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

Jukka Rahkonen jukka.rahkonen at mmmtike.fi
Wed Feb 11 09:59:48 EST 2009

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


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.


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