1. A method for determination of location coordinates of a selected position on or adjacent to the Earth's surface with improved accuracy, the method comprising the steps of: receiving GPS signals from N selected GPS satellites (N.gtoreq.3) and determining location coordinates (x.sub.gps, y.sub.gps, z.sub.gps) of a selected location on or adjacent the Earth's surface, where the coordinate z.sub.gps (t) is an estimate of a true elevation or vertical coordinate A(t) of the selected location relative to a selected vertical reference location at a time t; providing an altimeter reading z.sub.alt (t) that is an estimate of the elevation coordinate A(t) of the selected location relative to the selected vertical reference location at a time t; computing statistically determined estimates .sigma..sub.gps (t) and .sigma..sub.alt (t) of the standard deviations of the variables z.sub.gps (t) and z.sub.alt (t), respectively, for at least one selected time; defining an estimate A.sub.est (t) of the elevation coordinate A(t) of the selected position to be equal to z.sub.gps (t) if the standard deviations .sigma..sub.gps (t) and .sigma..sub.alt (t) satisfy a selected first criterion, defining A.sub.est (t) to be equal to z.sub.alt (t) if the standard deviations .sigma..sub.gps (t) and .sigma..sub.alt (t) satisfy a selected second criterion, and defining A.sub.est (t) to be equal to a value that lies between the value z.sub.gps (t) and the value z.sub.alt (t) if the standard deviations .sigma..sub.gps (t) and .sigma..sub.alt (t) satisfy neither the first criterion nor the second criterion; and calibrating the altimeter reading z.sub.alt against an accurate value for z.sub.alt at least once at a selected time t=t.sub.cal subsequent to a time at which an altimeter reading is initially taken.