Lab 3: Map Projections

Map projections enable people to view the three-dimensional, spherical Earth on a two-dimensional surface. There are several types of map projections, with each type preserving a different aspect of the Earth. Because map projections preserve different aspects of the 3D Earth in 2D, distortion of the Earth’s surface often occurs. Furthermore, each projection may have a different center with distortion occurring as you move further away from the center. Map projections are categorized into three main categories: conformal, equal area, and equidistant. In conformal projections, angles are preserved and the projection works best when looking at small areas. Equal area projections preserve the relative sizes of Earth’s features and shows area accurately. Equidistant projections preserve distance but only from the center of the projection.

This lab demonstrates how the Earth becomes distorted when viewed in the different types of map projections. I chose two conformal (Mercator and Gall Stereographic), two equal-area (Hammer-Aitoff and Bonne), and two equidistant (Equidistant Conical and Sinusoidal) projections. For this lab, I measured the distance between Washington, D.C. to Kabul, Afghanistan. The actual distance between the two cities is 6,877 miles. However, each projection provided a distance between the two cities that was quite different than reality. The differences in distance between Washington, D.C. and Kabul demonstrate how map projections can distort the Earth. For instance, I calculated the distance to be 10,119 miles in the Mercator projection. The difference between the projected and actual distance is 3242 miles. Mercator greatly exaggerated the distance between the cities. In Mercator, the center of the projection is the equator. Therefore, the least distortion will occur near the equator. Washington D.C and Kabul are in the northern latitudes, far from the equator, and are more distorted.

The distortion of land and distance in the northern latitudes is demonstrated by the size of the 30°x30° grid. In Mercator, the 30°x30° grid elongates as it moves away from the equator. Similar to Mercator, Gall Stereographic projects the spherical Earth into a plane and preserves only the angles. Distance between two points on a Gall Stereographic projection will be exaggerated because it does not care to maintain accurate distance or area. The 30°x30° grid in the Gall Stereographic projection is no longer a square but is a rectangle. As you move away from the equator, the rectangle becomes taller. It is not as elongated as the Mercator grid but distortion of the grid does occur. In the equal-area projections, Hammer-Aitoff and Bonne, the 30°x30° grid is also distorted. The grid curves in the Hammer-Aitoff projection. The curvature of the projection provides an oval shape, causing the most distortion at the equator. The difference in actual and measured distance is much smaller than the conformal projections. The Bonne map is heart shaped and the grid is distorted. However, Bonne provided the least difference in actual and measured distance. One explanation is that Bonne has the least distortion at the North Pole because that region resembles the spherical Earth the most. The Equidistant Conical projection map was the second closest measured distance to actual distance. The projection seems to be centered at the North Pole so areas closer to the pole are less distorted than areas further away. Furthermore, the map is circular and preserves the spherical aspects of the 3D Earth. Even though the Sinusoidal projection is categorized as an equidistant projection, it does have equal area projection aspects to it. Because of the aspects, the distance between the points is greatly exaggerated. The shape of the map itself also demonstrates the distortion because the poles are like the tip of triangles. In a Sinusoidal map, the relative size of landmasses is maintained while shape and direction are distorted. Overall, all of the maps show a distortion of the 30°x30° grid. Sometimes the grid is similar to the actual shape of a 30°x30° grid on the 3D Earth. However, when the grid does not seem similar, the distance between D.C. and Kabul become distorted.

Although map projections do exaggerate areas that are further away from its center, the projection itself is not the only reason the distance between the two points was not accurate. As mentioned in lecture, errors do occur. Human error is a possible reason I could not accurately measure the distance in projections meant to preserve distance. Even though ArcMap’s measurement tool has a ‘snap to feature’ option, unsteady human hands can lead to measuring errors. Furthermore, map scale affects the measurements. Through exploration, I found that map scale played a role in calculating distance. When I viewed the Equidistant Conical projection at a larger scale, the measurements became more accurate. The distance measured was 6,972 miles instead of 7,033 miles (what I measured before zooming in). Scale is important in map projections because the appropriate type of projection to use depends on the question that needs to be answered. For small areas, conformal maps seem to be the best choice. On the other hand, if distance is desired, then equidistant maps are the better option. Overall, in order to answer specific questions, one must choose the right type of map projection that will provide the least distorted, most accurate data.