ArcGIS Help - 2. Setting Display and Map Units

A map that is imported into ArcGIS has two sets of units associated with it.  Map units are defined by the creator of the map or digital image.  There is little you can do about them (except for setting them properly).  However, when you perform measurements on your map you can define the units the software will use to report the results.  These units are called the display units.
For example: most state maps and digital orthophotoquads have a map unit of feet. Your instructor requests that all measurements, buffers, distances etc. are reported in meters.  You can either measure everything in feet and do the conversions to meters later on in a spreadsheet, or you can let the software do the math by setting your display units to meters.

To set display units:

• In the table of contents select a layer.
• Go to View > Data Frame Properties
• Select the General tab
• Set Display Units to the desired option

To set map units:

• in the table of contents select the appropriate layer whose map units you want to change.
• go to view > data frame properties... > coordinate system
• Click on Modify.
• In the menu displayed on the right chose the appropriate linear unit from the drop down list.