[Geotiff] User defined datum: Transformation parameters

Bruce Raup braup at nsidc.org
Sat Feb 23 15:01:30 EST 2008

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.

Best regards,

On 2008-02-23 20:28 +0100,  Christian Zietz wrote:

> Hi,
> as far as I know there exists no official way to store transformation
> parameters (e.g. for a 7-parameter Helmert transformation) for a user
> defined datum in a GeoTIFF.
> So, before I go ahead and implement a solution using a private key ID:
> Did someone do this before? Is there an unofficial specification I can
> adhere to?
> Christian
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Bruce H. RAUP
National Snow and Ice Data Center
University of Colorado
449 UCB,  Boulder, CO 80309
Phone:  303-492-8814

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