[Geotiff] User defined datum: Transformation parameters

[Frank Warmerdam]

Bruce Raup wrote:
> Christian and others,
> 
> I haven't worked on this, but a closely related issue is that GeoTIFF 
> doesn't not seem to be able to separate the concepts of datum (used to 
> define longitude and latitude) and projection ellipsoid (used to map from 
> lon/lat to projected coordinates).
> 
> Petabytes of satellite image are stored in NASA archives where the 
> underlying lon/lat coordinates are based on the WGS-84 datum, but where 
> the projection ellipsoid is something else, such as a sphere.
> 
> The problem with only being able to store parameters for one ellipsoid is 
> that software can erroneously "decide" that it needs to do datum 
> conversion on unprojected lon/lat coordinates when display lon/lat or when 
> reprojecting, because it thinks that the lon/lat coordinates are defined 
> on the spheroid rather than WGS-84.
> 
> I'm wondering how well known this problem is, and whether there are plans 
> to augment the GeoTIFF spec to store the datum and projection ellipsoid 
> separately.  Or if I'm wrong about this gap in the spec, I would love to 
> be shown that as well.

Bruce,

I'm quite painfully familiar with this issue. When expressing such data
in a system that supports recording datum shift information I normally
express the ellipsoid as a sphere, make up a bogus datum identity (user
defined in geotiff terminology) and attach a preferred TOWGS84 shift
transformation of 0,0,0.

This is basically defining the coordinate system as the projection on the
sphere, but claiming the datum on that should be treated as WGS84.

If we did have a way of encoding a preferred shift to WGS84 we would be
able to do this quite easily in GeoTIFF.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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