Geoid – What is it? Why do we use it with Trimble TerraSync and GPS Pathfinder Office software?
What is the Geoid?
The geoid is a model of global mean sea level that is used to measure precise surface elevations (definition according to NOAA).
While we often think of the earth as a sphere, our planet is actually very bumpy and irregular.
See this NOAA article: What is the Geoid?
*A depiction of the United States geoid (lower 48 states). Areas in yellow and orange have a slightly stronger gravity field as a result of the Rocky Mountains.
Why do we use a Geoid for GPS Mapping calculations?
We use the Geoid in our GPS workflows when we wish to see precise measurements of elevation. Whether our workflow is to see this elevation LIVE in the field inside of Trimble TerraSync when connected to a Real-time GPS network or after Differential Correction/Post Processing via our office software (Trimble GPS Pathfinder Office).
If we did not use the Geoid file with MSL “Mean Sea Level” then our elevation data would have readings as HAE – “Height above Ellipsoid”.
Where can I download the Geoid file?
Geoid 12A is the current Geoid for the lower 48 states. Other Geoids are used for Alaska and Hawaii. Wisconsin and Minnesota also have their own Geoids if you are using “County Coordinate System”. Download from Trimble’s website.
- For the Lower 48 states (CONUS) using US State Plane Coordinates NAD83, NAD83 CORS96 or NAD83 2011 select and download Geoid12A Combined Conus
- For Wisconsin County Coordinate System choose: Geoid12A – Wisconsin
- For Minnesota County Coordinate System choose: Geoid12A – Minnesota
Where do I copy it to my Trimble GPS data collector to us with TerraSync (Geo 6000/7x or other data collector to be used with ProXRT or R2)?
Copy the Geoid12A file from your PC using “Windows or Microsoft Windows Device Center” to the following directory: Trimble Geo 7X handheld\Program Files\Common Files\Trimble\GeoData
Then inside of TerraSync go to your Coordinate System Settings: Select your State Plane Coordinate System, Datum, MSL and US Survey Feet. Once MSL “Mean Sea Level” is selected you can choose Geoid12A from the list.
- If using the Wisconsin County Coordinate System choose: G12-WI
- When using the Minnesota County Coordinate System choose: G12-MN
*By default the Geoid12A is not part off the TerraSync native installation. The Geoid12A might already be on your Geo 7X if you purchased during 2016 as we started to copy it onto those devices after installation of new sales of Geo 7x’s.
**The Geoid12A is already part of the installation of GPS Pathfinder 5.60 and higher. In your Coordinate System Settings inside of GPS Pathfinder Office Select your State Plane Coordinate System, Datum, MSL and US Survey Feet. Once MSL “Mean Sea Level” is selected you can choose Geoid12A from the list. Similar is true when selecting Wisconsin or Minnesota County Coordinate System. The you will choose either G12-WI or G12-MN.
A good test to be sure that the Geoid12A is working:
Test with TerraSync and or GPS Pathfinder Office with some test shots on several local known/certified control points.
- See our blog from October 31st 2016 “NGS CONTROL POINTS – RESOURCES TO FINDING THEM”
- Use a fixed Mono-Pole for your Geo and or Fixed 2 meter rod if using an external antenna with the Geo 6000 or Geo 7X (Sub Foot or Centimeter).
- Use a fixed 2 meter rod if using the ProXRT (Sub Foot) or R2 (Sub Foot or Centimeter).
- On the NGS data sheets be sure to look for the word “ADJUSTED” and current dates of last known verification of the control point.
- Some more current NGS Control Point Data Sheets may say Geoid12B but for use in the lower 48 states you can still use Geoid12A.