[Gdal-dev] Irregular geolocation grid

[Mathieu]

Hello,

Frank, you say that the "polynomial transformation stuff" (I guess you are
talking about the functions defined in gdal_crs.c) will work well only up to
3rd order. Could you please develop a litlle this assertion?

- What is the relationship between the number of control points, and the
"optimal" order? (For example, I notice that using order 3 with only 5 GCPs
is not good. When should I begin to use order 3 instead of 1 or 2?) 

- Is there, in practice, a maximum limit of number of control points, when
using 3rd order? 

- In partice again, why not using order 4? In the case of geo-referencing a
chart (which is very specific, from the mathematician point of view), for
example, is it proved that order 4 should be banished?

Mat

-----Message d'origine-----
De : gdal-dev-admin at remotesensing.org
[mailto:gdal-dev-admin at remotesensing.org] De la part de Frank Warmerdam
Envoyé : mercredi 14 avril 2004 16:43
À : gdal-dev at remotesensing.org
Objet : Re: [Gdal-dev] Irregular geolocation grid

Julien Demaria wrote:
> Hi,
> 
> I've searched about this in mailing list and found nothing :
> 
> I would to know if GDAL architecture is adapted or can be 
> modified/adapted/improved to handle rasters
> with geolocation from an irregular grid describing lat/lon for each pixel
?
> 
> (Maybe this is already handled in drivers such as envisat ? but I cannot 
> read Meris RR Geolocation in OpenEV or with gdalinfo...)

Julien,

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.

Best regards,
-- 
---------------------------------------+------------------------------------
--
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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