Working on a static render of my SRTM tiles. Decided to use gdaldem to do the color mapping, it does all I need.
First I needed gdal 1.7 to use gdaldem, but Debian still packages older versions. There’s a lot of instructions for compiling. Long story short, ./configure; make works fine. It’s 1 million lines of code, so I had a nice cup of coffee.
Here’s how to use gdaldem to turn an SRTM .hgt file into a PNG: gdaldem color-relief -alpha -of png N39W120.hgt nelson.cmap gdaldem.png
. There’s a variety of color map options, I hand-coded my own:
30000 180 0 0 255 4572 180 0 0 255 3049 130 80 80 255 3048 80 80 80 255 2438 0 0 0 255 2437 0 0 0 0 0 0 0 0 0 nv 0 255 0 255
That maps everything from 0-2437 meters (8000′) to transparent, then a grey ramp up to 10000′, then a red ramp up to 15000′, then solid red. Missing values (NV) are coloured green; need to deal with them later. The result looks like this
Now a bit of shell scripting to run the whole set:
for f in *.hgt; do ../experiment/gdaldem color-relief -alpha -of png $f ../png/nelson.cmap ../png/${f%.hgt}.png; done
It’s awfully fast, I’m impressed with GDAL speed. The output PNG isn’t so bad, either, the empty tiles end up being 2k or so each.
Next up: gdal2tiles.py to turn these into tiles I can serve for a slippy map. I might need to gdalbuildvrt first to combine all the files into one big virtual map.