What Does Height Really Mean? Part Ⅳ: GPS Heighting

摘要

This is the final paper in a four-part series examining the fundamental question, "What does the word height really mean?" The creation of this series was motivated by the National Geodetic Survey's (NGS) embarking on a height modernization program as a result of which NGS will publish measured ellipsoid heights and computed Helmert orthometric heights for vertical bench marks. Practicing surveyors will therefore encounter Helmert orthometric heights computed from Global Positioning System (GPS) ellipsoid heights and geoid heights determined from geoid models as their published vertical control coordinate, rather than adjusted orthometric heights determined by spirit leveling. It is our goal to explain the meanings of these terms in hopes of eliminating confusion and preventing mistakes that may arise over this change. The first paper in the series reviewed reference ellipsoids and mean sea level datums. The second paper reviewed the physics of heights culminating in a simple development of the geoid in order to explain why mean sea level stations are not all at the same orthometric height. The third paper introduced orthometric heights, geopotential numbers, dynamic heights, normal heights, and height systems. This fourth paper is composed of two sections. The first considers the stability of the geoid as a datum. The second is a review of current best practices for heights measured with the Global Positioning System (GPS), essentially taking the form of a commentary on NGS' guidelines for high-accuracy ellipsoid and orthometric height determination using GPS.