> Hamish wrote: > > > > I am working with MODIS Aqua imagery in HDF4 format. (level 3) > > ... > > > > I added a mini-tutorial here: > > > > http://grass.gdf-hannover.de/wiki/MODIS > > ... > > > > The Chlorophyll-a concentration data comes in log form, 0-65535. > > ... > > > > The metadata (available through gdalinfo) gives the formula and > > > > test points to convert to mg/m^3: > > > > r.mapcalc "${map}.chlor_a = 10^(($Slope * $map) + $Intercept)" > > > > > > > > ie a histogram of that starts high and exponentially drops to 0. > > > > By 2.0 there are few data cells left, but real data continues > > > > up to the max in areas having plankton blooms. > > ... > > > > So how to make nice color rules for that? 'r.colors -g' does the > > > > opposite of what I want, > > > > Glynn: > > > Odd. Theoretically, it should do exactly what you want. > Hamish: > > You are right, -g is the right thing to use, it's just that the data > > is /so/ logarithmic that it only brings out a little more detail and > > it appears like nothing happened. (it became clear after displaying > > a legend, changing the color rules, then redrawing) Glynn: > Strange. I would have expected to see exactly the same results from: > > r.colors color=bcyr map=$map > and: > r.colors -g color=bcyr map=${map}.chlor_a > > AFAICT, the -g flag should effectively "undo" the exponential > conversion. All of the constant factors (e.g. $Slope and $Intercept) > cancel out from the equations. > > Maybe the colour table needs more samples? r.colors is hard-coded to > use 100 samples: > > G_log_colors(&colors_tmp, &colors, 100); > > Do you get better results if you increase that number?
No, I get the same. I put some screenshots and r.univar info about the rasters here: http://bambi.otago.ac.nz/hamish/grass/bugs/r.colors/ You can see on that page the number of samples is not the problem (reducing it to 5 gives nearly the same result), the problem is the data is 90% done by 0.4, but the colr/ rules are only written out on integer values so they are always scaled linearly between the 0 and 1 colr/ rules. Hamish ____________________________________________________________________________________ You rock. That's why Blockbuster's offering you one month of Blockbuster Total Access, No Cost. http://tc.deals.yahoo.com/tc/blockbuster/text5.com _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
