Lab 5: Projections in ArcGIS

Map Projections
Distance Between Washington D.C. and Kabul
GCS WGS 1984: 6,944.09 miles

Significance, Perils, and Potential of Map Projections
The world is a three-dimensional sphere-like object spinning through space.  In order for humans to represent it in a useful form, we translate it to a two-dimensional projection on paper or a computer screen.  The problem with this translation is that some information will unfortunately be lost in the process.  Most map projections can preserve one or two properties such as the angles of the lines of longitude and latitude, the distance between places on the map along a parallel, or the area of each location.  When preserving one property, another will be distorted.  Another problem is that the larger the area covered on the map the greater the distortion.  The distortion is negligible when the effect of the curvature of the Earth isn’t felt on the area covered.  In order to decide which projections to use, a map reader needs to decide which properties are important for the maps use.  In this lab we looked at two versions of the three categories of projection: conformal, equidistant, and equal area to compare the distance between two cities, Washington D.C. and Kabul.

Looking at the conformal projection of the world, I used the Mercator and Gall Stereographic projections.  Conformal projections preserve the angles of the globe at the expense of the area and distance between locations.  The Mercator map is generated from a cylindrical projection giving it a rectangular shape.  The rectangular shape is the map shape we are most familiar with and so makes it the easiest to use.  However, the Mercator projection has two problems is it heavily distorts the area, making Antarctica appear larger than the rest of the continents combined, and it distorts the distance between the two cities with the measured distance approximately four thousand miles longer than the actual measured distance.  At the same time, the Gall Stereographic has a rectangular shape.  Conversely this projection doesn’t distort distance as much as the difference between the actual distance and the projection distance is less than two hundred miles.  The conformal projection is most useful in directionality and navigation as the angles between the longitudinal lines.  However, the distance between the different points could vary greatly so you would need another map to check the actual distances between locations.

Equidistant maps are projections that preserve the distance between locations on the globe.  For the exercise I used the Equidistant Conic and the Aitoff Projection.  The Conic projection takes the shape of a cone projection with the southernmost area of the globe heavily distorted around the edges.  However, this map does a great job of preserving the distances of points north of the equator close to the North Pole.  The Aitoff projection presents an ellipsoid with heavy distortion near the poles away from the equator.  Conversely there isn’t a lot of distortion of area on the map as the northern and southern poles taper in to a point.  For this reason the distance between the two cities is much distorted in comparison to the conic projection.  Depending on whether you were looking at the Northern Hemisphere or the equator, you would use either the Conic or Aitoff map.  However, if you were looking for a map that doesn’t distort area much, you would use the Aitoff map.

A big problem with most map projections is that they distort area, especially on world maps.  Many maps oversize Antarctica or distort the size of Greenland heavily.  In this exercise I looked at the Mollweide and Bonne projections.  The Mollweide projection has an elliptical shape that preserves the area of continents around the globe, but does not preserve the distance between points away from the equator.  The Bonne projection has a heart like shape that preserves the area of the continents while the distance between the cities is well preserved.  In this case the distance is preserved as the point in which the projection was based off of is the North Pole, keeping the distortion of Northern Hemisphere locations to a minimum.  However, this projection is an unusual shape for a map that makes getting information from it difficult.  At the same time, the projection has intense curvature and the appearance of discontinuities between the heart edges of the map. 

Looking at the different available projections that can made of the world, you can see the power of GIS at work.  You can take the same data and filter it through different projections in order to analyze different properties.  Each projection has its own positives and negatives and requires the map maker to decide which properties to prioritize and which to allow distortion.  Each projection changes the way in which we can see the world, which can allow for misleading information to be presented as fact.  It is important when making a map to clarify the distortions so that incorrect information will not be taken as fact.  At the same time, map projections allow us to take the complexity of our three-dimensional world and break it down into a form that is easiest for us to process.