PDOP stands for Positional Dilution of Precision. It indicates the accuracy of a 3D GPS position based on the number of satellites and the geometry of satellite positions. PDOP ranges from 0-99. The lower the number, the more accurate the data. Any position with a PDOP over 7 or 8 is probably not worth collecting.
Different GPS units require different PDOP limits. Recreational grade units (such as the Garmin or Magellens) do not have any limit, although a PDOP over 7 or 8 is not likely to produce results worth using. Mapping grade units (like our GeoXT) require a PDOP of 6.0 or less. Survey grade units (like our Hiper Pro) require a PDOP of 4.0 or less.
PDOP is typically shown on the GPS unit on the satellite page.