[Geotiff] User defined datum: Transformation parameters
braup at nsidc.org
Sat Feb 23 12:01:30 PST 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.
On 2008-02-23 20:28 +0100, Christian Zietz wrote:
> 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?
> Geotiff mailing list
> Geotiff at lists.maptools.org
Bruce H. RAUP
National Snow and Ice Data Center
University of Colorado
449 UCB, Boulder, CO 80309
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