[GRASSLIST:5229] Re: Floating point data
mash at econs.umass.edu
Sat Dec 14 11:14:18 EST 2002
I think that it's for color coding. A typical computer description of a color is three numbers (red blue green) that each range from 0 to 255, e.g.,
0 0 0 black (all colors turned off)
255 0 0 red (red turned on, blue and green off)
255 255 255 white (all colors turned on)
I may have the order and colors wrong but this is the gist of the 255 business. As you note (255 = 2^8 - 1) so I think that the computing origins of this color coding are from representation of colors with one byte of data.
Quoting Miha STAUT <mihastaut at hotmail.com>:
> I noticed that many of the commands reduce raster data to only 255 category
> values, at least r.info says so (e.g. r.average). Is it because of the
> floating point (like the following mail) representation or is it something
> else (e.g. like r.stats)?
> Miha Staut
> Ben Logan wrote:
> >I have some DEMs with 30 meter resolution, and the elevation values
> >appear to be in feet instead of meters like the 10 meter res DEMs. It
> >looked like I could convert the feet to meters with r.mapcalc, so I
> >ran it with
> >as the expression. It suceeded, but r.info says that my new map
> >(elev) only has 255 categories. The original (elev30) had 2441. It
> >seems to me that I should have the same number of categories...am I
> >missing something?
> In the above case, r.mapcalc will produce a floating-point map.
> Floating-point maps don't really have categories. The values can be
> coerced into categories using the map's quantisation rules, but only a
> few programs (e.g. r.stats) do this. Most programs just use the exact
> floating-point value.
> The quantisation rules can be changed with r.quant.
> Also, you can force r.mapcalc to generate an integer map using the
> "int(...)" operator, i.e.
> r.mapcalc 'elev=int(elev30/3.28)'
> I would expect that to have ~745 categories (one for each metre of the
> range, rather than one per foot).
> Glynn Clements <glynn.clements at virgin.net>
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