With the recent change within the Trimble Access and Trimble Business Center software regarding Geoid Model Interpolation, there are some very important updates that you should be aware of if you are using GPS equipment. This raises a few questions. Do you know which interpolation scheme you are currently using in your Geoid Models? Do you know which one you should be using? Over the past two years there has been a lot of dialogue about which Geoid Model to use, and also which interpolation scheme it should be set to. With the latest addition of the 2011 adjustment to NAD83 and the release of Geoid12A, we feel that it is a good opportunity to once again go over different Geoid Models, settings, and scenarios in which they should be used. The first thing to know is which Geoid Model goes with which adjustment. Here is the breakdown for how NGS is recommending their use:

NAD 83 (1996) – Geoid 03

NAD 83 (2007) – Geoid 09

NAD 83 (2011) – Geoid 12A

The other thing that needs consideration is the interpolation scheme that your Geoid Model is using. By default, the Geoid Models download with a local interpolation scheme, meaning that the latitude and longitude locations of the local ellipsoid are used to determine the Geoid separation at the location of the measurement, thus producing an elevation. When you are using State Plane or UTM with the NAD 1983 (Conus) (Mol) Datum Transformation, a locally interpolated Geoid Model works fine. But, if you are using UTM with the NAD 1983 (Conus) CORS96 (7P) datum transformation, or the new NAD 83 (2011) adjustment, you will need to use a WGS84 interpolated Geoid Model. You will also need to use the WGS84 interpolation with a Minnesota or Wisconsin county projection. Lastly, some local projections or calibrations may also require use of the WGS84 interpolation scheme.

The difference in using the local interpolation versus the WGS84 interpolation has to do with the Datum Transformation. Any projection that has a rotation, translation, or scale at the origin will require use of the WGS84 interpolation scheme in order to produce the correct elevations. It is very important to know which interpolation to use in order to receive accurate RTK elevations. This can be tricky in some regions because the difference between local and WGS84 Geoid derived elevations may not be as noticeable as other regions.