Spectra Precision Survey Pro CE v3.60 GPS User Manual - Manual (Page 11)

GPS Coordinates
7
• NGVD29
The first continental height datum in the United States was the
National Geodetic Vertical Datum of 1929 (NGVD29). According to
the technology of the day, this datum was based on normal gravity,
that is, the gravity field at the instrument when it was leveled.
Points along the coast were chosen and their elevation above sea level
was determined from a network of tide gauges. Spirit level networks
were then run across the country and closed on the opposite coast.
This datum contained a number of systematic errors including un-
modeled local gravity effects and refraction errors. Also, it was later
discovered that the “mean sea level” from the Atlantic to the Pacific
Oceans was not the same.
• NAVD88
In an effort to address these errors, the North American Vertical
Datum of 1988 (NAVD88) was realized from a single datum point in
Rimouski, Quebec. This datum is based on actual gravity, which
provides a better representation of true orthometric elevations. The
primary consideration in the choice of this datum point was to
minimize the recompilation of national mapping products. A side
effect of this choice is that the NAVD88 datum and the theoretical
level surface used to define GRS80 do not agree. The offset between
the NAVD88 vertical datum and the ITRF global geopotential model
is in the neighborhood of 0.27m
4 .
Survey Pro does not require choice of a specific vertical datum. For
RTK applications, elevations are solved relative to the base using the
vertical localization adjustment. Therefore, the vertical datum is
established by the datum of the base elevation.
4 Milbert D.G. Converting GPS Height into NAVD88 Elevation with
the GEOID96 Geoid Height Model
http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html p. 4