Understanding map units and display units

Map units are the units in which the coordinates for a dataset are stored. They are determined by the coordinate system. If the data is stored in a geographic coordinate system, the map units are usually decimal degrees (degrees, minutes, and seconds expressed as a decimal). If the data is stored in a projected coordinate system, the map units are usually meters or feet. Map units can be changed only by changing the data's coordinate system.

Display units are independent of map units—they are a property of a data frame. Display units are the units in which ArcMap displays coordinate values and reports measurements. You can set the display units for any data frame and change them at any time.

More about decimal degrees

Recall that latitude and longitude coordinate values are actually angle measurements. Angles are measured in degrees. For latitude-longitude coordinates, degrees can be expressed two ways: as degrees, minutes, seconds (DMS) or as decimal degrees (DD). In a GIS, decimal degrees are more efficient because they make digital storage of coordinates easier and computations faster.

Below is an example of how to convert a coordinate location from DMS to DD.

The latitude of London expressed in DMS is 51° 29' 16" North. To convert this location to DD, follow these steps:

1. Divide each value by the number of minutes (60) or seconds (3600) in a degree:

29 minutes = 29/60 = 0.4833 degrees
16 seconds = 16/3600 = 0.0044 degrees

2. Add up the degrees to get the answer:

51° + 0.4833 ° + 0.0044 ° = 51.4877 DD