[Gdal-dev] Irregular geolocation grid

[Eric Dönges]

Am 14.04.2004 um 16:43 schrieb Frank Warmerdam:

> GDAL can transport a set of many ground control points for 
> geolocation.  They
> may be regular (as would occur with the geolocation grid from 
> something like
> Envisat) or irregular as would occur with user collected control 
> points.
> However, there are no mechanisms in GDAL to make really effective use 
> of
> large number of geolocation ground control points.  The polynomial
> transformation stuff is only good up to 3rd order.  Having a huge 
> density
> of control points to feed that isn't really worthwhile.

A (possibly dumb) question: can the "polynomial transformation stuff" 
you refer
to above be used to map raster coordintes (pixels) to geographic 
coordinates ?
Reading the supplied documentation has lead me to believe that if the 
mapping cannot
be expressed in terms of an affine transform (which they can't in my 
case), the
application is responsible for deriving a useable transform from the 
GCPs supplied
by the dataset. Is this not the case ?

On a somewhat related note, is there any reason (besides the rather 
ugly math involved)
that GDALGCPsToGeoTransform() does not use all the GCPs available to 
determine an affine
transform ? I've written a least squares approximation that takes all 
GCPs into account
and thus does a somewhat better job (IMO, of course =8^) then the 
algorithm currently
used that I'd like to contribute if there is any interest for it.

--
Eric Doenges