[GRASS-dev] SRTM plus color table

[Hamish]

Michael wrote:
> GRASS has a lot of nice pre-defined color tables. But one that is
> lacking is an SRTM-plus color table. This uses the SRTM color table
> for land, but has enough contrast to also show the sea floor.
> Currently, only ETOPO2 has a decent color table for the sea floor,
> but it is so light in shallow water that the land/sea boundary is not
> easily discernible if combined with the STRM land colors. So here is
> an attempt to find a combination of SRTM land, plus a pleasing sea
> floor color table. Maybe someone can improve on it more. In any case,
> I'd like to suggest that something along this line be incorporated
> into the stock pre-defined color tables in GRASS.
[...]
> ---- SRTM plus color table -----
> -11000 0:0:0
> -5000 0:0:70
> -1000 0:10:130
> -50 0:70:225
> 0 0:191:191


Hi,

I fully agree that our deep sea colors need some work. The main
problem with that is if you wish to focus on the continental
shelves or the deep trenches. Almost all of the ocean floor is
shallower than 3000m, but there's the occasional dip to 10-11 km.
Also most of the continental shelf is shallower than 150-200m.
So a linear color scale doesn't work for all of that range without
either blurring out the detail in the shelf, or making everything
below 3km black. IIRC the old etopo and srtm color rules had one
focusing on the shelf and the other on the deep sea, but both
could be improved. (when it is just global elevation it seems
a little odd to name the rules after the datasource, unless the
rules came directly from them of course)

Perhaps a numerical approach is to mask off the elevation < 0 in
one of the global datasets and run 'r.colors -e' to find the nice
transitions? Probably some tweaks still needed by hand to make
sure the shelf breaks are still highlighted.


I would note that this one:
   http://grassold.osgeo.org/screenshots/images/earthquakes_small2.jpg

is the same as this,
   http://grass.fbk.eu/spearfish/earthquakes_small.jpg

just with the rules bumped to up the contrast, as I find the
default (observational) Blue Marble 2000 too dark.

And fwiw these were taken from NOAA's elevation + ice hybrid rules:
   http://adhoc.osgeo.osuosl.org/grass/earthquakes.png
but again a supplied R,G,B band image, not mathematical rules.


summary: a straight linear set of rules are not enough, for
best results we might need to do a histogram scale for the above
and below the shelf break, and somehow cleanly transition those
two together.

Also finding something which is nice on the computer monitor,
even when the colors are washed out on a presentation projector,
and still nice when printed to a page in black and white is
possible, but not always so easy to do.


best,
Hamish