The following is a brief explanation of ppm and what it means in the accuracy specification.  It is important to choose a CBS (Community Base Station) or CORS (Continuously Operating Reference Station) site as geographically close to your project as possible when differentially correcting your GPS data.

If you choose to do an "Internet Search" for these sites from within the Differential Correction module of Pathfinder Office™, they will be shown in order of their proximity to your project location.  

The distance between the base and the rover affects accuracy.  There is a degradation of one part per million (1 ppm) as the distance between the base station and rover increases.  Therefore, one millimeter of degradation occurs for every kilometer between the base and rover. 

Equivalents:

100Km = 62 miles
250Km = 155 miles
500Km = 310 miles

1M = 39.37 inches
100M = 328 feet
1000M or 1Km = 3280 feet or .62 miles

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