[GRASSLIST:4590] Re: different CELL integer types?

[William K]

Disappointing.  I understand not being able to change the existing 
format, but what about adding 1 and 2 byte signed cell formats?  When 
dealing with elevation data this can save everyone a lot of storage 
space (and probably speed up disk access some).

On Oct 20, 2004, at 10:28 AM, Glynn Clements wrote:

>
> William K wrote:
>
>> A little more info:
>>
>>> I've been importing a bunch of SRTM data recently and noticed that
>>> there seems to be (at least) 2 different integer cell types - 16 bit
>>> and 32 bit.  As imported, one tile takes up a maximum of 2.7MB in the
>>> CELL file, same as the source SRTM data file, and in the GRASS header
>>> file it's type is 1.
>>
>> I mean, the 'format:' line in the header file.
>>
>>> After I run it thru fillnulls, I round that back to integers with
>>> r.mapcalc, and the CELL file sizes double, and the GRASS header has a
>>> type of 3.
>>>
>> At least many end up like this.  I looked closer at a few, and it 
>> seems
>> that all the tiles with some negative values switch to the larger
>> integer size.  If the whole tile is positive, it stays as 16 bit.  Are
>> grass 16 bit ints not signed, so grass has to use 32 bit ints?  I can
>> mapcalc them to be all positive and they do switch back to 16 bit (or 
>> 8
>> bit for those low, flat tiles).
>
> Compressed integer rasters use 1, 2, 3 or 4 bytes, depending upon how
> many are needed. However, as you note, negative values always require
> 4 bytes.
>
> Essentially, the stream of ints is first converted to the external
> representation, which is 4 bytes, big-endian, with the topmost bit of
> the first byte being a sign bit.
>
> If the raster is uncompressed, the stream of bytes is then written out
> to the file.
>
> OTOH, if the raster is compressed, the next step is to determine the
> number of bytes required. If the first 1, 2 or 3 bytes of every cell
> in a given row are always zero, those bytes will be discarded. As
> negative values always have the topmost bit of the first byte set,
> they will always require 4 bytes. Consequently, any rows which include
> at least one negative value will be written out using 4 bytes per cell
> (although the actual data will still be subject to RLE compression).
>
> Unfortunately, this can't be changed without breaking existing raster
> maps.
>
> -- 
> Glynn Clements <glynn.clements at virgin.net>
>
>
-----
William Kyngesburye <kyngchaos at charter.net>
http://webpages.charter.net/kyngchaos/

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