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Quick way to prepare raster maps for use in QMapShack

This is translation of the original article in the blog: http://htrd.su/wiki.

Let us take http://loadmap.net as an example server for getting raster maps (military maps updated by the Goverment GIS Center) with OziExplorer references.

A major disadvantage of such maps is a border that overlaps neighbouring maps. To solve this issue A. Drozdoff wrote the small geocrop application (free and open source, patches welcome, *nix only) which helps to crop the maps.

A major advantage of QMapShack (QMS) is that it can open maps defined in a GDAL .vrt format file. A VRT file is a XML based map description containing different useful information about maps:

• links to the original map source (raster file in our case),
• reference description,
• other useful transformations.

The method used in geocrop in order to crop map borders works with VRT files too: cropping does not affect the original raster file transformation. The cropped file can be opened properly. The transformation speed is increased dramatically.

The geocrop build process is described in the GitHub (see link above). It requires the installation of developer packages for libgdal and libproj4. Building on Windows is not yet supported (any help is welcome). The application does not require any installation, simply copy it into ~/bin.

Having prepared geocrop 3 simple steps are required to prepare a map:

• Download the needed map sheet from http://loadmap.net, for example assume the 500-meter (scale: 1:50 000 or 1 cm = 500 meters) sheet K-53-027-A with raster image file K-53-027-A.png
• Download from the same location the OziExplorer reference file for the map: K-53-027-A.map
• Crop the border and create a VRT file with geocrop:

  `~/bin/geocrop -s 50k -f VRT K-53-027-A.map K-53-027-A.vrt`

Pay attention to the parameter -s - it allows to select the scale of the sheet:

50k (k - kilo) => 50 000 => scale 1:50 000.

The scale parameter allows to select suitable rules for the sheet content calculation and the detection of the border. Thus, this parameters is a mandatory.

Repeat these steps for all necessary sheets.

Next add the path to the maps to QMS and check the result.

On the following screenshot (for image with better quality see: http://htrd.su/wik) two sheets of a 200k map (2km) are displayed. The sheet boundaries can be seen but they do not affect the maps.

The podcast https://youtu.be/7mp35wYk0IQ describes the required procedure.

A single VRT file can be created out of several VRT files. In other words: it is possible to combine multiple VRT files for maps with the same scale into a single file (an “atlas”) and open only one VRT file for loading a whole series of raster maps.

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