[Tile-serving] DEM scaling

Yohan Boniface yohanboniface at free.fr
Fri Jun 28 09:51:09 UTC 2013

Hi tiles rendering masters :)

I'm preparing a worldwide service of the new born (and still burgeoning) 
"HOT style". You can have a preview here: 
It's a TileMill/Carto project, the repository is on Github [1].

We have decided to include terrain data in this rendering. Specifically:

- one hillshade layer (light source altitude 80°, default azimuth)
- one 25m contour lines layer

I'm now working with a 90m, 5x5° tile from CGIAR [2], as the HOT style 
preview is only on Hispaniola island. So everything is easy and fast. 
But now I need to scale it.

After some investigation and discussion, here is the process I have in 
mind for this:

* for the hillshade:
- get the 90m 5x5 tiles from CGIAR (820 zip files, so to do step by step 
not to overload the download service...)
- reproject them with gdalwarp
- merge them by group of four, with gdal_merge.py (according to MapBox 
[3] experience, tiles from 4096x4096 to 16384x16384 are optimal, and 
each one of the tiles is 6000x6000), taking care of -a_nodata
- create hillshade with gdaldem
- create overviews with gdaladdo
- create a vrt file with all those tif, using dgalbuildvrt (and with 
absolute paths ;))

* for the contour lines:
- use the same 90m 5x5 tiles
- create contour with gdal_contour, taking care of -snodata (output a shp)
- import the shp in PostGIS with shp2pgsql

Does this make sense?

If you have some advice, different experience, I would be glad to hear it :)

Thanks in advance!


[1] https://github.com/hotosm/HDM-CartoCSS
[2] http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp

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