[GRASS-user] Workflow of a classification project with orthophotos

[Jonathan Greenberg]

Nikos:

    Performing relative radiometric normalization is a *requirement* of 
applying a single classification to multiple images (also for change 
detection).  Unfortunately, it is not an algorithm that is available (to 
my knowledge), out-of-the-box, on ANY remote sensing platform (GRASS, 
ENVI, etc.).  However, you can do the radiometric normalization yourself 
-- the idea is that pixels in the overlap zone between two images which 
are invariant (e.g. have not changed in structure, spectral properties 
or, in more complex architectures like trees, sun angle) should be 
linearly related to their counterpart in the other image.  Assuming 
this, you can either manually choose a set of "psuedoinvariant" targets 
(pairs of pixels which are at the same location and are not changing) 
between the two images, and calculate an orthogonal regression to 
generate gains and offsets.  One of those images, therefore, becomes 
your "reference" and the other one your "target".  The gains/offsets are 
applied to the target image.

    There are automated algorithms for doing the pseudoinvariant pixel 
selection (search for "radiometric normalization remote sensing" on 
google scholar), or if you assume that the images do not change between 
dates and are WELL rectified to one another, you can extract the ENTIRE 
overlap zone between the two images and calculate the regressions based 
on those.  This last suggestion is probably the fastest, but also incurs 
the most error and I wouldn't neccessarily recommend it.

    This would be a VERY good algorithm to add to GRASS -- if anyone is 
interested in pursuing coding this, I can help design the algorithm 
(including which are the best automated invariant target selection 
algorithms).

--j

Nikos Alexandris wrote:
> After examining the mosaic I found multiple and big differences. I
> conclude that the producer did not perform any radiometric nor
> topographic corrections. It is a collage and not a mosaic :-)
>
> Is this the way it should be?
>
> Thank you,
> Nikos
>
>
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-- 
Jonathan A. Greenberg, PhD
Postdoctoral Scholar
Center for Spatial Technologies and Remote Sensing (CSTARS)
University of California, Davis
One Shields Avenue
The Barn, Room 250N
Davis, CA 95616
Cell: 415-794-5043
AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307