Scripts to automate conversion from DEM to contour shape files

Question

I am using the DEM files in the Cornell CUGIR repository to build contour layers in QGIS. The problem I have is that while it is easy to do, it requires many manual steps in QGIS that are both tedious and prone to error. I would like to know if there are any tools out there to automate the process.

Starting with a directory of DEM files, on a Unix box (in my case a Mac), here is what I would like to do:

1. convert the DEM files from metric to english units (meters -> feet)
2. create a contour layer with 100 ft intervals including labels for the elevation
3. create a contour layer with 20 ft intervals including labels for the elevation
4. apply the same style to all the created contour shape files

The tool I envision would either find all the applicable DEM files in a directory or take the list of files as a command line argument. I will write this myself if there isn't one out there somewhere already.

Answer 1

The gdal command line is great for this kind of thing. Using an ASTGTM DEM file, with heights in metres, to generate a shapefile with 20 foot contours (6.096m):

gdal_contour -a elev -i 6.096 astdem.tif astdem.shp

• -a elev: create a field called 'elev' with the contour height in metres;
• -i 6.096: create contours at 6.096m intervals (20 feet);
• astdem.tif - input DEM file;
• astdem.shp - output shapefile.

Create a new field with feet:

ogrinfo astdem.shp -sql "ALTER TABLE astdem ADD COLUMN el_ft integer(5)"
ogrinfo astdem.shp -dialect SQLite -sql "UPDATE astdem SET el_ft = round(elev * 3.2804,0)"

In QGIS, create nice labels and styles for your 20' and 100' contours (the el_ft field) and save the style file as a .qml. For each new contour shapefile you just copy and rename the .qml file with the same filename but .qml extension and it will automatically load in QGIS with the right style.

You could do this as a bash script, processing all the tif files in the directory:

!#/bin/bash
for DEM in $(ls *.tif); do
  gdal_contour -a elev -i 6.096 $DEM $DEM.shp;
  ogrinfo $DEM.shp -sql "ALTER TABLE $DEM ADD COLUMN el_ft integer(5)";
  ogrinfo $DEM.shp -dialect SQLite -sql "UPDATE '$DEM' SET el_ft = round(elev * 3.2804,0)";
  cp mylabels.qml $DEM.qml;
done