Geoid Height

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.

3D view of the geoid model

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

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 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

Orthometric height & Geoid Height

 

 

 

osedok

If you can’t explain it simply, you don’t understand it well enough.

Albert Einstein

8 Responses

  1. Rempu Rayat says:

    Hi, i just bought extension software for geoid height. Where do can see the height in map? Thanks.

    • osedok says:

      Hi there, go to Survey Settings and enable “Show speed & altitude”. If you have it already enabled the altitude displayed will be automatically adjusted to include geoid differences.
      As long as the ‘Geoid Height for GPS’ is switched on in Survey settings – all altitudes will be automatically recorded with geoid correction. You can compare your readings for the option switched on and off to see the difference.

  2. Kuloba Aron says:

    Hi provide me with Calculations about the geoid

  3. Daniel Flint says:

    How accurate is the measurement?

    • osedok says:

      Hi Daniel, high measurement accuracy is depending from number of factors – most important are the GPS device itself which I am not able to comment on, the second one is the Earth Gravitational Model itself. For the purposes of MapIt I am using the one published here by NASA http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html, but I am not able to provide more information in terms of accuracy in your area. You may get directly to the Earth Gravitational Model publishers to get some more information.

  4. Bruce Hedquist says:

    I’m not sure which geoid model I’m using. I tried importing the 2.5′ x 2.5′ one. I downloaded it off the net and put it in a folder as instructed. But the folder may not be under the program.

Leave a Reply