[GRASS-user] color anomalies after orthorectification

Stefan Kiefer st_kiefer at web.de
Fri Feb 21 14:01:20 PST 2014

as Markus wrote there are no overshoots with bilinear method. 
Nevertheless I agree with keeping cubic and forcing the range. 
Particular because I only rely on greyscale aerial photographs, so it 
doesent matter if its more or less black or white. Especially when 
humans can't distinguish more than 130 shades of grey. An apropriate 
rectification is more desirable.

Thanks again for the support and the discussion.

best regards


Am 21.02.2014 13:57, schrieb Markus Metz:
> On Fri, Feb 21, 2014 at 9:27 AM, . . <st_kiefer at web.de> wrote:
>> Hi Markus,
>> thanks for that hint, that might be the solution. As I must confess I did
>> not only try any of the other methods, believing cubic would yield the best
>> results because the scans are from an area with high energy of relief.
> I would leave it as it is. Bicubic gives nice results, but these
> overshoots must be taken care of. Your method of forcing values back
> to the range of 1,254 seems appropriate to me for handling resampling
> overshoots.
> Markus M
>> I will try this when I'm back in office and give a feedback of my
>> experiences then.
>> best regards
>> Stefan
>>> Markus Metz <markus.metz.giswork at gmail.com> hat am 21. Februar 2014 um
>>> 08:05 geschrieben:
>>> On Fri, Feb 21, 2014 at 12:26 AM, Stefan Kiefer <st_kiefer at web.de> wrote:
>>>> Hi,
>>>> I tried to orthorectify aerial photographs. That worked fine by now,
>>>> until
>>>> i got scans wich appear to be darker as before. At least this is the
>>>> first
>>>> observation I made. The effect occured with that scans is, that after
>>>> rectification some of the dark areas become white (or rather null,
>>>> querying
>>>> that cells result in -0.722622310236638).
>>> This should only happen with method=cubic or method=cubic_f. The other
>>> resampling methods nearest,bilinear,bilinear_f should not produce
>>> these resampling overshoots.
>>> Markus M
>>>> Has anyone had similar experiences when rectifying aerial photographs.
>>>> And
>>>> hopefully a hint how to avoid this damages. I suspect that the
>>>> orthorectified images become double precision whereas the imported scans
>>>> are
>>>> integer...
>>>> best regards
>>>> Stefan
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