Lab 5: Projections in ArcGIS

  

     Map projections are used to distort the surface of the earth into a two dimensional format that can be used for reference or navigation. There are numerous types of map projections such as conformal, equidistant, and equal area. Additionally, within each projection category, there are multiple ways to project the coordinate system for the respective map projection. Mercator and WGS 1984 PDC Mercator are two ways to maintain a constant linear scale in all directions, preserving the angles and distorting only excessively high latitudes on a conformal map projection. Equidistant conic and plate carree preserve distance from a standard point or line for equidistant map projections. Plate carree preserves distance from 0 degree parallel, while equidistant conic maintains scale along all meridians. Cylindrical equal area and Hammer-Aitoff preserve area by displaying the true proportions to the same areas on the earth for equal area map projections.
  
     Conformal maps, primarily Mercator projections, are often used for nautical navigation; however they are also often used by textbooks and for display purposes. This is impractical because it causes people to become familiar with a distorted world that has a grid coordinate system as opposed to a circular system. Equidistant maps are often used by atlases, airlines, and other aviation references. Similarly, equal area maps are commonly found in atlases and are used to reference regions that span in great directions such as Asia and the Pacific Ocean. Equal area maps are great when used to focus on a particular area, but are misleading when used to cover areas such as the entire earth.
  
     While completing this lab I noticed that while certain coordinate projections are classified under the same map projections, they won’t necessarily look the same. For example, Plate carree and equidistant conic appear significantly different although they both preserve distance. It’s just that they use different reference points.
  
     The true distance from Washington DC to Kabul, Afghanistan is approximately 6929.531 miles. As represented in each coordinate projection, the distance varies from 6,973.820 miles in the equidistant conic coordinate system, to 10,140.596 miles in the plate carree coordinate system. I found it strange how although they both use an equidistant map projection, their distances vary severely (by more than 3,000 miles). Besides the way the coordinates are projected or referenced, another reason for this discrepancy may be the way the distance was measured. When using a map of the world to determine distances between cities, it may be difficult to get the same exact reference point at each location when measuring the distance. A pixel off may affect the result by a hundred miles or more. Ultimately, from what I have learned in this lab, there are different projections for different situations and that it is best to use the most accurate projection in regards to the context for which you need the map, with the largest scale available.