Standards and specifications for vertical control networks, part 4.
This is the fourth article in a short series of articles on vertical control. Professor Reilly has done an excellent job of presenting the standards and specifications for geodetic leveling in the previous three articles. I will briefly address two topics in this article—vertical datums, NGVD 29 and NAVD 88, and GPS vertical control. Yes, that’s right; GPS can be used to establish vertical control.
First, a vertical datum is a reference surface, e.g., a representation of the geoid, to which heights are referred. The basic difference between vertical datums used in the United States is how they were realized. Other differences occur because of distortions due to local adjustments, different corrections applied to leveling data, , e.g., refraction correction, and type of height (either Helmert or normal).