Outputting heights based on the geoid or an approximation to the geoid
When you want heights output from the receiver that are based on gravity datums rather than the ellipsoid that GPS typically outputs, there are two methods:
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Use of a geoid model file, which relates the ellipsoidal height to a geoidal (orthometric) height that is a constant gravity surface.
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Use the Trimble Business Center software to subgrid the file. It produces a *.ggf file, which you must rename to a *.sgd file.
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Import it into an application file in the Configuration ToolBox utility.
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Use the Configuration ToolBox utility to select the coordinate system to upload the *.ggf file.
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Use the Configuration ToolBox utility to load that file directly, if your computer is connected to the receiver. Otherwise, import the application file using the web interface (select Receiver Configuration / Application Files).
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In many cases the inclined plane method is used as it is often easier to get this from a site calibration.
If three or more control points are well distributed around the site, then the Siteworks site calibration process gives the option to fit an inclined plane to map the GPS ellipsoidal heights to local datum heights.
To import the relevant file, see Outputting local site or map projection coordinates (NEE).
This geoidal height will agree with local height datums more accurately. It is possible to upload a subgridded geoid model file rather a global geoid model file. A global geoid model file is too large to load into receivers, so you need to reduce its size.
Once it is loaded into the receiver, the receiver can then output the elevation values. For example, the NMEA GGA message will now output geoidal heights.
