Lab 5 - Map Projections

 

Map projections such as the various projections portrayed above are useful tools in terms of displaying the Earth's spherical surface as an image on a flat surface. This allows the users to judge distance, navigational directions, and approximate locations in the real world by using such map projections. The various map projections that are available out there allow different projections to be made for different purposes: if only a nautical heading is needed, the conformal projections can be used, if only distance is needed, an equidistant projection could be used. With the need to answer the various geographical questions available out there, more tools such as the numerous projections allow for more ways to analyze geographical data.

The existence of the various projections available out there are the result that no projection is perfect - different projections are used for different reasons. As a result, without the basic knowledge of map projections, a user may have a difficult time in obtaining accurate information from the improper projection. For example, the actual distance from Kabul to Washington D.C. is about 6,944 miles. The most ideal projection to be used would be an equidistant projection. However, even though we have selected the most ideal projection, the projection itselfis very limited in displaying the distance accurately. To be more specific, an azimuthal equidistant map projetion would be more desired because the equidistant map, in measuring distances, is most accurate only when the locale of interest (the starting point for the measurement) is located in the middle of the sphere, otherwise distances would become greatly distorted. For example, the equidistant conic map is the more accurate measurement for the distance between Kabul and Washington D.C. of the two equidistant maps. Had the center of the map been focused on either Kabul or Washington D.C., the distance between the two would have been more accurately measured.

While equidistant maps are useful for displaying the distances on the map accurately, they still may distort the actual size of the landmasses drastically depending on how far away they deviate from the center. In order to preserve the actual size of the landmasses then, an equal area projection would be more useful. However, with equal area projections a compromise must be made in displaying accurate distances. For the Bonne projection, it relies on the central meridian being the point of reference, the farther east and west we deviate from the meridian, the more distorted the water becomes (which may cause an inaccurate measurement between distances). However, since the area is preserved, the distance between Kabul and Washington D.C. wasn't drastically affected because they are relatively close to the meridian. As for the Hammer-Aitoff projection, it uses the equator as the central point of reference, the rest of the parallels of the equator are curves - the farther north and west we deviate, the more distorted the distance from North to West becomes. Once again, the distance according to the aforementioned projection is 8,447 miles, a mere 1,000 something odd miles off from the actual distance.

Lastly, the final map projection that must be discussed is the conformal map projection. These map projections are used to preserve local angles - meaning they are great for navigational purposes, when a heading is needed. The drawback behind these projections is that the farther we deviate from the equator and the middle of the map, the more distorted the shapes become, thereby rendering distances to be unusable due to its large variations with different map projections. The Mercator projection is the most common projection used - it preserves local angles but not the shape or size of continents that deviate far from the equator. Hence, the distance from Kabul to Washington D.C. calculated in the Mercator is 10,006 miles. In comparison, the Gall Stereographic projects a sphere onto a flat surface of the Earth. The projection may not distort the shape and size of landmasses, however, the distortions are not as great as that found in the Mercator map. In addition, for the Gall Stereographic projections, the projection uses 45 degrees N and S of the equator as points of interest, which provides the map with an overall sense of balance on the map - no section will become drastically distorted. The distance measured between Kabul and Washington D.C. by the computer on the Gall Stereographic map is 7,162, much closer to the actual value than some of the other projections.

Based on the discussion mentioned above, there are limitations as to what a user can do on ArcGIS without delving too in depth with the content. Despite these limitations, because different situations call on different maps, there still exists a multitude of map projections available to analyze the data. As long as the user considers what is the feature that is being compromised on the map projection and what the purpose of the projection is, ArcGIS can become a very powerful tool in geographic analysis.