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) + $Intercept)" > > > > > ie a histogram of that starts high and exponentially drops to 0. By > > > 2.0 there are few data cells left, but real data continues up to the > > > max in areas having plankton blooms. > ... > > > So how to make nice color rules for that? 'r.colors -g' does the > > > opposite of what I want, > > Glynn. > > Odd. Theoretically, it should do exactly what you want. > > You are right, -g is the right thing to use, it's just that the data is > /so/ logarithmic that it only brings out a little more detail and it > appears like nothing happened. (it became clear after displaying a > legend, changing the color rules, then redrawing)
Strange. I would have expected to see exactly the same results from: r.colors color=bcyr map=$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? -- Glynn Clements <[EMAIL PROTECTED]> _______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev