Lab 5: Projections in ArcGIS

To create a map, one must decide how they will represent the real world on a 2D plane.  To do this, map projections are necessary, as they are the representation of the surface of our ellipsoidal earth on a flat plane.  Projections enable us to look at our Earth in a more convenient 2D plane, instead of a 3D globe.  The parameters of a projection include datums (the geometrical representation of the earth), standard lines, central lines, and false easting and northing.  Though it is impossible to preserve all aspects of a sphere when projecting it onto a plane, certain properties of the real world can be conserved, such as the perpendicular intersection of parallels and meridians (conformal projections such as the Stereographic and Mercator seen above), distances from the center of the projection to every other location (equidistant projection such as the Azimuthal and Sinusoidal above), and the proportion of areas (equal area projection such as the Hamme Aitoff and Goode's Homolosine above).

One of the perils of map projections is that, as mentioned earlier, it is impossible to preserve all properties of the Earth when projecting it onto a plane, and so the projection invariably looks different from the real, ellipsoidal Earth.  Each projection type has different potentials and pitfalls, depending on the properties that are preserved, and those that are not.

In conformal protections, the main advantage is that local angles are preserved, making them useful for navigation and large scale maps.  However, a disadvantage is that areas at the poles and equator and significantly distorted, as seen in the Mercator and Stereographic projections above.  Areas at the poles appear much larger than they actually are, while those at the equator appear much smaller.  Because of this, mercator maps are not well suited to be general reference maps on a world scale.

In equidistant maps, because distances are conserved from a given point, they are useful for evaluating situations where accurate distances from a certain location are important, such as airline travel routes, seismic and radio maps, and the North Korean missile crisis.  One issue with this map is that only distances from the given point are correct, while all other distances are not, as demonstrated by the different distances between Washington DC and and Kabul measured on the equidistant map projections (Table 1).  Furthermore, these maps are not useful for looking at areas of entities.  This is particularly evident in the Azimuthal Equidistant projection above.

Table 1. Distance between Washington DC and Kabul by Projection Type

Equal area maps are be useful for calculating the occurrence of a phenomena in particular areas, such as a thematic maps.  One downfall is that maps cannot be equal-area and conformal, therefore local angles are cannot be preserved in equal area projections.  Furthermore, distances are not necessarily accurate in equal area maps, as seen in Goode's Homolosine projection above.