Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual - Manual (Page 242)

User’s Manual – GPS Mode
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datum not programmed into the database, you can use the Projection
Key-In Wizard to create a custom Molodensky or similarity datum
transformation.
Vertical Datums
GPS satellites orbit the Earth’s center of mass, while objects on the
surface of the planet are affected by the force of the local gravity field.
Although it is possible to accurately model the orbits of satellites
about the Earth’s center of mass, modeling the local gravity field is
much more difficult because of the unequal distribution of masses
within the earth.
Everyone knows that water flows downhill from a higher elevation to
a lower one. However, water will not always flow from a higher
ellipsoid height to a lower one. Ellipsoid height is simply the altitude
above the reference surface, and may not match the slope of the local
gravity field. When surveying with GPS, you need to correct for the
local gravity field to convert measured ellipsoid heights (h) into
orthometric elevations (H). This is usually done with a geoid model.
Survey Pro can use several different geoid models to convert local
ellipsoid heights into elevations in a particular vertical datum. Most
geoid models are initially based on the global equipotential surface
used in the definition of the initial WGS84 datum. Below is a
description of some vertical geoid models and datums.
• EGM96
The National Imagery and Mapping Agency publishes the global
geopotential model EGM96 5 . This geopotential model was used to
generate the worldwide 15-minute geoid height grid data file,
WW15mGH.grd. This file contains geoid separation values at 15-
minute intervals for the entire globe and provides a good estimate of
geoid slope corrections.
• 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
5 http://164.214.2.59/GandG/wgs-84/egm96.html