[FWTools] Compressing geoid model?
Charles Karney
charles.karney at sri.com
Tue Jun 7 09:31:53 EST 2011
For egm2008-5.pgm...
Other lossless image compression schemes:
pamtotiff -flate saves 7.5%
pnmtopng -compress 9 saves 10%
Other general compression schemes:
gzip -9 saves 10%
zip saves 10%
bzip2 -9 saves 44%
inno setup + lzma/ultra64 saves 42%
Incidentally the 1" grid file egm2008-1 generally compresses better
(because the data is smoother). In particular lzma/ultra64 reduces it
size by 74%.
The pgm format is of course wonderful in that it permits random access.
This is potentially important for the 1" grid data (which is about
0.5GB). (And for GeographicLib, I wanted also to avoid a dependency on
a tiff library.)
On 06/07/11 09:36, Mikael Rittri wrote:
> 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?
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