There is a lot more interest in three dimensional surfaces these days and this is also reflected in the open source geospatial world. This afternoon at FOSS4G in Barcelona, Frank Warmerdam gave a talk on the subject of Vertical Datums: Introduction and Software Review to a standing room only audience.
He summarized the different models for elevation measurement including the traditional elipsoidal, tidal and orthometric. (The image shows the divergence between the geoid (EGM 96) and elipsoidal (WGS 84) models for representing the Earth.) He described some of the challenges in working with vertical coordinate systems, such as the difficulty in relating EPSG to OGC vertical coordinate systems.
The good news for the open source geospatial community is that Frank is working on extending several open source libraries to support vertical datums including
* Liblas
* Libgeotiff
* GDAL
* PROJ.4
He mentioned that CS-MAP already supports vertical datums and in particular provides extensive transformation grid files (gtx) for transforming between different models. Proj.4 and CS-MAP are both part of the OSGeo MetaCRS project that Frank chairs.

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