| Re: MODIS L1B - 250m [message #29810] |
Tue, 12 March 2002 08:52 |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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Thomas Ohde wrote:
> We want to use Level 1B Calibrated Radiances 0.25km data (MOD02QKM) of
> http://acdisx.gsfc.nasa.gov/data/dataset/MODIS for our research in the
> Baltic Sea, but we had a striping with mismatch in the coast line (bow-tie
> effect).
>
> Are there any software in IDL or ENVI for the correction of the MODIS
> LEVEL-1A data?
Maybe I'm just being defensive, because it implies that there was an
error in my geolocation code, but I don't think "correction" is the
right term. The radiances you see are, to the accuracy of the L1B
calibration, the correct radiances. The specified locations are, to
about 50m accuracy, the correct ground locations from which those
radiances were collected. The bow-tie effect is a real characteristic of
the way the data was collected, not an error that needs correction. I
apologize if I sound a bit defensive about this.
Liam Gumley has written some code for correctly displaying MODIS image
data, try <ftp://origin.ssec.wisc.edu/pub/MODIS/IDL/>.
The best way to handle the butterfly effect in MODIS image data is to
treat each scan of data (which corresponds to 40 consecutive scan lines,
at 250m resolution) as a seperate image, 10 km wide at the center, 20km
wide at the outer edges, and a few thousand kilometers long. Each such
image will overlap the preceeding and following image. Create each image
seperately, and then decide how you want to handle the overlaps.
Averaging in the overlap region is best, but it's quicker to just let
later scans overwrite parts of the image that were filled in by the
earlier scans.
Note: the satellite's altitude above the ground varies slightly. It
sometime flies low enough that there are actually gaps between scans
near the center of the scans. It sometimes flies high enough that as
many as three different scans of data overlap the same ground position,
near the outer edges of the scans.
That's just in normal operations; when the spacecraft is flying with a
yaw near 90 degrees, as many as a hundred scans of data can overlap at
the same point; I was called in once to help resolve a problem caused by
this. It turned out that they were trying to process MODIS geolocation
data during a maneuver, and they were using an algorithm that kept track
of the overlaps in fixed sized arrays that allowed for a maximum of 10
overlaps. They weren't really interested in processing the maneuver
data, they just hadn't bothered to check.
Please note that you must NOT interpolate the geolocation data across
scan boundaries. Just last week I got a complaint from someone about the
accuracy of our geolocation code; it turned out that he had made
precisely that mistake. The resulting pictures had bizarre ripples of
distortion running across them, at the boundaries between successive
scans. You can interpolate within a scan, but since the geolocation data
is at 1km resolution, you'll have to extrapolate it at the edges of each
scan when displaying 250m data.
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