4.1. Geographic coordinate systems

A geographic coordinate system is defined by the ellipsoid and datum that are used to calculate longitudes and latitudes which represent locations on the Earth's surface (Snyder, 1987). It is represented in degrees, minutes, and seconds north or south of the equator (latitude) or east and west of the equator (longitude) (i.e. Fribourg, Switzerland is at 46° 48’ 00’’ N, 7° 09’ 00’’ E), and is the simplest solution for representing places on the globe (Burrough and McDonnell, 1998). Decimal degrees are also used to represent latitude/longitude coordinates and can be calculated with the following equation: Decimal Degrees = Degrees + Minutes/60 + Seconds/3,600 (http://support.esri.com/en/knowledgebase/techarticles/detail/27215) (i.e. same location above would be represented as 46.8 N, 7.15 E). There are various online sources for calculating conversions between latitude/longitude and decimal degrees (e.g. http://transition.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html).The most commonly used geographic coordinate system is the World Geodetic System (WGS) 1984 (Fig. 8). However, geographic coordinate systems do not transfer nicely into two dimensions, causing major distortions in distance, area, shape, or direction (Snyder 1987; Seeger, 1999) on a map or computer screen. To solve these issues, projected coordinate systems are used.

Fig. 8. A map of the world shown in the WGS 84 geographic coordinate system.figure008