# Determining the Geoid model used by GPS receiver

I have the following NMEA output by Holux M-241 GPS receiver:

``````\$GPGGA,144014.000,2227.0987,N,11410.1382,E,2,6,1.67,32.6,M,-1.3,M,0000,0000*7B
``````

I know that WGS84 height is higher than Geoid height 1.3 meter. But how can I know that which Geoid model it is using?

Also, is it a database stored in the GPS chip for Geoid height and WGS84 height conversion?

• by default, the GPS height is usually given relatively to WGS 84. What makes you think this is not the case here ? – radouxju Jan 27 '14 at 8:05
• In the above NMEA response, there are two heights. One is ellipsoid height, which is the height you mentioned. One is geoid height, which is calculated from a function of coordinates and ellipsoid height. What I mean is the geoid height. – Harold Chan Jan 27 '14 at 8:11
• > To call WGS 84 simply an ellipsoid isn’t quite accurate. The WGS 84 GPS systems we use have a geoid component as well. The present WGS 84 system uses the 1996 Earth Gravitational Model (EGM96) geoid and is the best-fitting ellipsoid to the geoid model for the selected survey points in the set. (Obe & Hsu, Post GIS in Action, p. 157) – geotocho Mar 25 at 21:47

We don't know what geoid model is used, or how is the undulation being calculated.

In the Global Positioning System Fix Data section of the NMEA sentence information page, it is described as Height of geoid above WGS84 ellipsoid, without specifying what geoid model is being used.

But the codification of the geographic coordinates lets us to ask the online calculation of geoid undulations based in the GeoidEval utility of GeographicLib, which returns:

``````Geoid height:

lat lon = 22.45164 114.16897 (22°27'06"N 114°10'08"E)
geoid heights (m)
EGM2008 = -2.5657
EGM96   = -2.3861
EGM84   = -1.9314
``````