i.median colortables

[Michael Shapiro]

Each raster map that is produced has a number  of  support  files
automatically  created.  At one time a histogram file was created
automatically.  with a count of each value that  occured  in  the
rastermap.  This  process  slowed  the system down enough that it
wasn't worth keeping for the few instances that a histogram might
be  needed.  r.support  can  be  used  to generate this histogram
support file.

A simpler support file which just keeps track of the minimum  and
maximum  values  in  the  raster  map is always produced. This is
called the range file. Both the histogram file (if there is  one)
and  the  range  file  (there  should  always be one) live in the
cell_misc directory. For example, if the  raster  map  is  called
"soils",  the  the  range  file  is cell_misc/soils/range and the
histogram is cell_misc/soils/histogram.

If i.median is producing weird results and you think it is due to
the  lack  of  the  histogram  (note  it is a warning only), then
create histograms using r.support.

|
|
|I've been trying to use i.median to produce false infrared
|images from Landsat tapes, and am given error messages like
|"WARNING: Histogram for [file name in PERMANENT] missing"
|What produces these histograms? I've run g.support and r.stats
|but neither seems to save a histogram.
|
|A possibly related question: when I have run i.median (ignoring
|the no-histogram error) the same color is applied to a nu
ber of
|different data values, so that the resulting image has only a few
|different colors in largish clumps. Michael Shapiro wrote a few
|weeks ago that "The  number  of  colors in the color tables for raster 
|maps depends only on the the number of categories in the raster map."
|How does GRASS determine how many categories are in a composite
|map like that produced by i.median, and is it possible to set this
|number in some other way to produce a colortable with more different
|values? Would the lack of a histogram cause GRASS to produce a more
|limited colortable?
|
|Thanks for any advice on this question --
|
|Nick Cahill
|nick at ikaros.harvard.edu
|
|
|


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Michael Shapiro                        U.S. Army CERL                  
email:   shapiro at zorro.cecer.army.mil  Environmental Division          
phone:   (217) 352-6511  ext 526       P.O. Box 9005                   
fax:     (217) 373-7222                Champaign, Ill. 61826-9005
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