Week 6: Projections in ArcGIS

Map projections are important in GIS because they project geographic data clearly. Even though the Earth is a 3D object, we still must project in 2D form many times. But this causes problems because all map projections introduce some sort of mathematical distortion from shape to area to distance or direction. We must know how certain map projections distort our globe and how to use the preserved state of the projection for our benefit because depending on the purpose of the map, some distortions are acceptable and others are not.

Equal Area maps preserve, you guessed it, the area. The Bonne projection is a pseudo-conic projection. It has segmented meridians and the meridians bend at the equator. The parallels are plotted as concentric, equally spaced circular arcs. It is particularly well suited to regional and continental mapping, but shape distortion around the edges limits its use in global maps. In the Cylindrical Equal Area map with straight parallels and meridian but appear to be stretched North-South near the equator and stretched East-West near the poles. Even with this distortion it is one of the more practical for online mapping and supported by ESRI.

Equidistant Map projections aim to maintain distance. for the Conic Equidistant map the meridians are straight equidistant lines, converging at the North Pole. "Compared with the sphere, angular distance between meridians is always reduced by a fixed factor, the cone constant." Parallels are arcs of circle, concentric in the point of convergence of meridians. But the consequence of this is that parallels cross all meridians at right angles meaning distortion is constant along each parallel. In a Equidistant Cylindrical projection all the meridians have a constant scale that are equal to the standard parallels's, so all parallels are equally spaced. It is still commonly used in digitized textures ("skins") of earthly and planetary features.

Conformal map projections preserve angles locally. The Sinusoidal prjection retains property of equivalence of area. Areas on a map are proportional to same areas on the Earth. "Distances are correct along all parallels and the central meridian(s). Shapes are increasingly distorted away from the central meridian(s) and near the poles." But because of this, a Sinusoidal projection map is not perspective or equidistant.The Eckert VI projection is used for maps of the world. It is pseudocylindrical and equal area. "The central meridian and all parallels are at right angles, all other meridians are sinusoidal curves." There is an increase of shape distortion at the poles but the scale is correct at standard parallels of 49:16 North and South.

Map projections rule, but probably not as much as my TA: Erin Streff, middle name Elizabeth.