[Proj] Several versions of the EGM96 grid

[Kristian Evers]

All,

Just a word of caution. It has recently come to my attention that there's in fact several versions of the egm96_15.gtx grid being distributed. Thomas discovered it whilst trying to debug some failing tests. One version of the grid is distributed with OSGeo4W and the other is distributed on the OSGeo download website: http://download.osgeo.org/proj/vdatum/egm96_15/egm96_15.gtx

It is not clear to me which of the two versions of the grid is the correct one, but it is obvious that they are different. The image at https://i.imgur.com/eG80Wv5.jpg  shows the difference between the two grids (download.osgeo.org - OSGeo4W). There is upwards of a meters difference in some places. Quite significant, and potentially a problem for some users. In the OSGeo4W-grid the grid values seem to be limited to two decimals, whereas the other grid has more decimal places (4-7, normal floating point behavior). I have a suspicion that the two grids are offset slightly in the east-west direction, but haven't been able to confirm it.

Has anyone on the list got any insight as to which of the two versions of the grid is the correct one?

Cheers,
Kristian
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