Converting Single Point Elevations from WGS84 Geoid to WGS84 Ellipsoid

[HeatherKyle]

Hello,

I am writing an application in C# using the ArcGIS Engine SDK to provide ground elevation information at specified locations.  I'm using a RasterCatalog to store DTED elevation data.  To retrieve the elevation height at a desired location, I create a Surface out of the Dataset in the RasterCatalog that contains the location and then call the get_Z method, passing in the location coordinates.  From a little research I've learned that DTED elevation height information is in reference to the EGM96 geoid model of the earth.  (This model is similar to the one used for the WGS84 geoid model so I consider them interchangeable.) 

This process works well but there are times when I am more interested in the the elevation above the WGS84 ellipsoid rather than above the geoid.  I've been told by ESRI representatives that vertical datum conversions are already built into the ArcGIS Engine 9.3.1 SDK in the Projection Engine but I have not been able to locate it.  Has anyone else been able to perform this conversion on a point to point basis?

Thanks,

cm

[MelitaKennedy]

Hello,

I am writing an application in C# using the ArcGIS Engine SDK to provide ground elevation information at specified locations.  I'm using a RasterCatalog to store DTED elevation data.  To retrieve the elevation height at a desired location, I create a Surface out of the Dataset in the RasterCatalog that contains the location and then call the get_Z method, passing in the location coordinates.  From a little research I've learned that DTED elevation height information is in reference to the EGM96 geoid model of the earth.  (This model is similar to the one used for the WGS84 geoid model so I consider them interchangeable.) 

This process works well but there are times when I am more interested in the the elevation above the WGS84 ellipsoid rather than above the geoid.  I've been told by ESRI representatives that vertical datum conversions are already built into the ArcGIS Engine 9.3.1 SDK in the Projection Engine but I have not been able to locate it.  Has anyone else been able to perform this conversion on a point to point basis?

Thanks,

cm

Hi,

The Projection Engine SDK (C or Java) is included with the ArcSDE SDK. The functionality is there to convert values using an EGM96 file. I've attached a C program that can convert EGM96/WGS84 geoid heights to WGS84 ellipsoid heights. Even though it says version 10 in the beginning, it works at 9.3.1. The particular vertical transformation is 10084 ("WGS_1984_To_EGM_1996_Geoid_1").

There are FORTRAN programs to access a geoid file here:

http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html

In the past, some people have converted geoid grid file into a raster and then queried the raster for the conversion value.

Melita