In taking the GPS positioning, the HDOP is an index of quality for data and it should be smaller than 1.5 (150 in pax counter mobile).
Another quality index could be number of Satellites, Sats, which should be greater or equal 5 to be sure to have taken precise GPS data.
http://www.crowtracker.com/hdop-gps-position-errors/
https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation)
