Costa Rica shaded relief

Shaded Relief

This is an example of how to create a shaded relief raster with a vector map overlay (using SVG and d3.js).

Step 1 was to create the raster. I used tiled GeoTiffs from the SRTM project, downloading four tiles that completed a map of Costa Rica. To combine the tiff files into a single raster with the correct projection and dimensions, I used gdalwarp:

gdalwarp \
  -r lanczos \
  -te -250000 -156250 250000 156250 \
  -t_srs "+proj=aea +lat_1=8 +lat_2=11.5 +lat_0=9.7 +lon_0=-84.2 +x_0=0 +y_0=0" \
  -ts 960 0 \
  srtm_19_10.tif \
  srtm_20_10.tif \
  srtm_19_11.tif \
  srtm_20_11.tif \
  relief.tiff

The t_srs option sets an albers equal area projection that will center on Costa Rica. The te option defines the extent of the map, using SRS coordinates. I don’t fully understand how this works and used some trial and error. Note that the x/y has a ratio of 1.6, the same as the intended output resolution (960x600).

Note that the projection here mirrors the projection set in index.html.

Step 2 is to create, from this GeoTiff file, two images: one, grayscale, that represents “shade” — given a certain direction of sunlight, it simulates the effect of light on the relief map:

gdaldem \
  hillshade \
  relief.tiff \
  hill-relief-shaded.tiff \
  -z 4 -az 20

The second image is a “color relief” that maps certain colors to certain elevations. The color_relief.txt file provides this information in the format: elevation r g b.

gdaldem \
  color-relief \
  relief.tiff \
  color_relief.txt \
  hill-relief-c.tiff \

These files are combined using the program hsv_merge.py:

hsv_merge.py \
  hill-relief-c.tiff \
  hill-relief-shaded.tiff \
  hill-relief-merged.tiff