dem_water DEM waterbody flattening

This tool processes a digital elevation model together with raster files for waterbodies and modifies the DEM to flatten the waterbody areas while keeping an overall consistent elevation model.

Warning

Much of this program is highly experimental and not very robust. It will crash or produce nonsense results when called with the wrong options. Best to consider it just a demo for the moment.

Compiling the program

dem_water requires the CImg image processing library to be built and should be compilable on all platforms supported by the library. You need to download this library separately.
Copying CImg.h to the source directory is sufficient.

It also uses GDAL for loading image files with geo-coordinates.

Program operation

To correctly process waterbodies they need to be fully within the processing area of the program. Normally this means a significantly larger area needs to be processed while the output can only be used for a smaller area. There are two operating modes for this

• Tiled mode where a typical 1x1 degree DEM tile and its 8 neighbors are processed while the results are only for the central tile this is invoked by use of the -is? options.
• General mode using the -i option where the full area is only cropped by a single pixel and the output needs to be manually cropped further as needed.

Some of the input data is expected to cover the full processing area, some only the smaller output area. This is mentioned in the options description.

Program options

the program offers the following command line options:

• -i DEM input file. Full size (alternative to -is?)
• -is1 to -is9 DEM input 1x1 degree tiles (alternative to -i)
• -m Main waterbody mask image file, expected to be 32 bit signed integer with unique value for every waterbody values of -1 are interpreted as ocean. Full size. (required)
• -ms Supersampled waterbody mask with byte valued pixels between 0 and 255. Small size. (optional)
• -b Bathymetry DEM to use for ocean areas. Small size. (optional)
• -o Output image file name (required)
• -p File for optput of POV-Ray mesh (optional)
• -e File for error point list in GeoJSON format (optional)
• -ce File for coastline error point list in GeoJSON format (optional)
• -sr DEM input resolution in tiled mode. Either 1 or 3. Default: 1
• -r Radius to taper off elevation changes. Default: 2.1
• -z Elevation value to use for the Ocean. Default: 0
• -lm Minimum elevation value to allow for land. Default: -32767
• -w Wall height to generate around water areas. Default: 0
• -t Elevation difference threshold for the calculation of waterbody levels. Default: 50
• -et Elevation difference threshold for error point generation. Default: 80
• -lon tile longitude Default: 0
• -lat tile latitude Default: 0
• -3 generate POV-Ray mesh in true 3d (i.e. curved earth surface). Default: off
• -oc Generate output using CImg rather than in hgt format. Default: off
• -n Noise to add to the output data (useful to mitigate integer meter discretization artefacts). Default: off
• -debug Generate a large number of image files from intermediate steps in the current directory for debugging. Default: off
• -h show available options

Using the program

The program requires you to separately rasterize the waterbody data. This can be done using something like the following:

gdal_rasterize -ot Int32 -init -1 -burn 0 -ts ... -te ... $COASTLINE_SOURCE $WATER_MASK

gdal_rasterize -a "$LAKES_ID" $LAKES_SOURCE $WATER_MASK

To generate the supersampled input mask just rasterize at higher resolution and scale down using gdalwarp or ImageMagick.

By default the program generates the output DEM in hgt format. You can change that when using the general mode using -oc.

Error point output is pretty self explaining. Attributes included are the ID from the input data and the maximum error.

If you try to use the POV-Ray mesh output you will almost certainly run into accuracy issues.

Legal stuff

This program is licensed under the GNU GPL version 3.

Copyright 2013 Christoph Hormann