The term map is also used to describe a GIS Project or View. A map can provide an interpretation of features on the earth s surface. In a GIS, locations on the earth s surface described by points, lines, and polygons are defined by a series of x,y co-ordinates. A degree of latitude on the earth's surface is about 69 miles. A degree of longitude is about 69 miles at the equator and undefined at the poles, but any point on the surface rotates through a degree of longitude in about 4 minutes of time.
Decimal degrees are the decimal representation of fractions of degrees.
30 minutes equal half a degree, and 30 seconds equal half a minute. Co-ordinate systems must be consistent between map layers. For any database to be useful for spatial analysis, the database must be registered to a recognised global co-ordinate system. The accuracy of measurements , e.g., of distance, done on a map is affected by the type of map projection used to draw this map. The accuracy with which a given map scale can depict the location and shape of map features is known as resolution .