Hi, I am working with MODIS Aqua imagery in HDF4 format. (level 3) http://oceancolor.gsfc.nasa.gov/PRODUCTS/
The Chlorophyll-a concentration data comes in log form, 0-65535. Plotting the data with a bcyr color rules makes it look great. I added a mini-tutorial here: http://grass.gdf-hannover.de/wiki/MODIS 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)" Extended r.univar results + percentile=33,67 of the chlor-a map: n=16522204 null_cells=20802596 min=0.01 max=64.5654 range=64.5554 mean=0.263613 mean_of_abs=0.263613 stddev=1.1348 variance=1.28777 coeff_var=430.478 sum=4355475.7290696017 first_quartile=0.0717828 median=0.129749 third_quartile=0.211782 percentile_33=0.0893581 percentile_67=0.176154 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, 'r.colors -e' shows some more detail but isn't really right. A custom one using 'r.univar precentile=,,,,' cues shows some more but isn't smooth and is a pain. Any ideas? Could G_log_colors() be turned on its head to make a G_exp_colors() function and associated 'r.colors -x' flag? Would it work? I can convert the colors given here: http://oceancolor.gsfc.nasa.gov/PRODUCTS/colorbars.html from 0-255 to min->max using the 10^(mx+b) formula, and then pass those as rules to r.colors: http://trac.osgeo.org/grass/browser/grass-addons/raster/r.colors.tools/palettes So this isn't a urgent need, but could be a useful feature. thanks, Hamish _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
