[GRASS-user] Raster histogram or r.info for raster min/max values

Hamish hamish_b at yahoo.com
Tue Jul 21 02:20:14 EDT 2009


Frank wrote:
> I am trying to perform a dark pixel correction on a Landsat ETM
> mosaic consisting of 15 scenes. For each band in each scene I
> need to determine the minimum brightness value and then subtract
> that 'bias' from the entire band. I have figured out how to use
> r.mapcalc to do the subtraction, but I am getting what appears to be
> conflicting results when I attempt to calculate the minimum value.

did you try yet the i.landsat.rgb module? you may look inside the
script with a text editor to see/adjust the method used.

example: http://grass.osgeo.org/screenshots/rs.php

It uses the r.univar module's percentile= option to chop away the
bottom x% of pixel values, rather than assume the minimum value seen
is worth keeping and not just an outlier.

Note that it does not touch data values, it only adjusts the colors.

> My operating system is Windows Vista Business and I am using GRASS
> 6.4.0svn under the wxPython user interface. My dilemma is this:
> If I run r.info on Band 3 (visible red) for a particular
> scene I get this result:
> Range of data:    min = 1  max = 255

note r.info works on the map's metadata (so full extent), while r.univar
(and most other raster modules) work on the current region settings (zoom

also if current region resolution does not match the map's native
resolution some resampling will happen which may cause it to skip cells,
and so the minimum value might be hidden.
(see g.region help page and raster intro help page)

the command shell (dos box) way would be:
 g.region rast=your_map   # (zoom to match extent of that map)
 r.univar your_map

> Given that this band is visible red I would expect a minimum value
> somewhat greater than 1 -- a value of between 5 and 10 would be more
> appropriate for this region.
> If I display the raster histogram (using the Analyze button on the Map
> Display window), the x-axis extends anywhere from around 10 to around
> 235 (in a small window) or from 6 to around 247 (if I span the window
> across both monitors).

....hmmm so the wxGUI histogram works on the Map Display region instead
of the computational one?  (I'm not an expert on the ins and outs of the
GUI, not sure if that is true or not, nor which way is more appropriate)

try "zoom to current layer"

> And, yes, I know the x-axis units are 'in tens'.

the axis scaling is dynamic dependent on the data.

> Varying the size of the window has a major effect on the
> histogram! It is not a visual error - the first tic clearly changes
> relative to the y-intercept.
> Which result do I believe? 

The d.histogram module which generates that is a little bit crusty,
but all it does is run and plot up the results of r.stats. So I'd
suggest to run r.stats.
> I do not trust the minimum values from r.info.

Trust them, but realize that it represents the entire map, not just
the current zoom settings.

> What I would really like is to generate a summary of the
> number of pixels with each brightness value from 0 to 255.
> Is there an easy way to do that?


g.region rast=your_map.red # (eg red band, all bands should be the same)
r.stats -c your_map.red

then cut and paste output into your favorite plotting program to create
the histogram.

I don't think/know that this would work on Windows, but for Mac and
Linux users a way to recreate d.histogram using the d.linegraph module:

g.region rast=your_map.red
r.stats -c your_map.red | cut -f1 -d' ' > Xs
r.stats -c your_map.red | cut -f2 -d' ' > Ys
d.mon x0
d.linegraph x_file=Xs y_file=Ys

(dev note: it seems d.linegraph and d.histogram share code, if either is
to remain for GRASS7 recent bug fixes to d.histo should be ported over to
d.linegraph. personally I wouldn't mind to see PyPlot replace them both)

> I have only been using GRASS a couple weeks and have not
> figured out how to script it yet, so would prefer the
> solution to involve the UI.

ok, so I have failed badly in that regard but hopefully it helps explain
some of the underlying logic and give you an idea of what to look for
in the menus.

(dev note- TODO: get module synopsis PDF* to show GUI menu hierarchy
location for each module name. prototype code can be found in
find_menu_hierarchy() in tools/module_synopsis.sh)
[*] http://grass.ibiblio.org/gdp/grassmanuals/grass64_module_list.pdf



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