LAB 5 - Projections in ArcGIS

Map projections are two-dimensional representations of the three-dimensional, spherical earth. The specific type of projection is important to note when viewing a map because of the particular distortions that accompany them. Distortions are inevitably caused by the application of a 3D image onto a 2D plane. Depending on the region being studied, different projections can be utilized in order to avoid distortions from occurring in the area of interest. Additionally, projections preserve specific attributes on maps such as distance and area. Three projections represented in this lab are equidistant, equal area, and conformal. All have their own particular potential and pitfalls. Each type is classified depending on the properties that they aim to preserve.

Conformal maps are designed to preserve angles locally. This has the effect of maintaining the proper shape of map features while, unfortunately, distorting their relative sizes. In order for both size and shape to be preserved, a globe is the only appropriate medium. The Mercator and Gall maps displayed in this lab are two types of conformal projections. The former is often used for nautical purposes because of its ability to represent lines of constant course, rhumb lines, as straight segments. However, the Mercator projection does not maintain the size or shape of landforms as the scale increase. Gall projections cause distortions of areas approaching the poles; forms near the equator preserve their size and shape.

Equal area maps preserve the size of forms at the expense of the shape. In addition, shape, angle, and scale all may be distorted in this type of projection. Straight meridians and parallels are an aspect of these maps. The meridians are equally spaced whereas the distances between the parallels vary according to where they occur on the map. The first type of equal area projection exhibited is Cylindrical Equal Area projection. This projection is often used as an intermediate mathematical step in the construction of other equal area projections. The second is the Hammer Projection the parallels are curved but there is no stretching at the center of the map. Equal area projections are used when examining maps for the purpose of comparing the relative proportions of landforms.

Equidistant projections are the third type represented in this lab. These projections are designed so the earth is centered in a way in which at any given point on a straight line radiating from the center to any other point represents the shortest distance and can be measured to scale. However, the distance between two points (neither of which are located on the line radiating from the center) will be distorted. The first of the equidistant projections is the equidistant cylindrical in which all meridians are standard, equally-spaced vertical lines and parallels are horizontal, equally-spaced, equally-long lines. The final map represented in this lab is the Sinusoidal projected map. All the parallels are straight lines and are also sine curves with the exception of the prime meridian. This type of projection is often used to show tropical latitudes.