Mean Sea Level heights in Trimble Positions Desktop Extension

Mean Sea Level heights in Trimble Positions

All heights collected in previous versions of Trimble Positions software were expressed as heights above WGS84 ellipsoid (HAE). The new Positions Desktop add-in and Positions Mobile extension add support for Mean Sea Level (MSL), also known as orthometric height.

Every time a project is created or edited in the Positions Desktop add-in or Positions Mobile Project Center extension it is possible to specify if GIS feature heights will be calculated as HAE or MSL. If GIS features need to have MSL heights, it is necessary to define how the difference between HAE and MSL (known as geoid separation) will be calculated. There are two ways to specify geoid separation:

  • Specifying a numeric value for the geoid separation to be applied. The formula to calculate heights is: HMSL = HHAE – NGS, where NGS is the numeric value specified.
  • Specifying a file containing a geoid model. Trimble Positions uses geoid models from Geoid Grid files (*.ggf). These files contain tables of geoid-ellipsoid separations that are used with the GPS ellipsoid height observations to provide an estimate of elevation.

Elevation settings specified in the Positions Desktop add-in will be applied each time the “Update Features” function is used on sessions for that project. This applies to all types of projects supported by the Positions Desktop add-in, including data collected from ArcPad.  In ArcGIS for Windows Mobile workflows, these elevation settings will also apply to the “Feature Height” metadata value which can be stored in an attribute using the Auto Fill settings in Mobile Project Center (populated in the field and updated in the office during “Update Features”).

If GIS features collected in ArcGIS for Windows Mobile with the Positions Mobile extension need to have MSL heights, then appropriate elevation settings must be specified in the Positions Mobile Project Center extension before a project is saved and deployed to devices. Changing a project’s elevation settings only in the Positions Desktop add-in will not be applied to mobile devices until the project is re-saved in Mobile Project Center and re-deployed.

Geoid (*.ggf) files are not installed with Trimble Positions software. It is the customer’s responsibility to find files that satisfy their needs. Trimble provides a good selection of geoid files and related utilities at Selected geoid files need to be downloaded (and unzipped) to the folder:



It is also the customer responsibility to deploy geoid files specified in projects to mobile devices. The Positions Mobile extension will look for referenced geoid files in the device folder:

\My Documents\Positions

Elevation Settings in Positions Desktop

Elevation Settings in Positions Desktop