SPOT image processing

[Michael Shapiro]

(1) How do you know the maps have all zeros in them. The best way to
tell is to run r.describe in the target location on each of the maps.
(The UTM coordinates for the lon/lat coordinates are right.)
Perhaps the resultant files are too big and you have filled up your
disk and that is causing the write errors. Check you disk usage.

(2) Images can't be rectified in lat/lon (yet) since i.rectify doesn't
know about curvilinear coordinate systems.

(3) i.tape.other extracts what you ask it to. If the tape has
non-image data mixed with image data (leader pixels) you can get
those pixels extracted as part of the image (perhaps 33 pixels).


|This is a problem that concerns SPOT image rectification in GRASS4.0.
|We have a Spot image of North Benin (63-328) with the following lat
|long coordinates for the corner pixels:
|upper left (c1) N0111933 E0023045
|upper right (c2) N0111435 E0030304
|lower left (c3)  N0104742 E0022331
|lower right (c4) N0104244 E0025545
|
|with m.ll2u we calculated the utm coordinates for these points, using
|spheroid wgs84 in zone 31. Results (east north) (without decimals):
|c1 446802   1252051
|c2 505578   1242854
|c3 433527   1193378
|c4 492254   1184160
|
|We created a location BENIN with default region limits in utm:
|north  1383127
|south  663305
|west   228285
|east   610674
|and resolution of 20x20 meters.
|We created a mapset SPOT in this location.
|We extracted the image from tape with i.tape.other creating the normal
|output:b.1, b.2, b.3 all with similar rows and columns and regions. A
|group was created and targeted to the relevant utm location and mapset
|(BENIN SPOT).  i.points was used to identify the four corners and
|administer the correct coordinates. finally i.rectify was started.
|Having tried this in various ways and running r.support on both input
|and output does not help: the result is always the same: a
|geographically rectified rastermap with appropriate parameters (region
|and resolution) and with the cell_file of considerable size (around 10
|Mbyte) and all support files but with ALL PIXELS HAVING CATEGORY 0 (no
|data).  We ran both i.rectify option 2; "determine the smallest region
|which covers the image", using the entire BENIN area as target location
|and we also created a much smaller target location just the size of the
|geographic region that the rectified image would| cover and then opted
|for version 1: "use the current region in the target location"; again
|empty rastermaps as result.  The following error message was mailed by
|grass: "GIS error: unable to write row 0" (somtimes other row numbers
|were indicated). Questions:
|
|1 any solutions for this problem;
|2 can an image be rectified in lat long (i.points seems to ask for utm,
|state plane or xy)
|3 i.tape.other seems to shift the image 33 colomns to the right; how
|can this be avoided