GPS Overview Appendix B
MPC User Manual Rev 0D 83
Figure 80: Illustration of GPSCard Height Measurements
From the above diagram, and the formula h = H + N, to convert heights between the ellipsoid and
geoid we require the geoid-ellipsoid separation value. This value is not easy to determine. A world-
wide model is generally used to provide these values. NovAtel GPS receivers store this value
internally. This model can also be augmented with local height and gravity information. A more
precise geoid model is available from government survey agencies e.g. U.S. National Geodetic Survey
(http://www.ngs.noaa.gov
) or Geodetic Survey of Canada (http://www.geod.emr.ca).
Why is this important for GPS users?
The above formula is critical for GPS users as they typically obtain ellipsoid heights and need to
convert these into mean sea-level heights. Once this conversion is complete, users can relate their GPS
derived heights to more “usable” mean sea-level heights.
B.3 GPS Positioning
GPS positioning can be categorized as follows:
1. single-point or relative
2. static or kinematic
3. real-time or post-mission data processing
A distinction should be made between accuracy and precision. Accuracy refers to how close an
estimate or measurement is to the true but unknown value; precision refers to how close an estimate is
to the mean (average) estimate. “Accuracy versus Precision” on Page 84 illustrates various
relationships between these two parameters: the true value is "located" at the intersection of the cross-
References:
1 Topography
2 Geoid (mean sea level)
H = GPSCard computed height above/below geoid
N = Geoidal Height (undulation)
3 Spheroid (ellipsoid)
h = GPS system computed height above the spheroid
N = h - H
