Spectra Precision Survey Pro v4.5 Ranger User Manual - Manual (Page 217)

GPS Overview
207
Vertical Coordinate Systems
GPS measurements provide ellipsoid heights. Most survey projects
require orthometric elevations. To convert heights into elevations,
you need to correct for the difference between the surface of the
reference ellipsoid and the level surface representing the gravity
field.
The procedure to convert heights (h) to elevations (H) involves the use
of a geoid model. The geoid is a theoretical surface that approximates
mean sea level. If one knows the separation between the reference
ellipsoid and the geoid, called the geoid undulation (N), then one can
determine orthometric elevations from ellipsoidal heights.
Geoid Models in TDS Software
Survey Pro has several geoid models in the coordinate system
database. All of the geoid models use data files in geoid grid file
(*.GGF) format.
Note: To use a geoid model, geoid data files (*.GGF) must be in the
Geodata directory.
• In the U.S.A., Mexico, and the Caribbean you can use the NGS
Geoid96 or the NGS Geoid99 models. This coverage includes the
continental United States, Alaska, Hawaii, Mexico, and the
Caribbean.
• In the continental USA you can use the new NGS Geoid03 model.
Terrain
Geoid
Ellipsoid
H
N
h
h=H+N
Fig. 6: The Height Equation