[gdal-dev] Re: [Fwd: Re: Warping an already georeferenced image with control points

Even Rouault even.rouault at mines-paris.org
Thu Feb 12 13:42:26 EST 2009


(I'm CC'ing the list)

I'm not sure what you mean with adapting the pixelsize, now that the output 
has only GCPs and no more geotransform matrix.

As far as including this option in baseline gdal_translate.cpp, I'm currently 
not really enthousiastic, as there are lots of way to get the result achieved 
(what is done with that patch could be done similarly with GDAL python API 
for instance) and the choice of corners is something rather arbitrary. Why 
should you trust the georeferenced coordinates of the corner points as 
reliable GCPs ? Why not some regularly discretized points along the edges ? 
Or a regular grid over the whole raster ? etc. etc. So, I'd prefer hearing 
from other GDAL developers or users that this patch is a sensical addition 
before commiting it.


Le Thursday 12 February 2009 11:47:00, vous avez écrit :
> Hi Even,
> Thanks a lo. Just one small addition: the pixelsize has also to be
> adapted, else you get an image with false dimension. If it's not too
> much to add that, I'm going to try it out and will let you know the
> results. If it works, I have indeed a practical solution for my problem.
> Even so, I am not convinced that this is an uncommon problem. Consider a
> very basic GIS functionality: edge matching.  You have digitized a map
> from many sheets, and at the end the georeferenced images don't match
> precisely at the edges. Edge-matching functions (ArcGIS has lots of
> them) ensure that the edge of a feature on one map perfectly matches the
> feature edge on the adjacent map. This is exactly the kind of thing you
> can do with gdalwarp using control points on a georeferenced image. It
> is also more or less the thing I want to do with my historical maps.
> Would this be an argument to preserve the boundaries of a georeferenced
> image, when adding control points?
> Jan
> Even Rouault wrote:
> > Jan,
> >
> > I concur with Jukka's analysis. Your need is rather uncommon, and I also
> > thinks it shouldn't be a default behaviour. However, adding that
> > capability to gdal_translate is quite easy. I've attached a patch for
> > gdal_translate.cpp that adds the capability of adding the 4 corners as 4
> > additional GCPs with the "-add_corners_as_gcps" option. You might give it
> > at try (it applies cleanly on GDAL 1.6.0 and trunk). I'm not sure yet if
> > it is valuable enough to include it in baseline gdal_translate.cpp.
> >
> > Best regards
> >
> > Le Wednesday 11 February 2009 15:59:48 Jukka Rahkonen, vous avez écrit :
> >> Jan Hartmann <j.l.h.hartmann <at> uva.nl> writes:
> >>> Sorry to keep moaning about this, but I need an indication what's going
> >>> on here. Mind, I don't need an immediate solution: for the time being I
> >>> have a workaround. Just an idea whether this a real problem, a dumb
> >>> question, something that can be handled in the foreseeable future (or
> >>> not), perhaps with adequate funding. Everything is better than talking
> >>> to a blind wall.
> >>>
> >>> Sorry again Frank,
> >>>
> >>> Jan
> >>
> >> Hi,
> >>
> >> I think that your need to keep the original extents but add more ground
> >> control points inside the image frame is rather uncommon. Doesn't it
> >> mean that you trust that the image corners are correctly warped to a new
> >> projection, but there are local distortions in the middle of image which
> >> should be corrected with a few extra ground control points? For my mind
> >> it shoudn't be the default behaviour of gdal but it might be usable as
> >> an user selectable option sometimes.
> >>
> >> I know that missing gcp's at the image corners often leads to very odd
> >> result with polynomial warping because the formula shoots over.  Even
> >> unaccurately placed gcp's could help a lot in preserving the original
> >> shape of the map. I guess that you are perhaps playing with scanned
> >> historical maps?  I have a few old scanned parcel maps which are
> >> covering just the area of the farm, and two of the map corners are just
> >> white background. It is impossible to measure any real ground control
> >> points from the corners because there is nothing on the map to compare
> >> with, and warping with all gcp's on the middle area makes really funny
> >> looking curves into the hand drawn rectangle framing the original map.
> >>
> >> -Jukka-
> >>
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