Q: Why are my geocorrected files upside down? Do I need to manually rotate images to make north "up"?

A:

Flight lines in level 1 data are as the sensor sees them (ie. appearing in the order seen by the sensor). If the aircraft is flying south, the first lines in the data will be the most northerly with later lines further south. If the plane is flying east, the first lines will be the most westerly, with later lines heading east. Navigation data is included with the level 1 imagery and can be used when producing level 3 georeferenced images. You don't need to do any manual rotation to compensate for flight direction yourself when making level 3 images - that's handled by azgcorr.

Level 3 images are corrected for flight direction and will appear with north "up" in any viewer that reads the georeferencing information correctly (e.g. ENVI).

If you see level 3 images where "up" is along the flight direction rather than north, this means your viewer hasn't correctly read the georeferencing information. The most common case is loading the level 3 HDF directly into ENVI. There is no well defined standard for georeferencing in HDFs so ENVI cannot read the information there and shows data in file order, meaning some flight lines may appear with south "up". To avoid this problem, export the data to a BIL or GeoTIFF file and load that into ENVI instead, e.g.:

 # BIL file (recommended), creates .hdr file too
azexhdf -v -h level3_file.hdf -Be level3_file.bil

 # GeoTIFF - won't work if filesize > 2GB
azexhdf -v -h level3_file.hdf -G level3_file.tif

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Last modified 10 years ago Last modified on Jul 2, 2009 2:20:07 PM