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 us ed to provide these values. NovAtel GPS receivers s tore this value
internally. This model can also be augmented with local height and gravity informatio n. A more
precise geo id model is avail able from government surv ey 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 impor tant for GPS users?
The above formula is critical for GPS users as they typically obtain ellipsoid heights and need to
convert thes e into mean sea-level heights . Once this conversion is comp lete, 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. sing le- poi n t or rela tiv e
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 unkno wn value; precision r ef e rs to h o w c los e a n e sti m ate is
to the mean (average) estimate. “Accuracy versus Precision” on Page 84 illustrates va rious
relationships between these two parameters: the true value is "located" at the intersection of the cross-
References:
1 Topography
2 Geoid (mea n sea level)
H = GPSCard computed height above/below geoid
N = Geo idal Hei gh t (undulation )
3 Sphero id (ellip soid )
h = GPS system computed height above the spheroid
N = h - H