Orthometric Height in “Mapit->Survey Settings” is available for Mapit Pro users and has been added to allow recording of altitude as orthometric height  which approximates so called global mean sea level and represents local vertical datum surface.

The Geoid  is the shape that the surface of the oceans would take under the influence of Earth’s gravity and rotation alone, in the absence of other influences such as winds and tides. This surface is extended through the continents (such as with very narrow hypothetical canals). All points on a geoid surface have the same effective potential (the sum of gravitational potential energy and centrifugal potential energy). The geoid can be defined at any value of gravitational potential such as within the Earth‘s crust or far out in space, not just at sea level. The force of gravity acts everywhere perpendicular to the geoid, meaning that plumb lines point perpendicular and water levels parallel to the geoid if only gravity and rotational acceleration were at work.

Several local geoid grids are available and ready to use in Mapit out of the box including GEOID12B (United States), PLGEO11 (Poland), OSGM15 (Great Britain) or HT 2.0 for Canada.

We have added also support for custom Geoid grids in *bin (US National Geodetic Survey format) – simply convert your local grid to *bin format and rename the file to LOCAL_GEOID.bin then place in Mapit->Resources folder. When the file is ready enable Mapit->Orthometric Height and select last option on the list. Your grid will be used to calculate the orthometric height in Mappit. Please see details how to convert your grid to appropriate format further down in this article.

If you would like to understand more the vertical datums please see excellent video by Dave Dole available on Geospatial Users Group youtube channel:

The geoid shape passes through the Earth’s crust and is determined from data collected all over the world about the Earth’s gravity field.  The vertical distance between the geoid and the ellipsoid is called the geoid height.  This height can be either negative of positive.  Differences in height between the geoid and ellipsoid (geoid heights) range from roughly -100 to +100 meters.

The best view yet of Earth’s geoid obtained by GOCE (strongest gravity measurements in yellow, weakest measurements in blue). Credits: ESA/HPF/DLR.

If you would like to record the orthometric height instead of the ellipsoidal GPS height which assumes the Earth to be totally homogeneous as if there be would no mountains or trenches this extension will do the job and no post processing is needed.

The picture below explains the differences between the ellipsoidal and orthometric height.

Orthometric height

The embedded geoid model is using Worldwide 15 Minute Binary Geoid Height File to calculate the geoid offset.

You can check the difference between the WGS84 (GPS) altitude and the Mean See Level using the program on this website:

http://geographiclib.sourceforge.net/cgi-bin/GeoidEval?input=55.00+-3.00&option=Submit .

You can download the local geoid grid e.g. from https://www.eye4software.com/hydromagic/documentation/geoid-models/ and convert to the *bin format using Geoid File Conversion Utility .

Geoid File Conversion

Geoid Height are also included in POINT layers export when GNSS stats are enabled for export.
GPS Info screen now displays geoid used and applied geoid height.

Orthometric height & Geoid Height