This invention relates to a method for updating digital maps and for matching global navigation devices to a digital map. Such navigation devices rely upon GPS signals (20, 24) from satellites (22, 26). One well-documented cause of position error in navigation devices arises from the phenomenon of GPS multi-path. It has been observed that GPS multi-path errors in the latitude/longitude direction are highly correlated with errors in altitude. By comparing altitude value of GPS probe data with reference specifications for altitude, unreliable probe data (outliers) can be easily identified and culled. Such techniques can be used as well by a mobile navigation device to confirm a match to a particular road segment and if not revert to other positioning techniques such as inertial guidance systems and the like. If the local altitude is not reliably known, an estimation can be derived directly from the collected probe data.