13.16. dem_mosaic

The program dem_mosaic takes as input a list of DEM files, optionally erodes pixels at the DEM boundaries, and creates a mosaic. By default, it blends the DEMs where they overlap.

Usage:

dem_mosaic [options] <dem files or -l dem_files_list.txt> \
  -o output_file_prefix

The input DEMs can either be set on the command line, or if too many, they can be listed in a text file (one per line) and that file can be passed to the tool.

The output mosaic is written as non-overlapping tiles with desired tile size, with the size set either in pixels or in georeferenced (projected) units. The default tile size is large enough that normally the entire mosaic is saved as one tile, in the format output_file_prefix-tile-0.tif. Alternatively, one can pass to the -o option an output file name ending in .tif. Then the mosaic will be written with this exact name, without appending tile-0.tif. (This will fail if the tool decides there is a need for more than one tile.)

Individual tiles can be saved via the --tile-index option (the tool displays the total number of tiles when it is being run). As such, separate processes can be invoked for individual tiles for increased robustness and perhaps speed.

The output mosaic tiles will be named <output prefix>-tile-<tile index>.tif, where <output prefix> is an arbitrary string. For example, if it is set to results/output, all the tiles will be in the results directory.

By the default, the output mosaicked DEM will use the same grid size and projection as the first input DEM. These can be changed via the --tr and --t_srs options.

The default behavior is to blend the DEMs everywhere. If the option --priority-blending-length integer is invoked, the blending behavior will be different. At any location, the pixel value of the DEM earliest in the list present at this location will be kept, unless closer to the boundary of that DEM than this blending length (measured in input DEM pixels), only in the latter case blending will happen. This mode is useful when blending several high-resolution “foreground” DEMs covering small regions with larger “background” DEMs covering a larger extent. Then, the pixels from the high-resolution DEMs are more desirable, yet at their boundary these DEMs should blend into the background.

To obtain smoother blending when the input DEMs are quite different at the boundary, one can increase --weights-blur-sigma and --weights-exponent. The latter will result in weights growing slower earlier and faster later. Some experimentation may be necessary, helped for example by examining the weights used in blending; they can be written out with --save-dem-weight integer.

Instead of blending, dem_mosaic can compute the image of first, last, minimum, maximum, mean, standard deviation, median, and count of all encountered valid DEM heights at output grid points. For the “first” and “last” operations, the order in which DEMs were passed in is used. With any of these options, the tile names will be adjusted accordingly. It is important to note that with these options blending will not happen, since it is explicitly requested that particular values of the input DEMs be used.

If the number of input DEMs is very large, the tool can fail as the operating system may refuse to load all DEMs. In that case, it is suggested to use the parameter --tile-size to break up the output DEM into several large tiles, and to invoke the tool for each of the output tiles with the option --tile-index. Later, dem_mosaic can be invoked again to merge these tiles into a single DEM.

If the DEMs have reasonably regular boundaries and no holes, smoother blending may be obtained by using --use-centerline-weights.

Example 1. Erode 3 pixels from input DEMs and blend them:

dem_mosaic --erode-length 3 dem1.tif dem2.tif -o blended

Example 2. Read the DEMs from a list, and apply priority blending:

echo dem1.tif dem2.tif > image_list.txt
dem_mosaic -l image_list.txt --priority-blending-length 14 \
  -o priority_blended

Example 3. Find the mean DEM, no blending is used:

dem_mosaic -l image_list.txt --mean -o mosaic

Example 4. Write with the exact output name, without using the tile-0.tif extension:

dem_mosaic dem1.tif dem2.tif -o blended.tif

Command-line options for dem_mosaic:

-h, --help

Display the help message.

-l, --dem-list-file <filename>

Text file listing the DEM files to mosaic, one per line.

-o, --output-prefix <string>

Specify the output prefix. One or more tiles will be written with this prefix. Alternatively, an exact output file can be specified, with a .tif extension.

--tile-size <integer (default: 1000000)>

The maximum size of output DEM tile files to write, in pixels.

--tile-index <integer>

The index of the tile to save (starting from zero). When this program is invoked, it will print out how many tiles are there. Default: save all tiles.

--tile-list <string>

List of tile indices (in quotes) to save. A tile index starts from 0.

--erode-length <number-of-pixels (default: 0)>

Erode input DEMs by this many pixels at boundary before mosaicking them.

--priority-blending-length <number-of-pixels (default: 0)>

If positive, keep unmodified values from the earliest available DEM except a band this wide measured in pixels inward of its boundary where blending with subsequent DEMs will happen.

--hole-fill-length <number-of-pixels (default: 0)>

Maximum dimensions of a hole in the output DEM to fill in, in pixels.

--tr <resolution>

Output DEM resolution in target georeferenced units per pixel. Default: use the same resolution as the first DEM to be mosaicked.

--t_srs <proj4-string>

Specify the output projection (PROJ.4 string). Default: use the one from the first DEM to be mosaicked.

--t_projwin <xmin ymin xmax ymax>

Limit the mosaic to this region, with the corners given in georeferenced coordinates (xmin ymin xmax ymax). Max is exclusive.

--first

Keep the first encountered DEM value (in the input order).

--last

Keep the last encountered DEM value (in the input order).

--min

Keep the smallest encountered DEM value.

--max

Keep the largest encountered DEM value.

--mean

Find the mean DEM value.

--stddev

Find the standard deviation of DEM values.

--median

Find the median DEM value (this can be memory-intensive, fewer threads are suggested).

--nmad

Find the normalized median absolute deviation DEM value (this can be memory-intensive, fewer threads are suggested).

--count

Each pixel is set to the number of valid DEM heights at that pixel.

--georef-tile-size <projected-units>

Set the tile size in georeferenced (projected) units (e.g., degrees or meters).

--output-nodata-value <double>

No-data value to use on output. Default: use the one from the first DEM to be mosaicked.

--ot <type (default: Float32)>

Output data type. Supported types: Byte, UInt16, Int16, UInt32, Int32, Float32. If the output type is a kind of integer, values are rounded and then clamped to the limits of that type.

--weights-blur-sigma <integer (default: 5)>

The standard deviation of the Gaussian used to blur the weights. Higher value results in smoother weights and blending. Set to 0 to not use blurring.

--weights-exponent <float (default: 2.0)>

The weights used to blend the DEMs should increase away from the boundary as a power with this exponent. Higher values will result in smoother but faster-growing weights.

--use-centerline-weights

Compute weights based on a DEM centerline algorithm. Produces smoother weights if the input DEMs don’t have holes or complicated boundary.

--dem-blur-sigma <integer (default: 0)>

Blur the final DEM using a Gaussian with this value of sigma. Default: No blur.

--extra-crop-length <number-of-pixels (default: 200)>

Crop the DEMs this far from the current tile (measured in pixels) before blending them (a small value may result in artifacts).

--nodata-threshold <float>

Values no larger than this number will be interpreted as no-data.

--force-projwin

Make the output mosaic fill precisely the specified projwin, by padding it if necessary and aligning the output grid to the region.

--save-dem-weight <integer>

Save the weight image that tracks how much the input DEM with given index contributed to the output mosaic at each pixel (smallest index is 0).

--save-index-map

For each output pixel, save the index of the input DEM it came from (applicable only for --first, --last, --min, --max, --median, and --nmad). A text file with the index assigned to each input DEM is saved as well.

--threads <integer (default: 4)>

Set the number of threads to use.