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Dear Grass Developers<BR>
<BR>
I have been having a problem using module i.atcorr to process Landsat 5 TM bands 1 and 2. The resulting rasters are empty for these two bands. For other bands the result seems to be correct. There are many posts talking about this topic, but I did not find an answer. So I tried using Grass 6.4 and Grass 7.0. Unfortunately the same error occurred. Next I tried changing some parameters of the input file. What I found is that changing the time of acquisition of the image the algorithm was able to perform the calculations and return acceptable results. Bellow I show the content of input file A, with which the module did not work, and input file B, with which the module worked. Notice that the only difference is an increase of one hour in the time of acquisition of the image. By the way, it also worked if I decreased the acquisition time by one hour. The same occurred to other images from other dates. From this result I infer that the problem may rely on the calculations of sun position.<BR>
<BR>
I also tried contacting Mauro Antunes, the author of the original code in Fortran. He was my teacher, so I have a close relation with him. I asked him if he was interested in taking a look at the C++ code implemented in GRASS, but he is not willing to do it. He has been working on the Fortran code during the last years and provided me with a copy. Using his Fortran code and the same input file that I used for i.atcorr gave the expected results.<BR>
<BR>
I hope this is of any help for fixing this old issue in the i.atcorr module. Unfortunately I know neither C++ nor Fortran to help finding the error.<BR>
<BR>
The content of input file A is:<BR>
<BR>
---<BR>
7 # geometrical conditions = Landsat 5 TM <BR>
12 26 16.34 -54.125 -28.87 # month day hh.ddd long lat (hh.ddd is in decimal hours GMT) <BR>
1 # atmospheric mode = Tropical <BR>
1 # aerosols model = Continental Model <BR>
20 # visibility [km] (aerosol model concentration) <BR>
-0.3 # mean target elevation above sea level [km] <BR>
-1000 # sensor on board a satellite <BR>
25 # band 1 (0.430-0.560)<BR>
---<BR>
<BR>
The content of input file B is:<BR>
<BR>
---<BR>
7 # geometrical conditions = Landsat 5 TM <BR>
12 26 17.34 -54.125 -28.87 # month day hh.ddd long lat (hh.ddd is in decimal hours GMT) <BR>
1 # atmospheric mode = Tropical <BR>
1 # aerosols model = Continental Model <BR>
20 # visibility [km] (aerosol model concentration) <BR>
-0.3 # mean target elevation above sea level [km] <BR>
-1000 # sensor on board a satellite <BR>
25 # band 1 (0.430-0.560)<BR>
---<BR>
<BR>
Kind regards,<BR>
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-- <BR>
<BR>
Alessandro Samuel-Rosa<BR>
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PhD Candidate Graduate School in Agronomy - Soil Science<BR>
Federal Rural University of Rio de Janeiro<BR>
Seropédica, Rio de Janeiro, Brazil<BR>
---<BR>
Guest Researcher ISRIC - World Soil Information<BR>
Wageningen, the Netherlands <A HREF="mailto:alessandro.rosa@wur.nl">alessandro.rosa@wur.nl</A> | Phone 0031 06 4435 9563<BR>
---<BR>
Homepage: <A HREF="http://soil-scientist.net/">soil-scientist.net</A> Skype: alessandrosamuel <BR>
<BR>
<BR>
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