19 - Mapping

Abstract

Mapping the spherical Earth onto a flat surface requires compromises. Different map projections can preserve directions, distances, areas, or shapes, but not all four simultaneously. This chapter contains examples of map projections that are commonly used to plot Earth-referenced data. Interactive Data Language (IDL) can plot maps using a wide variety of map projections. Once the mathematical transformation for the map projection is set up, data can be superimposed on the maps using contours, images, or symbols. The map projection is selected with the MAP_SET procedure. MAP_SET can take three optional, positional parameters, P0LAT, P0LON, and ROT. The first two parameters specify the latitude and longitude of the point on the Earth's surface that is mapped to the center of the map. The third parameter specifies the rotation of the map around that point. Some projections have additional keywords associated with them to set specific properties of those projections. The cylindrical equidistant projection is particularly simple. Longitude is plotted on the abscissa and latitude on the ordinate. Both scales are linear. An alternative global projection that does not distort areas at high latitudes is the Hammer equal-area projection. In atmospheric and oceanic applications, it is often required to plot maps of either the northern or southern hemisphere, usually to focus on data in the middle and high latitudes. There are several types of polar projections to choose from. Two that are frequently used are the azimuthal-equidistant and orthographic projections. The other polar projection illustrated here is the orthographic projection.