Coordinate System

A coordinate system is a way of identifying the location of a point on the earth’s surface. Latitude/longitude is one type of coordinate system. Hundreds of other coordinate systems are used around the world to identify locations in specific countries or regions. Different coordinate systems are based on a variety of projections, geodetic datum and other reference information.

With the exception of latitude/longitude, coordinate systems are tied to a particular projection and datum. Map projections are attempts to portray the surface of the earth or a portion of the earth on a flat map. Since the earth is a curved surface, if we want to represent it on a flat map, we need some way of converting locations on the curved surface to corresponding locations on a flat map. A map projection is the way to accomplish this. However, distortions of size, distance, scale and area always occur to some extent, so some projections are better for certain regions or land mass sizes than for others. No flat map can perfectly represent the curved earth surface. Over the years, literally hundreds of map projections have been devised in an effort to achieve the lowest possible distortion for a given area and location on the earth’s surface. Many countries have created custom map projections that minimize map distortion of their country but are useless elsewhere.

Datum defines the reference systems that describe the size and shape of the earth. Hundreds of datum have evolved over the centuries, including newer ones based on satellite measurements. Depending on your study area, you may encounter maps based on a variety of datum and coordinate systems.

EDX software supports a wide assortment of global and national coordinate systems, with the option of choosing from a variety of different map projections and datum. Also supported are un-projected x,y linear systems for indoor studies. With EDX software, choosing a coordinate system automatically converts all map layers (including images) to that system so layers correctly line up on your map. This valuable feature has not been incorporated into any of the common GIS mapping packages.

The Coordinate System dialog box shown here is accessed by clicking on Map>Coordinate System. Normally, most data that is available for North America is in a latitude/longitude format, using Transverse Mercator projection and WGS 84 datum. Unless you’re working in a British National Grid or Australian National Grid format you should not have to change the settings here. 

Some coordinate systems contain more than one system member. For example, for Portugal there are three systems from which to choose, but for latitude/longitude there are no system members. Latitude/longitude is not tied to a particular projection or datum, so you will need to select these items. All other coordinate systems are tied to a projection and datum, so they will be grayed out; you can see what they are but you cannot change them.