Week 6: Map Projections

Map Projections show both the wide variety of uses of geographic data and the potential problems with analyzing that data. At first glance it seems odd that there would be such a wide discrepancy in the distance between Kabul and Washington DC along the maps. Although the great elliptic, which measures the root that would actually be traveled by a person on the earth, remains constant, the planar distance has a 4,000 mile variance, with an error of over 3,000 miles in some maps. It is surprising that map projection choice can have such a large effect.

However, when we look closer it begins to become clear why this happens. When going from a 3 dimensional globe to a 2 dimensional map, certain features will necessarily be lost in translation. Since we have a wide variety of uses for maps it begins to make sense. In some situations maintaining equal area will be important, and we will use the Eckert II, while in others shape will be most valuable and we will use a Stereographic projection. Since we were focusing on measuring distance in this lab, it is therefore unsurprising that some maps are better than others. Different tools are required for different situations, and the wide variety of projections makes it possible to take the same geographic data and use it to answer a wide variety of questions about various attributes.

However all this variety does have its drawbacks, however. The wide spread of results means that it is harder to trust that what you are measuring conforms to reality. Even with the equidistant map projections there was a large discrepancy in planar distance. With Equidistant Conic the planar distance from Washington DC to Kabul was 6,957 miles but with Plattee Carree it measured 10,098 miles. Since these maps are both supposed to be equidistant this result reinforces that every map projection will give slightly different results for planar measurement, making it hard to determine what the true value is.

For all the drawbacks of geographic projections, they are a valuable tool. While planar measurements may be inaccurate, the Great Elliptic distance is constant among all projections, and is a fairly reliable tool. In addition the different maps are helpful for visualization when examining different phenomenon. For example, if I was trying to look at the relative sizes of the United States and Brazil, the cylindrical equal area map would be very helpful while if I was trying to figure out the distance between two points I could use a Plattee Carree. Different map projections help us use a single geographic data set into a wide variety of maps and make it possible to visualize a wider variety of phenomena.

Larger Version of the Image available Here: http://www.flickr.com/photos/aroch/7219679272/in/photostream