[GRASS-dev] SRTM plus color table

[Hamish]

Michael wrote:
> So, a little trial and error
> experimentation produces this:
...
> ...with the following color rules.
> 
> -11000 0:0:0
> -8000 0:0:50
> -5000 10:10:70
> -3000 30:30:100
> -1000 70:70:170
> -100 100:100:200
> 0 150:150:255
> 0.1 57:151:105
> 100 117:194:93
> 200 230:230:128
> 500 202:158:75
> 1000 214:187:98
> 2000 185:154:100
> 3000 220:220:220
> 5000 250:250:250
> 8850 255:255:255
> nv 255:255:255
> default 255:255:255

I'll respond the long way, with some personal cosmetic
observations and critique on ocean colors. YMM(& probably will)V.

my test data is the etopo1 global elevation dataset.
See Global Datasets on the grass wiki for links.

r.colors etopo1 color=<>

etopo2 -- ocean colors are not too bad. land colors much too
green then harsh transition to white.


srtm -- only the shallow shelves are visible, most of the ocean
is black. Georges Bank off Cape Cod and the North & Baltic Seas
show up quite well (often < 100m). ~600m is already black, but
for a global view 3000m should still be visible. I find srtm on
land to be too brown, but in coastal areas it can look ok, e.g.:
  http://adhoc.osgeo.osuosl.org/livedvd/docs/_images/grass-fractal.png


elevation -- relative % based, only intended for land.


terrain -- a mix of the green from etopo2 with the black oceans
of srtm! worst of both worlds? elevation based.


haxby -- specifically created by W.F. Haxby for highlighting
terrain on the sea floor, but for segments of the sea -- not
the full range. See also "Digital images of combined oceanic and
continental data sets and their use in tectonic studies" in AGU's
EOS from a couple months ago:
  http://onlinelibrary.wiley.com/doi/10.1029/EO064i052p00995/abstract
The haxby rules are relative % based scale, so you need to crop
out the ocean first:
  r.mapcalc "etopo1.sea = if(etopo1 <= 0, etopo1, null())"
  r.mapcalc "etopo1.land = if(etopo1 >= 0, etopo1, null())"

(but even then the scale is too wide, just zoom in on one part
of the ocean floor to use them)


SRTMplus -- the shallow seas have a rather coral-reef look to
them, structure of the deep sea seems a bit blurred out.


proposed (above/dropbox'd): the moonlight effect of the sea
floor is nice, although it's a little bit purplish. You can see
a bit more structure in the Baltic, and still some structure on
Georges Bank and the Hudson Canyon. Around the Bahamas the
transitions are a bit harsh.  Like "srtm" I find the land to be
too brown for my taste.


One thing I often do with ocean floor plots is to put the land in
greyscale, or as a greyscale shaded relief, with the blue->black
ocean. That bypasses any high desert vs. ice cap coloring issues.
 (shaded relief works best in a non-LL projection)

d.erase 60:60:60
r.colors etopo1.sea color=etopo2
d.rast -o etopo1.sea
d.rast -o etopo1.land.shade    # lowered resolution to 0:05 during creation else it looks to granular


in summary, etopo2 is my favourite for the deep oceans, but may
be too bright if you are strictly looking at the upper 100m of the
continental shelves, in that case "srtm" may be better.

adding the following 100m rule to "etopo2" seems to look nice for
adding detail in the shallow seas:  (e.g. east coast of the US
and the North/Baltic Seas)

-100:100:100:225

(see attached screenshot; but you really need to zoom into a
shallow sea like the Bay of Fundy or the Baltic to really see
the difference)  It makes the Med look pretty nice too, but that
already looked nice with the "etopo2" rules.

I propose to add the above rule to "etopo2".


regards,
Hamish


-----
fwiw,
G> r.univar etopo1.sea -e percent=0,10,20,30,40,50,60,70,80,90,100
minimum: -10803
maximum: 0
range: 10803
mean: -3433.38
standard deviation: 1716.47
...
0th percentile: -10803
10th percentile: -5286
20th percentile: -4902
30th percentile: -4532
40th percentile: -4226
50th percentile: -3890
60th percentile: -3509
70th percentile: -2958
80th percentile: -1863
90th percentile: -253
100th percentile: 0
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