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
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Davis, CA 95616
Cell: 415-794-5043
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