[GRASS5] zlib compression for CELL maps
woklist at kyngchaos.com
Wed Jan 4 10:32:20 EST 2006
At least for SRTM rasters, it depends on whether there are negative
elevations (common around coastal areas). I noticed this a while
back and wondered about it. I don't remember the details, but it
came down to what order of integer is needed - 1, 2 or 4 bytes,
unsigned - to represent all the values in the region. BUT, if there
were negative integers it always used a signed 8 byte integer (or
something large like that). Compression came into it also I think,
again I don't remember details.
So SRTM tiles in coastal areas are often much bigger than you'd
expect, though with the new version 2 SRTM that shouldn't be as big a
problem since water areas have been filled with a flat 0 elevation,
or an appropriate elevation for lakes.
I don't know about the GRASS_INT_ZLIB variable (env? compile?), but I
was under the impression that I couldn't do anything about it.
On Jan 4, 2006, at 8:44 AM, Jaro Hofierka wrote:
> I have a problem with a size of integer (CELL) maps. It is strange
> that the size of compressed integer maps is bigger than the same
> maps in floats. Recently, we tried to import srtm files and
> floating point maps took much less disk space then integers :-(
> I tried to use the GRASS_INT_ZLIB variable, but the compression
> flag in a header file is still set to 1 (should be 2?). Problably I
> missed something. I re-compiled GRASS using zlib.
> Many thanks for any hints!
William Kyngesburye <kyngchaos at kyngchaos.com>
"Oh, look, I seem to have fallen down a deep, dark hole. Now what
does that remind me of? Ah, yes - life."
More information about the grass-dev