[FWTools] Compressing geoid model?
Mikael Rittri
Mikael.Rittri at carmenta.com
Tue Jun 7 08:36:51 EST 2011
Hello,
I have been trying to compress the geoid heights of EGM 2008,
starting from the 5 arc minute resolution that is available
via GeographicLib, http://geographiclib.sourceforge.net/html/geoid.html
I thought tif would be a nicer format than the pgm format provided by
GeographicLib, and I hoped that tif compression would help.
However, I have tried these commands:
gdal_translate -ot UInt16 -co "COMPRESS=NONE" egm2008-5.pgm egm2008-5-uint16-nocompression.tif
gdal_translate -ot UInt16 -co "COMPRESS=LZW" egm2008-5.pgm egm2008-5-uint16-lzw.tif
gdal_translate -ot UInt16 -co "COMPRESS=PACKBITS" egm2008-5.pgm egm2008-5-uint16-packbits.tif
and got the following sizes of the tif files:
egm2008-5-uint16-nocompression.tif 18252 KB
egm2008-5-uint16-packbits.tif 18377 KB
egm2008-5-uint16-lzw.tif 23659 KB
where the original file was
egm2008-5.pgm 18235 KB
So, PACKBITS makes the result 0.7 percent larger than NONE,
which is perhaps not a surprise, since run-length encoding
should not be suitable for this kind of data.
However, LZW makes the result 29.6 percent larger than NONE,
which surprised me. I would have thought the the geoid data
should be fairly easy to compress (although I know almost
nothing about how LZW works).
Am I missing something obvious? Is there some other
(GDAL-readable) compressed format that would be better
for this kind of data?
Best regards,
Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com
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