Lab 5: Projections in ArcGIS

Map projection is the process of converting a spherical model onto a planar model, while preserving certain spatial characteristics, such as distance, area, or shape. Originally conceived by projecting from within the sphere, it enables the creation of accurate flat maps. However, one must be aware that not one projection type can accurately translate all the spatial characteristics from the sphere to the plane. Thus we have different projection types, namely conformal, equidistant, and equal area, each with distinct advantages and disadvantages that are used accordingly. All these different types, regardless of their accuracies and inaccuracies, all enable the convenient representation of maps on a planar surface, allowing easy distribution and storage of maps that would not be possible with globes.

Conformal map projections, such as Mercator and Gall stereographic above, preserve shape and local angles, creating a system of orthogonal latitude and longitude gridlines. Mercator specifically represents rhumb lines derived from an initial bearing as straight lines, and stereographic preserves the shape of circles. However, conformal maps distort area, which is made obvious by the disproportionate size of Antarctica in both Mercator and Gall stereographic examples. Equidistant map projections, such as cylindrical and conic above, represent accurate distances along designated lines and outward from the center. However this type of projection significantly distorts area sizes, and does not necessarily show true distances of the points along the center, as we can see in the inaccurate distance between the Americas and Australia in the equidistant conic example. Equal area map projections, such cylindrical and sinusoidal above, preserve respective areas but fail to accurately represent latitude-longitude grid angles. Cylindrical, also known as a Gall-Peters projection, only represent true distances along the 45th parallels north and south. On the other hand, sinusoidal represents the area of the Earth as the area between two symmetrically rotated cosine curves. We can clearly see in both cylindrical and sinusoidal equal area examples that these gridlines are distorted, simply by comparing both to the conformal Mercator example.

For the purpose of our lab, in which we measured the distance between Washington, D.C. and Kabul, we clearly see the distinctions between the types of map projections. To determine bearing we can look at both Mercator and Gall Stereographic conformal map projections. We see that the linearity in all directions dictates that traveling southeast in a straight line will conveniently get me from D.C. to Kabul. To determine true distance we can look at both cylindrical and conic equidistant map projections. We see that the even mapping of longitudes and latitudes dictates that the true distance between D.C. and Kabul is around 5,065 to 6,941 miles. And as obvious as its name says, we can accurately assess the areas of the United States and Afghanistan by referring to both cylindrical and sinusoidal equal area projections. I am certain that interchanging any of these projection types with the data we are seeking, will only result in false measurements.

All map projections have both positive and negative implications depending on what information is being sought. Because it is obviously impossible to accurately translate all spatial data from a sphere to a plane, each projection will preserve some characteristics while significantly distorting the rest. As mentioned before each map projection has its proper use, which is why any spatial analysis can be rendered erroneous if the wrong type of projection is chosen. This is the reason why it is important to be aware of how each map projection was created, and what spatial data it excels in translating from the three dimensional to the two dimensional domain.