities and an increase in the local tax base. In fact, gentrification may be actively encouraged by local governments who frequently offer tax and other incentives for large development projects in economically distressed communities. Tax increment financing (TIF) districts, for example, are frequently used to help incentivize private development. In a TIF district, future property tax revenue increases over typically a 10-30 year period are captured and used toward the funding of a new development or infrastructure project. Although TIFs districts originated in California, they are no longer permitted there. They are, however, allowed in all other U.S. states.

Inner city areas, such as here in downtown Boston, have become increasingly desirable. Late-stage gentrification often involves complete replacement of older structures with new up-scale residential and commercial units. Credit: Author.

To help reduce the potential displacement caused by gentrification, one of the most effective things a local government can do is require that new private developments of a certain size contain a minimum proportion of affordable housing units. In many cases this won’t reduce displacement 100%, but it will help maintain a higher degree of neighborhood diversity and inclusivity than would otherwise be possible.

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The Earth’s Largest Rivers Sat, 18 Nov 2017 20:48:27 +0000 The world’s largest rivers by discharge volume, drainage area, and length When it comes to geographical extremes, one of the most common questions posed by

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The world’s largest rivers by discharge volume, drainage area, and length

When it comes to geographical extremes, one of the most common questions posed by children and adults alike is “what is the largest river in the world?” The word “largest,” however is an ambiguous term when it comes to rivers and many other natural features. Does largest mean the longest river, the one that has the largest drainage basin, or the river that discharges the most water? While we could simply rank the world’s rivers according to these three measurements and leave it at that, we felt that the relative sizes of the rivers could be best interpreted using infographics. Here we rank the 15 largest rivers in the world by length, drainage area, and discharge volume.

The Amazon River system is the largest in the world in terms of discharge, drainage area, and overall length.

A river’s discharge volume refers to the amount of water (usually measured in cubic meters or kilometers) that flows out of a river into another water body in a given amount of time. As you can see from the figure below, the Amazon river is by far the largest river in terms of discharge volume. In one second, the Amazon River discharges on average 175,000 cubic meters of water into the Atlantic Ocean. This is more water discharged per second than the next four largest rivers combined.

The Amazon River discharges so much water primarily because 1) it’s drainage basin covers the largest area of any river on Earth (see below), and 2) it is located in the world’s largest remaining tropical rainforest, which receives on average 108 inches of rain each year, or about 270 centimeters!  Note that the Congo is also located in an area of tropical rainforest, but it does not cover as much area. Other rivers, like the Yangtze and Mississippi, may be nearly as long as the Amazon, but they have smaller drainage basins and are found in more temperate climates that receive less rainfall.

World’s 15 largest rivers by water discharged. The size of the squares represent the volume discharged.

World’s Largest Rivers by Drainage Area

All rivers and streams have a drainage basin, also known as a watershed or catchment basin. Drainage basins are the area of land surrounding a water body that collects, or catches, precipitation and funnels the water, either on the surface or under the ground, to that water body. Typically, the larger a river’s drainage basin, the more water it will carry, and the greater its discharge volume will be.  Thus, drainage area and discharge volume are often related. However, some rivers are located in arid climates that receive very little rainfall, while others are in tropical regions that receive a lot of rainfall.

The Amazon River is once again ranked number one for having the largest drainage basin in the world, some 6.9 million square kilometers. This area covers a vast portion of the Amazon Rainforest, which receives a great deal of rainfall. As discussed above, the combination of a vast drainage basin and a wet, tropical climate, makes the Amazon River stand out from the rest..

The world’s 15 largest rivers ranked by drainage area. The size of the squares represent the area of each river’s drainage basin.

World’s Longest Rivers

Measuring river length is not quite as straight forward. Which river is the longest depends a great deal on how you measure it. At some point, as you trace the course of a river from the mouth to the headwaters you’ll have to decide which tributary to follow. If measured all the way to the beginning of the longest tributary, then the Amazon is likely the world’s longest river at nearly 7,000 km long.  This is, however, sill a matter of some debate, so consider rivers very close in length here to be approximately equal. This includes the Amazon and Nile, Yangtze, and Mississippi, and the Yenisei, Yellow and Ob. There’s a rather significant difference in length between the Mississippi and Yenisei, and between the Ob and the Congo, however. Note that some rivers are part of the same “river system,” which means that they combine at some point prior to emptying into a body of water. An example would be the Mississippi-Missouri river system, which joins near St.Louis, Missouri and eventually empties into the Gulf of Mexico.  

The 15 largest rivers in the world by total length (km).

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The Nitrogen Cycle Thu, 16 Nov 2017 18:56:06 +0000 The flow of nitrogen through the Earth system is critical to the health of ecosystems around the planet. Living organisms need nitrogen to produce molecules

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The flow of nitrogen through the Earth system is critical to the health of ecosystems around the planet. Living organisms need nitrogen to produce molecules like amino acids, proteins and nucleic acids.

There is a large quantity of nitrogen on Earth, and it can be found in many different forms. Most of the nitrogen is located in the atmosphere in the form of a gas; N2. Unfortunately, N2 is very unreactive and cannot be used by most living organisms to produce proteins. Animals (including humans) rely on eating plants and other animals to take up nitrogen. Plants can absorb nitrogen only from the soil when it is in a solid form and soluble in water, like ammonium (NH4+) or ion nitrate (NO3-). Plants and animals depend on other processes to convert the nitrogen in the air to nitrates in the soil. The various processes that move nitrogen from the air to soil to living organisms and back to the air again form the nitrogen cycle.

The nitrogen cycle. Credit: USGS.


  1. The atmosphere is the largest store of nitrogen and holds about a million times more nitrogen than all living organisms combined. 78% of the air is nitrogen
  • Nitrogen fixation (N2 to NH3 / NH4+): Nitrogen as a gas (N2) is extremely stable and it takes a lot of energy to turn into nitrate, like volcanic action or lightning discharges. However, some bacteria are able to break the bonds of the two nitrogen atoms and combine it with hydrogen to form ammonia (NH3 or NH4+). The bacteria use the enzyme nitrogenase, which only functions if there is no oxygen around, usually under layers of soil. Rice grown in wetlands have a mucus around their roots on which microorganisms grow. Legumes form nodules on their roots where rhizobia bacteria live that fix atmospheric nitrogen. These plants, and the plants around them, benefit from the ammonia created.
  1. The soil is store of nitrogen in many forms; nitrates, nitrites, ammonium. The conversion from one form to another takes place in the soil.
  • Nitrification (NH3 / NH4+ to NO2- and then to NO3-): different types of bacteria work on this process in two steps. First, the ammonia will be converted into nitrites that then can be converted into nitrates by other bacteria. This is an important step in the nitrogen cycle because nitrates and nitrites are much more mobile in soils and are more readily absorbed by plants than ammonia.
  • Assimilation: Plants take up the ammonia and nitrates through their roots and use them to make amino acids. These are then used to make plant proteins that help them grow.
  1. Plants are a source of nitrogen for animals who consume them (herbivores). Carnivores then eat herbivores and obtain nitrogen this way. Nitrogen is essential for all life; amino acids, proteins and DNA all require nitrogen.
  • Ammonification: Organic matter gets broken down either through the digestive canal of animals in the form of excrement or in the soil after death. Microorganisms (decomposers) use dead organic material for energy and turn organic nutrients into amino acids, DNA or chlorophyll, then back into ammonia. This nitrogen can then be taken up by plants again into the same cycle, or go through denitrification.
  • Denitrification: Denitrifying bacteria turn ammonia back into N2, nitrogen gas, which gets released back into the air. Denitrification only occurs in soil where there is little oxygen, such as waterlogged soil and wetlands.

All the various processes discussed above form one large cycle, which keeps approximately the same amount of nitrogen in every reservoir. The soil releases as much nitrogen to the atmosphere as it receives. Plants take as much from the soil as they return to the soil after decay. However, humans are interfering with the cycle and have altered it, mainly by burning fossil fuels and using large quantities of nitrogen for agricultural purposes.

Human impact on the nitrogen cycle

Burning fossil fuels releases nitric oxide into the atmosphere, causing smog and acid rain, and adding to the greenhouse effect warming the planet. The high-temperature combustion also fixes a small amount atmospheric nitrogen abiotically. Every year humans cause up to four times as much nitrogen fixation by burning fossil fuels than lightning does; about 20 billion kg of fixed nitrogen per year.

Humans apply millions of tons of nitrogen fertilizer to crops each year.

The cultivation of soybeans, peas, and other crops that host symbiotic nitrogen-fixing bacteria have added to the amount of fixed nitrogen caused by human activities. Large areas of natural and diverse vegetation have been turned into monocultures of rice and soybeans adding twice as much of new biologically generated nitrogen to the soil as combustion does; averaging 40 billion kg of nitrogen per year.

Humans use nitrogen fertilizers to help grow crops. This represents the largest human contribution to new nitrogen in the nitrogen cycle, with 80 billion kg of nitrogen per year. The fertilizers are often leached from the soil into the groundwater system, as well as streams and rivers. Livestock produce huge quantities ammonia as a waste product, which enter the soil and leach into bodies of water.

These human activities have serious and long-term consequences for the environment. Nutrients like calcium and potassium in the soil are lost causing fertility problems. Plants adapted to low-nitrogen soils are lost, as are the animals depending on these plants. Water bodies with excessive ammonia will have reduced oxygen levels, causing loss of biodiversity and changes in the food web. Most commonly, algae will grow in abundance, killing many forms of life. Excessive amounts of nitrates in the water make it unsafe for consumption. Today, nearly 80% of the nitrogen found in human tissues originated from synthetic fertilizers.


David Tilman, 1997, Human Alteration of the Global Nitrogen Cycle: Causes and Consequences
Anne Bernhard, 2010, The Nitrogen  Cycle : Processes, players, and human impact.
John A. Lamb, Fabian G. Fernandez, and Daniel E. Kaiser, 2014, Understanding nitrogen in soils
Plant and Soil Sciences Part 5: Nitrogen as a Nutrient
Fondriest, 2010, The Nitrogen Cycle

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Models of Urban Form & Development Wed, 15 Nov 2017 19:30:41 +0000 Cities grow and evolve over time, forming patterns guided by an array of physical, economic, political, and technological influences. While no two cities are exactly

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Cities grow and evolve over time, forming patterns guided by an array of physical, economic, political, and technological influences. While no two cities are exactly alike, and urban form – the arrangement or layout of urban land uses; offices, houses, shops, roads, etc. – can vary significantly from one region to another, several urbanists have tried to identify and explain patterns of urban structure and urban growth at various points in time. While cities have evolved, so to has our understanding of their form and function. Here we take a brief look at a just a few of the models – the idealized and simplified representations of reality – that have been developed over the last 100 years to describe and explain the structures of humanity’s greatest artifiact: our cities.

Ernest Burgess’ (1925) Concentric Ring Model attempted to explain broad patterns in urban development patterns.

Concentric Ring Model

Models of urban form developed in the early 20th century emphasized a monocentric (single-centered) form. In the land use model developed by Ernest Burgess in 1925, the city was thought of as a series of five concentric zones radiating out from the center of the city, or the central business district (CBD). The first and most central zone, which included the CBD, was considered the heart of the region both in terms of commerce and culture. This first zone was also where all major transportation lines converged. The second zone contained a mix of residential, commercial and industrial areas. According to the Burgess model, this zone was typically occupied by low-income residents and was likely to contain slums and other blighted areas. The three outer rings of the city were exclusively residential, with high-income residents most likely to live in the two outermost rings. In the Burgess model, all urban development occurs from the central city outward.

Burgess based his concentric zone model on early 20th century industrializing Chicago.

Sector Model

Expanding upon the Burgess model, Homer Hoyt in 1939 developed a sector model of urban form that considered direction as well as distance from the CBD. Hoyt observed that rent levels throughout 25 U.S. cities did not usually vary by concentric ring, but rather by sections radiating out from the central city. The general pattern suggested that rent levels (and, presumably, land values) within each section of the city, like slices of a pie, were more similar than those found among concentric circles.

Hoyt reasoned that urban growth must occur from the center outwards along particular corridors or wedges. High-income residential areas, for example, move successively outward from their initial location within the central city in a direction that maximizes their access to major transportation routes and recreational or scenic amenities such as water fronts, open countryside and commercial centers. Directional growth resulted in the sectoral partitioning of the city not only in terms of rents and land values but also land uses. Areas with high property values effectively excluded undesirable uses, such as heavy industry, from developing within that sector.

Homer Hoyt’s sector model of urban form (1939)

Multiple Nuclei Model

By the 1940′s, the automobile had begun to transform American cities, facilitating the broad dispersal of people into suburban enclaves ever further from the city center. Cities that once had a very clear monocentric urban form began to evolve into a more complex patchwork of suburban centers strung along major road and highways. Observing this transformation, Chauncy Harris and Edward Ullman in 1945 developed their multiple nuclei model, in which they envisioned a polycentric (multi-centered), or polynucleated, urban form.

According to the multi-nuclei model, the city is composed of a number of sections, each with its own functional specialization. Different land uses and economic activities tend to form around the city center in no universal concentric order or direction, but rather assemble according to the complex interaction of four primary variables.

Harris and Ullman’s (1945) Multiple Nuclei Model reflected the patchwork of land uses for in the automotive city.

First, many land uses require access to specialized facilities or locational amenities. Large retail shopping centers, for example, require access to major transportation routes. Second, some activities, such as those associated with the financial, insurance, and real estate industries (FIRE), benefit from agglomeration, and tend to cluster together in central locations. Third, the incompatibility of certain land uses, such as heavy industry and high-income residential housing, assures a certain degree of regional differentiation.

Lastly, high land values restrict the development of all but a few land uses in certain locations (e.g. office buildings in CBDs or large retail outlets at major highway intersections). These nodes of similar economic activity form either as the expanding city envelops surrounding settlements, or when new nodes develop in accordance with the variables discussed above. Thus, according to Harris and Ullman’s model, the city does not simply grow outward from the CBD, but rather evolves from the complex integration and development of separate functional nuclei.

Urban Realm Model

Expanding on the multi nuclei hypothesis, James Vance proposed the urban realms model in 1964. According to the model, the city is composed of autonomous nuclei (or urban realms) largely independent of the traditional CBD or central city. Vance argued that polynucleated belts of urban development formed over the previous century primarily due to the extrodinary growth in population and areal extent of cities, as well as the introduction and mass adoption of the automobile.

Vance’s (1964) urban realm model increased the scale to an entire metropolitan area with several different urban nodes.

The size, character, and structure of urban realms depend on four primary factors. First is the topography of the landscape. Mountains, water bodies, and other natural features can both direct the spread of urbanization and influence the type of development within an urban realm. The second factor is the size of the metropolitan region, with larger urban areas tending to have larger, more numerous, and more differentiated urban realms. Third is the level and character of economic activity within each realm, and fourth is the layout of infrastructure and overall accessibility within and between realms.

Particularly important in the model is the presence of major circumferential highways and other transportation corridors that link the various urban realms. Airport connections allow urban realms to connect with other cities, providing additional economic independence from the central city. As urban realms become more powerful, the core-periphery relationship begins to weaken, resulting in what Vance described as a “sympolis” rather than a metropolis. Many large cities have distinct urban realms, including the greater Los Angeles area, whose western urban realm – the “Inland Empire” of Riverside and San Bernardino – is larger than all but a few U.S. cities with a population of over 4 million.

The major urban realms within the Los Angeles metro.

White’s Revised Concentric Model

Michael White (1987) provided a revised Burgess model to reflect new social, economic, and political forces influencing urban growth. White envisioned late-20th century urban form as a complex patchwork of concentric zones, corridors, epicenters (i.e. nuclei or nodes) and enclaves. The CBD, though having maintained its position as the economic and cultural heart of most large cities, had become more specialized in finance and management as retail and other activities migrated to the suburbs. Surrounding the CBD is the zone of stagnation, which White argued suffers not only from lack of investment, but also slum clearance, highway construction and relocation of industry to more peripheral locations. At one time it was expected that the CBD would expand and revitalize the zone of stagnation, but in most cities the CBD has expanded up rather than out.

Most of the remainder of the city from the zone of stagnation outward is composed of a patchwork of wealthy enclaves and (mostly poor) immigrant pockets held together by a spatially diffuse realm dominated by the middle class. Dotting the urban landscape are clusters of specialized activity, such as industrial parks, universities, and hospitals that can exert significant influence on local land use patterns. Finally, as standard among polycentric models, there are epicenters and corridors of economic activity along major transportation routes, especially where radial and circumferential highways intersect. These nuclei and corridors “form a latticework that extends over the entire urban region,” that increasingly “challenge the hegemony of downtown” (White 1987).

The Megapolitan Model

A new model of urban structure developed by Robert Lang and Paul Knox (2009) moves up in scale and beyond the single metropolis. They argue that the expansion of metropolitan areas, and the transportation linkages that bind them, have given rise to larger trans-metropolitan urban agglomerations, termed megapolitan regions. To be classified as a megapolitan region, neighboring metropolitan areas must share at least 15 percent of new commuters from 1995 onward.  Using this criteria, Lang and Knox (2009) have identified nine megapolitan areas in the U.S. The concept of the megapolitan region borrows from Pickard’s (1970) urban regions, described as areas with “high concentrations of urban activities and [an] urbanized population,” and Lewis’ (1983) ‘galactic metropolis,’ which contains “varying sized urban centers, subcenters, and satellites [that are] fragmented and multimodal, with mixed densities” (Lang and Knox 2009).

The multi-metropolitan or “megapolitan” urban model proposed by Lang and Knox (2009).

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Green Roofs Tue, 14 Nov 2017 19:03:13 +0000 A green roof is one that’s been covered with a layer of soil and vegetation. Growing plants on rooftops can replace some of the vegetation

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A green roof is one that’s been covered with a layer of soil and vegetation. Growing plants on rooftops can replace some of the vegetation that was removed when the building was constructed. Installing green roofs reduces the negative impacts of development while providing numerous environmental, economic, and social benefits. Green roofs can absorb rainwater, provide insulation, create habitat for wildlife, contribute to a more attractive urban environment, and reduce the urban heat island effect.

Hanging Gardens of Babylon. Source: Wikipedia.

The earliest documented roof gardens were the hanging gardens of Babylon in modern-day Syria, considered one of the seven wonders of the ancient world. Germany was one of the first countries to embrace green roof technology in modern times and constructing green roofs has grown in popularity throughout the world.

There are two different types of green roofs: intensive and extensive. Intensive green roofs include trees, shrubs and are limited to flat roofs. They are high maintenance and high cost.  Extensive roofs include herbs, grasses, mosses and drought tolerant plants. They are generally lower maintenance and cost less. However, they may not provide the same aesthetic benefits.

How Green Roofs are Built

Building a green roof can be a bit of an engineering challenge; it’s important that a building is first inspected to make sure it can structurally accommodate the extra weight. If so, a waterproof layer and a root resistant layer is first placed down on the rooftop. Excess water should be drained into the roof’s gutters, and the angle of sunlight should be appropriate for plant growth. Due to wind exposure, intense sunlight, moisture stress, severe drought, and elevated temperatures, green roofs can be a challenging location for some plants to grow. Plant selection must take into consideration site, micro-climate, and aesthetic factors.

Typical layering used in constructing green roofs.

Benefits of Green Roofs

Green roofs are associated with a variety of benefits. Due to root uptake and surface storage, excessive water problems can be reduced. Stormwater can be absorbed by the vegetation and released slowly over several hours, reducing the impact of flooding at ground level.

The vegetation provides habitat for birds, insects, and animals, creating small ecosystems and microclimates. The flowering plants allow pollinators such as bees to be introduced into the urban environment as well. While these ecosystems are small, they can increase the biodiversity found in cities.

A green roof on Chicago’s city hall. Credit: National Geographic.

Green roofs contribute to improvement of urban air quality and can even help cool the urban environment. In urban areas, vegetation can reduce the impact of the urban heat island. Vegetation has largely been replaced by impervious, man-made surfaces in built-up areas. These surfaces retain heat, leading to an increase in temperature compared to the suburban and rural areas, especially at night. This effect can be reduced by increasing the reflection rates of incoming radiation or by increasing vegetation cover. A regional simulation model using 50% green-roof coverage distributed evenly throughout Toronto showed temperature reductions as high as 2°C in some areas (Bass et al. 2002).

Another advantage of green roofs is their ability to provide aesthetic and psychological benefits for the urban populations. They can provide visual improvements, spaces for relaxation and restoration, and promote physical and psychological health. They can also be used for urban agriculture with food production providing nutritional, economic and educational benefits.

Green roofs reduce sound and air pollution by absorbing sound waves and harmful gases from the traffic below. The vegetation can filter out atmospheric pollutants before they reach ground level. Plants can also act as a sound buffer, preventing noise from above such as airplanes from reaching the residents on the top floor.

Finally, green roofing can be cost effective. Due to the waterproof membrane covering, the vegetation shields the roof from UV light and physical damage. This can extend the lifespan of a roof by over 200%.

Green Roofs Around the World

Germany and Scandinavia have been the forerunners at implementing green roofs. Green roof have received government supported since 1970’s. The US, however, is beginning to catch up. Chicago has more green roofs than any other US city with a total of approximately 7 million square feet of coverage.

Washington D.C has set a goal of 20% green roof coverage by 2020. Toronto is the first city to mandate green roofs on any industrial or residential building with over 21,500 square feet. By doing so, it is estimated that the city would save $37 million a year in savings on storm water management, energy bills and costs associated with the urban heat island effect.

San Francisco passed a bill in January 2017, which will require between 15 and 30% of roof space on construction projects to be devoted to green roofs. In parts of the US, including New York, incentives for green roofs in the form of tax benefits have been issued to promote the installation of vegetation on buildings.


Bass, B., Krayenhoff, ES., Martilli, A., Stull, RB. and Auld, H.2003 The impact of green roofs on Toronto’s urban heat island Pages 292 – 304 in Proceedings of the First North American Green Roof Conference: Greening Rooftops for Sustainable Communities; 20–30 May, ChicagoToronto (Canada) Cardinal Group

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The Ozone Layer Tue, 14 Nov 2017 01:01:22 +0000 The atmosphere that surrounds the Earth consists of multiple layers. The layer closest to the ground is the troposphere. This is where we live out

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The atmosphere that surrounds the Earth consists of multiple layers. The layer closest to the ground is the troposphere. This is where we live out our lives and where nearly all weather occurs. Commercial aircraft typically fly just below the tropopause, the boundary between the troposphere and the layer above: the stratosphere. It is in this layer of the atmosphere that you find the ozone layer, or ozonosphere.

The ozone layer is not a uniform blanket of ozone in the atmosphere, but rather a broad vertical area where ozone is present in relatively high concentrations. It is located about 25 km above the surface of the Earth.  Ozone makes up only 0.00006 percent of the total of the atmosphere, or 0.6 parts per million. In the ozone layer, however, this concentration is much higher at up to 15 parts per million.

Molecules of ozone are composed of three oxygen atoms (O3) and form naturally in the presence of ultra-violet (UV) radiation. In the stratosphere, ozone acts like a screen, filtering out all the UV-C radiation and most of the UV-B. Ninety-nine percent of the ultraviolet light reaching the earth’s surface is UV-A. Without the ozone layer, the entire planet would be bombarded with potentially lethal UV-B radiation. It therefore acts as a critical shield, protecting the biosphere near the surface from solar radiation.


Graph of ozone concentration throughout the atmosphere. Source: NASA.

The hole in the ozone layer

Concentrations of ozone in the atmosphere vary by location, but also change depending on seasons and climate. In general, the least ozone is found in tropical regions where the troposphere is thicker than at the poles. However, there is one place with even less ozone: Antarctica. In 1984, scientists discovered that there was a “hole” in the ozone layer – a region of relatively low ozone concentration – centered above Antarctica. News of the hole spread quickly, and more research was soon conducted to investigate what had caused the dramatic decline in concentration. In 1987, scientists were shocked to learn that the hole in the ozone layer had increased in size and was now as large as the continental US.

Concentration of ozone over Antarctica in 1978 (left) and 2016 (right). Credit: NASA.

There are a few reasons why the hole in the ozone layer was located over Antarctica. A polar vortex isolates the air above Antarctica, so that any chemicals that get caught down there have trouble dispersing. Additionally, during the winter, Antarctica is for a time in perpetual darkness, and the lack of UV light reduces the production of new ozone. Every spring the hole in the ozone layer is at its maximum. In September, after the Antarctic winter, measurements may be as low as 0 Dobson units in some areas.

CFCs, such as those formerly found in aerosol sprays, have been banned in many countries.

Ozone-reactive chemicals called chlorofluorocarbons (CFCs) have been used in aerosol sprays, solvents, and packing materials due to their low cost and durability. Unfortunately, when these molecules are released into the atmosphere they react with ozone in the stratosphere and deplete the ozone layer. CFCs are considered among the most potent ozone-depleting chemicals produced by people.

Chemicals can stay in the air for more than 10 years before they reach the ozone layer. Consequently, there is a significant delay between the release of these chemicals and their effects. It can then take a hundred years for chlorofluorocarbons to be broken down in the atmosphere, and therefore the consequences of using these chemicals may be felt for a long time. Some of the consequences of a reduced ozone layer are:

  • More skin cancer in people and animals and quicker aging of skin
  • Decreased immune system in organisms
  • Damaged crops and a decrease in yield
  • Reduction in growth of phytoplankton, the foundation of the food chain
  • Cooling of the Earth’s stratosphere and climate change

Research and measuring the hole in the ozone layer

To slow the damage we are causing to the ozone layer, a plan was formulated and signed by the international community in 1987. The Montreal Protocol on Substances that Deplete the Ozone Layer would phase out the production and consumption of ozone-depleting chemicals and eliminate the use of chlorofluorocarbons. Since then, more research has been done and more evidence has emerged on the devastating effects of anthropogenic (man-made) chemicals on the ozone layer.

Since the discovery of the hole in the ozone layer, governments around the world have initiated projects to keep better track of the ozone layer. The EASOE experiment in 1991 and 1992 found that the ozone concentration in Europe was at its lowest ever that year, but scientists suggested that it may be due to the eruption of the Pinatubo volcano in the Philippines, which ejected a vast amount of hydrogen chloride into the atmosphere. Typhoon Yunya, however, occurred shortly after and washed most of the chemical out of the atmosphere before it could reach the stratosphere. Nevertheless, the eruption impacted the climate and global temperatures for a few years. From 1993 to 1995, the SESAME project showed that ozone levels in Europe had dropped another 30%. This time scientists concluded that the coldest winter in 30 years had caused the decline in ozone levels.

In 2017, scientists from NASA and the National Oceanic and Atmospheric Administration (NOAA) in the U.S. announced that the hole in the ozone layer above Antarctica was the smallest it has been since 1988. This is still two and a half times the size of the hole in 1987 when scientists were shocked by its size. It was expected as well. Warm stratospheric temperatures in 2016 and 2017 minimized cloud formation in the stratosphere and constrained the growth of the hole. Unfortunately, scientists also pointed out that “the smaller ozone hole extent in 2016 and 2017 is due to natural variability and not a signal of rapid healing.” Real recovery is not expected to happen before 2070.


National Oceanic and Atmospheric Administration (NOAA)
NASA Ozone Watch
Paul Ward, The Antarctic Ozone Hole
Plan of action on the ozone layer: A joint publication of the World Meteorological Organization and the United Nations

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Vector vs. Raster Data Models Sat, 11 Nov 2017 19:50:35 +0000 Within geographic information systems (GIS), there are two main ways to model features across space: vectors and rasters. Both types of data models have unique

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Within geographic information systems (GIS), there are two main ways to model features across space: vectors and rasters. Both types of data models have unique advantages and disadvantages, and come with assumptions and limitations. Geographic research often involves analyzing data represented as one or both types of data models.

Vector Data Model

Imagine a stick figure for a moment. Stick figures are essentially vectors — their head, legs, body, and arms are made of points, lines and polygons; the three ingredients of a vector data model. Ultimately, all vectors are created by points, the basic building block of lines and polygons. Points represent an exact location but do not themselves take up space. Combining points in a linear fashion will create a line segment. Technically, lines are composed of an infinite number of individual points, so that any location along a line can also be associated with a single point, or exact location (typically on the Earth’s surface). Finally, line segments can be combined to form shapes with three or more sides, known as polygons.

Vectors are composed on points, lines, and polygons, as in this map of parcels in Omaha, NE.

Most geographic features can be represented as some combination of points, lines, and polygons. Which of the three is best used to represent a given feature, however, often depends on scale. For example, a relatively close-up (large scale) view of a city often requires a polygon to show the extent of the municipal boundaries or built-up area. If the feature takes up much of the territory or map being viewed, then a polygon is probably most appropriate. If you zoom out to the national level (small scale), however, it makes sense to represent cities as discrete points since they take up little space at such a broad scale.

Linear features like streams and roads are most often represented as line segments since they are much longer than they are wide. However, just like with cities, if you zoom in far enough you’ll see that all roads and streams have some width. In a vector data model, these linear features may be represented by a line at small scales and polygons at large scales (i.e., zoomed in), or they might be represented by lines at all scales. Often it depends on how much detail is actually needed to do a particular analysis or convey certain information.

Vectors are frequently used in all kinds of applications. One common arena is urban planning, where parcels and buildings are often represented as polygons, roads as lines or polygons, and small features like fire hydrants and telephone poles are represented by points. The U.S. Census Bureau also relies heavily on the vector data model, with census units like block groups and census tracts represented by polygons. All census data is then tied to these units for analysis.

Raster Data Model

Imagine zooming in really close to a digital photograph. What you’ll see eventually is that the entire photograph is actually composed of thousands (if not millions!) of small boxes known as pixels. A one megapixel image, for example, contains exactly one million pixels. Digital images are rasters; information is encoded in a continuous layer of grid cells arranged in a matrix across a surface. Every grid cell in a raster data layer is associated with one or more attributes or values. This can be a categorical value such as grass, trees, or water, or a numerical value such as inches of rainfall. Importantly, every space in a raster has a value, even if that value is zero or “no data”.

Vector data is composed of grid cells, each with an associated value.

While vectors are quite good at representing individual features such as buildings or streams, rasters are ideal for modeling variables that vary continuously over the Earth’s surface, such as land cover, rainfall, elevation,  and concentrations of air pollution. Rasters can also be very useful in calculating landscape change and estimating values across a surface. If, for example, you would like to know how much annual precipitation has changed over a certain period of time and across a given landscape, you could simply subtract one precipitation value from another for each grid cell to a get a change value. Additionally, if you want to estimate precipitation across a broad area but only have actual measured values at a few point locations, you can create a surface of estimated values in the form of a raster. This is a common type of spatial analysis that can be accomplished using different methods within a GIS.

Raster datasets come from a variety of sources, but one of the most common forms is remotely-sensed imagery. Remote sensing involves collecting data at a distance. In geography this often involves photographing or otherwise sensing the Earth’s surface using elevated platforms such as planes, helicopters, and satellites. Remotely sensed images can be imported into a GIS where they can then be viewed and analyzed using a variety of techniques. One common procedure is re-classifying remotely-sensed imagery in order to emphasize certain features in the landscape or assess the size of, or change in, a certain attribute.

A 30×30 meter resolution raster dataset of land cover data in the Chicago region. Note that urban land cover is red. Source: USGS.

Raster or Vector?

Deciding whether to use a vector or raster data model in your work entirely depends on the data you have at hand and what your goals are for displaying and/or analyzing the data. There are many analysis that make use of both data models or require the conversion of one to another. While conversion is a common procedure, it’s recommended that any translation between raster and vector be kept at a minimum to avoid accumulating error in your spatial model.

Land cover represented as a raster (left) and as a vector (right). Source: UConn library.

The size of the dataset may also be a consideration, as raster datasets can be quite large and difficult for some computers to process in a timely fashion. Often it is recommended to use vector data unless modeling a continuous surface. Furthermore, when using a raster data model it is important to use cell sizes that are appropriate for your analysis (i.e. an appropriate resolution).

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Glacial Landforms and Landscapes Sat, 11 Nov 2017 00:31:15 +0000 Over the last 2.5 million years, during the geological epoch known as the Pleistocene, no other force on Earth has shaped and reshaped the landscape

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Maximum extent of the Laurentide ice sheet. Source: Kansas Geological Survey.

Over the last 2.5 million years, during the geological epoch known as the Pleistocene, no other force on Earth has shaped and reshaped the landscape as much as ice. During this period, and up to the end of the last ice age about 12,000 years ago, the Earth has undergone numerous periods of continental-scale glaciation. Each time the ice sheets advanced and retreated they sculpted the landscape on a massive scale.

Even today, large ice sheets cover Greenland and Antarctica and hundreds of smaller mountain or alpine glaciers are found at high elevation all over the world. Though most of these glaciers are currently in retreat due to a warming climate, they continue to erode and shape the landscape.  In this article we consider the major landforms and landscape features produced by the erosional and depositional action of glaciers, both today and in the past.


Glaciers often act like huge conveyor belts, with ice flowing from an area of accumulation (where more ice forms from snowfall than is lost from melting, evaporation, etc.) to a zone of ablation (where more ice is lost than accumulates). For alpine glaciers, this means the ice flows downhill through a valley, and for ice sheets, ice flows from the center of the sheet toward the edges. As the ice moves, it frequently picks up an drags along rocks, soil, and other debris (collectively called glacial till). Regardless of whether the glacier is advancing or retreating, the ice will continue to move this material toward the edge, where it is deposited into mounds called moraines.

Medial moraines form between ice flows in this Alaska glacier. Source: Don Becker, USGS.

If the glacier forms a pile of till where it has advanced the furthest, this is called a terminal moraine. Often glaciers will not retreat quickly from this most advanced position, and will therefore build up a significant moraine at its terminus. As the glacier retreats it may also pause from time to time, when accumulation again equals ablation. Each time the glacier stops for a while, it is able to construct additional mounds of till known as recessional moraines. Thus, in a post-glacial landscape it isn’t unusual to have one terminal moraine and several recessional moraines.

Moraines can also develop along the sides of glaciers as material falls downhill from higher elevations and accumulates along the glaciers’ margins. These lateral moraines, as they are called, are commonly found among alpine glaciers. If two or more valley glaciers (alpine glaciers flowing downhill and through a valley) join, lateral moraines can get caught in between the ice flows forming what is then called a medial moraine. Large valley glaciers formed from the merger of multiple smaller ice flows may look stripped due the presence of medial moraines.

Ridges and Valleys

The Matterhorn in the Alps is a well-known horn, sharpened by the erosional work of ice. Credit: Go Trotting Switzerland, Flickr.

As they slowly migrate downhill under the influence of gravity, alpine glaciers tend to carve out steep valleys and ridges. The constant grinding of glacial ice and snow against either side of a mountain ridge causes the ridge to become steeper and sharper over time, forming what is known as an arête. Material is also removed from the valley itself, creating wider and wider U-shaped valleys. At the end of the last ice age, some of these U-shaped valleys along the coast became inundated with water, forming what we know today as fjords. There are, however, many dry U-shaped glacial valleys, including the spectacular Yosemite Valley in California.

Many alpine glaciers originate in bowl-shaped depressions high in elevation at the head of mountain valleys. These depressions, called cirques, can give rise to cirque glaciers (those actually within the cirque) and valley glaciers that move downhill between mountain ridges. Where three or more cirques come together, a very steep mountain peak may form, known as a horn. The Matterhorn in the Alps is a well-known example.

Glacial Lakes

When glaciers retreat and begin to melt, they often leave behind depressions in the ground that fill with glacial meltwater. These [mostly shallow] depressions in the ground are called kettles, and when they fill with water they are known as kettle lakes. Kettle lakes often fill with sediment and are usually not very deep; however, as is the case in Minnesota, they can occur in large numbers in the post-glacial landscape.

Some glacial lakes can be much larger in size than the average kettle lake. The Great Lakes in North America are the largest glacial lakes in the world, and formed around the end of the last major ice age (~12,000 BCE) when the Laurentide ice sheet left massive depressions in what is today the Great Lakes basin. As the ice sheet retreated, meltwater filled the depressions carved by ice and continued to be feed by groundwater. Other lakes have formed by huge waterfalls draining off the sides of continental glaciers, scouring away the land underneath.

Many of the “10,000 lakes” in Minnesota were formed by glaciers, most notably kettle lakes. Source: Google Maps.

Meltwater has also accumulated on top of and alongside continental ice sheets, forming lakes such as the former Lake Missoula in present day western Montana. The lake was massive; it covered 3,000 square kilometers and contained about half the volume of Lake Michigan. The water was held back by an enormous ice dam, some 600 meters tall, than was breached several times, causing massive floods across the northwest U.S. A couple hundred cubic kilometers of sediment were scoured out by these floods, forming broad canyons.

Glaciofluvial Deposits

An esker in Labrador, 1986. Credit: Gord McKenna, Flickr.

Glaciers are often quite ‘dirty’, carrying with them tons of rock, sediment, and other debris. Much of this material is simply dropped by the glacier itself (such as the case with moraines), but some is transported away by glacial meltwater coming off the top of the glacier, and from underneath. Meltwater that forms on the surface of a glacier can flow down to the base by entering deep well-like holes in the ice known as moulins. This water may be channeled along the underside of the glacier through tunnels, which then empty out along the edges of the glacier.  Meltwater can carry a significant amount of sediment, which then gets deposited either underneath the glacier or out into the adjacent landscape as an outwash plain. The outwash plain, also called a sandur, consists of a layer of mixed sediment and debris deposited both directly by the glacier and by glacial melwater.

Occasionally, the channels of water flowing underneath glaciers become clogged with sediment, much like old pipes in your home can become clogged with minerals and other materials. Once the glacier retreats, a narrow, snake-like hill known as an esker is left behind. Eskers rise above the landscape like inverted stream channels, many even appearing to meander back and forth like a river. Mound-like features known as kames may also form as sediment gets trapped and piles up underneath a retreating glacier.


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History of Maps Wed, 08 Nov 2017 03:05:02 +0000 Ancient Maps Maps are one of the oldest forms of human communication, perhaps even pre-dating the written word. Cave paintings dating back almost 20,000 years

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Ancient Maps
Cuneiform tablet from Mesopotamia showing map of the local village and surrounding landscape.

Maps are one of the oldest forms of human communication, perhaps even pre-dating the written word. Cave paintings dating back almost 20,000 years have been found that contain simple astronomical maps of the stars and constellations. Aside from cave paintings, the oldest surviving maps were carved into stone or clay tablets more than 4,000 years ago in a region known as Mesopotamia. Located between the Tigris and Euphrates Rivers in modern day Iraq, Mesopotamia is known as the “cradle of civilization.” The area saw the rise of the Sumerian civilization and the Akkadian, Babylonian and Assyrian empires.

The ancient maps focused on important features of the landscape including the location of hills, mountains, valleys, villages, canals, rivers and other water sources, gates, walls, and houses. The ancient Egyptians used advanced surveying techniques to produce accurate maps showing boundaries, routes, and the locations of mineral deposits and other important features. One of the best preserved maps of ancient Egypt, the Turin Papyrus, included a map legend and depicted various features using different colors.

Left half of the Turin papyrus map. Courtesy of J. Harrell, Wikipedia.

Greek & Roman Maps

The Greeks used observations and mathematics to create maps of the entire globe as it was known at the time. The Greeks did in fact consider the Earth to be a sphere, rather than a flat plane. In the 4th century BCE, the Greek philosopher Aristotle made the argument for a spherical Earth based on the observations that 1) during the lunar eclipse the Earth casts a circular shadow on the Moon, 2) ships dip below the horizon as they sail out to sea rather than simply disappearing from view, and 3) not all the stars can be seen from one point on Earth. Later, Eratosthenes used the angle of sunlight at different latitudes to calculate the circumference of the Earth and was only about 2 percent off the true measurement.

Ptolemy’s world map, circa 150 CE. Source: The Catalogue of Illuminated Manuscripts.

Greek and Roman map-making reached it’s pinnacle with Claudius Ptolemy (90–168 CE), who used a coordinate system of lines running north-south (longitude) and east-west (latitude) in order to locate fixed positions on the Earth’s surface — a fundamental cartographic method still in use today. Ptolemy’s main geographic work, Geography (or Geographia), included his most famous map, a world map showing Europe, the Middle East, northern Africa, and western Asia. While ground-breaking, Ptolemy’s maps contained inaccuracies partly due to a miscalculation of the Earth’s circumference.

Roman maps tended to be practical, showing the locations of towns, roads and other infrastructure vital to the administration of the empire. The map below, Tabula Peutingeriana (Peutinger Map), is a 13th century copy of a 1st century Roman map showing the empire’s extensive road network, the cursus publicus.

Small section of the Tabula Peutingeriana showing Rome’s impressive road network, the cursus publicus. Source: Bibliotheca Ausgustana.

Chinese Maps

Yu Ji Tu, a map carved into stone in the year 1137 during the Song Dynasty. Source: Library of Congress.

The earliest maps in China date back to the 4th century BCE.  Many of the early maps were drawn on sheets of silk or wooden and were used to depict river systems, administrative boundaries, and locations of natural resources like timber and minerals. Pei Xiu (224–271), known as the “Ptolemy of China,” produced maps with a geometrical grid and graduated scale to more accurately determine location and the distance between points. In the 12th century, during the Song  Dynasty, detailed maps of China and the surrounding region were etched into stone. These  stone stele maps (named after the Stele Forest of Xian where they were found) are impressively detailed, with intricate coastal boundaries, major river systems, and hundreds of settlements.

Maps of the Middle Ages

In Europe, map-making and cartography were dominated by the church during the Middle Ages (roughly from 400 to 1400 CE). The works of classical cartographers like Ptolemy were largely forgotten or ignored in favor of more simplistic, theology-based depictions of the Earth’s surface. The so-called “T-O” map is perhaps the definitive example of the medieval map.  It is a circular map (the “O”) of the world divided into three parts by a T-shaped (the “T”) divide.

The basic layout of a typical T-O map (Meyers Konversationslexikon, 1895).

The section of the map above the T represented Asia; to the left of the T was Europe, and to the right was Africa. In addition to depicting the three known continents, the sections also represent the portions of the Earth apportioned to the three sons of Noah: Shem (Asia), Japheth (Europe) and Ham (Africa). At the very center of the map was Jerusalem, the birthplace of the three major Abrahamic religions —Judaism, Christianity and Islam. The lines of the T represented the known world’s major waterways, which included the Mediterranean Sea (the upright portion of the T running east-west), the Don River (the northern portion of the T), and the Nile River (the southern portion of the T).

Despite the simplistic “disk-like” world view illustrated by the T-O map, it was common knowledge throughout the Middle Ages that Earth was a sphere rather than a plane. However, at the time it was believed that no one could inhabit the southern portion of the globe due to the torrid climate of the equatorial region.

The Hereford Mappa Mundi, created circa 1300 in England, is a typical “T-O” map with Jerusalem at center, Europe at bottom left and Africa on the right. Credit: Richard of Haldingham.

Maps of the Renaissance & the Age of Exploration

By the early 1400s there was renewed interest in the works of the classical cartographers. The works of Ptolemy, including his maps and books, became widely distributed with the introduction of the printing press in 1436. At the same time these classic maps and carographic methods were being rediscovered, technological advancements in shipbuilding and navigation heralded a new and unprecedented era of exploration and discovery.

With the new information provided by explorers such as Dias, Columbus, Cabot and Magellan, German and Purtugese cartographers took the lead in expanding Ptolemy’s world map and drawing new nautical maps for sailing. Following Christopher Columbus’ voyage in 1492, cartographers began to include the America’s in maps of the world. In 1507, the German cartographer Martin Waldseemüller produced the first map of the world, titled Universalis Cosmographia, with the the label “America.”

The Waldseemuller map provided the first geographic reference to America. Source: Cornell U. Library.

The Portuguese cartographer Diogo Ribeiro is credited with the first “scientific” world map, the 1527 Padrón real. The map, which shows much of eastern North and Central American and almost all of South America, was created using the latest information obtained from Magellan’s voyage around the world. Ribeiro’s map is the first to show the (nearly) full extent of the Pacific Ocean, and to depict eastern North America as a single contiguous landmass.

Gerardus Mercator (1512–1594), a well-known Flemish cartographer, developed the Mercator map projection in 1569. As with all cylindrical projections, the lines of latitude and longitude meet at right angles on the Mercator projection. Maps based on the Mercator projection were eventually adopted by the seafaring community, which relied upon accurate directions, particularly over long distances. One major disadvantage of the Mercator projection is the east-west and north-south stretching that becomes increasingly pronounced toward the poles. This size distortion causes landmasses in the mid and upper latitudes to appear larger, and landmasses around the equator to appear smaller, than the actually are.

Gerardus Mercator’s world map, 1569. Not the absence of Australia, which was discovered by Europeans until 1770.

Modern Maps

With the advent of better surveying equipment and other scientific tools, such as the telescope, sextant and chronometer (to tell time), maps have became increasingly accurate and detailed over the last 300 years. Only in the last 100 years, however, has much of the Earth’s surface been mapped accurately. The rapid development of flight in the early 20th century allowed large tracts of land to be sensed remotely and mapped according to photographs taken from the air. By the end of the century, satellites orbiting the Earth could scan the entire surface within just a few days. Today, cartography is still both an art and a science, but it is done mainly on computers rather than on the drawing board. First developed in the 1980s, computer-based Geographic Information Systems (GIS) allow users to not only create maps, but to also analyze, store, and manipulate spatial information.

The Earth as seen from NASA’s Terra satellite using the MODIS sensor. Credit: NASA.


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The Geography of Religion Tue, 07 Nov 2017 02:59:35 +0000 There are few things that both unite and divide people as effectively as organized religion. Up until modern times, nearly every civilization, society and empire

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Geographic distribution of the world’s largest religious groups. Source: The Independent, Statista.

There are few things that both unite and divide people as effectively as organized religion. Up until modern times, nearly every civilization, society and empire has been affiliated with a particular belief system. Religion is one of the defining features of a culture, and one that can affect nearly every aspect of daily life, including clothing, diet, occupation, recreation and education. Although a wave of secularism in modern times has caused organized religion to wane in many regions of the world, it still remains a powerful geopolitical force and important aspect of billions of people’s lives.

The sharing of a particular set of beliefs and values has effectively united groups of people throughout history, and because groups of people are usually associated with particular geographic areas, religion too is linked to specific regions. Much like language, religion has been one of the defining means by which groups of people have differentiated themselves from other groups of people. In this way, religion helps to form a cultural border around a group of people that separates them from other cultural groups, and helps maintain solidarity within. Integrated with other aspects of culture, religion has formed the basis of  countless civilizations and empires, and has been one of the dominant forces in the development of the modern nation-state.

Islam began in eastern Saudi Arabia around 600 C.E. and spread outward rapidly over the next 200 years.

Religion, like other aspects of culture, exhibits a very distinct geographic pattern. The spatial distribution of major religious groups results from the migration of people and the spread of religious ideas from one place to another. Much like language, organized religions, and the religious ideas and values that give rise to them, typically originate at some central location and diffuse outward, often evolving into sub-religious groups (much like regional dialects) as they expand. Islam, for example, originated on the Arabian peninsula in modern day Saudi Arabia around the year 632 C.E., and spread outward as far west as Spain, as far east as Turkmenistan, and as far south as Sudan, in just three centuries. Through military conquest and the spread of the new Islamic religion, the Arab Empire  became one of the largest in history by the end of the 8th century.

While all religions have a region of origin, many also center around a particular holy city and/ or group of sacred spaces. Jerusalem, for example is a holy city of the three major Abrahamic religions: Judiasm, Christianity and Islam. In Islam, however, Mecca is the most holy city on Earth; it is the birthplace of the Prophet Muhammad, the founder of Islam. In Christianity, Rome is home to the Papacy and has served as the center of the Roman Catholic Church for over 1,600 years. Christianity is an example of a religion that arose in one place (modern day Israel), found a new cultural center (Rome), and is now almost entirely absent from its place of origin.

For many religions, rivers and other natural features of the landscape serve as sacred spaces and hold special meaning for followers. For example, the Jordan River along the eastern border of Israel is sacred to Christians, while Ganges River is sacred to Hindus and Mount Fuji is sacred to Shintoists in Japan. There are also countless sacred structures, many concentrated in holy cities, built to honor deities and other religious figures.

The early spread of Christianity was focused around the Mediterranean basin.

Although there are dozens of organized religious throughout the world, only three are considered major universalizing religions because they actively seek to convert non-believers and members of other religious groups: Christianity, Islam and Buddhism. The desire to universalize their teachings has had a profound impact on the size and geographic distribution of these religions. It is no accident that the three religions have over 4 billion followers, more than half the world’s population.

The Ganges River in northern India is a sacred place according to the Hindu religion. Credit: Sergio Carbajo, Flickr.

Organized religions that are not universalizing (or proselytizing) tend to remain relatively small or at least bound to a particular geographic region. They are also usually associated with a single cultural group. Hinduism, with about 1 billion followers, is the largest non-universalizing religion, but is limited to India and parts of southeast Asia. As suggested by its size and reach, Hinduism was in fact a universalizing religion thousands of years ago when it spread eastward from the upper Indus Valley. Shintoism, Judiasm and Sikhism are examples of smaller non-universalizing religions also associated with distinct cultural communities.

In addition to religious values, the spread of organized religions was also the result of technological and economic forces. Today, the populations of Middle and South America are overwhelmingly Roman Catholic. Roman Catholicism is not only a universalizing religion, it happened to be the dominant religion of Spain and Portugal, the two dominant seafaring nations in the 15th and 16th centuries, and the first two countries to reach the New World. Once there, the Spanish and Portuguese conquerors and missionaries set about converting the local populations to Roman Catholicism. Further to the north, however, English, Danish and Dutch immigrants settled the East Coast and Midwest regions of the present-day United States, bringing with them their Protestant Christian religion.

Although much later waves of immigrants from Ireland, Italy and France would increase the proportion of Roman Catholic followers in North America, Protestanism would remain the largest religion in the United States and much of Canada. The Protestant sect of Christianity is also the dominant religion in other regions settled by northern Europeans, including South Africa and Australia.

Today, Christianity – both in terms of number of followers (about 2 billion) and land area – is the largest major organized religion. It’s followers can be found throughout the world, but most notably in North and South America, Western and Southern Europe, Russia, Australia/ New Zealand, and parts of Sub-Saharan Africa. Islam is the second largest religion, with about 1.5 billion followers. Followers of Islam (Muslims) can be found throughout northern and eastern Africa, the Middle East, central Asia, and much of Indonesia.

Both Buddhism and Hinduism have between 500 and 1.5 billion followers and their followers collectively cover much of southern and eastern Asia. Many regions of the world have been moving away from organized religion and towards secularism (freedom from religious rule and teachings). These areas include most notably China, Europe and Russia, and parts of North America. The percentage of the non-religious has reached 90 percent in China and parts of Scandinavia.

Percent of people not affiliated with a particular religion by country. The non-religious make up 90+ percent of some nations. Source: Wikipedia.

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