Some MapSends has built-in topo database. The database is stored in the \MAP\BLX folder in the *.blx files. Format of the *.blx files is very complex (to my opinion this is the most complex of Mapsend's formats). Topographic data composed in the series of data chunks, each chunk represents 128x128 matrix of heights. Chunks in turn, composed from 15 smaller matrices to build 128x128 matrix. Moreover, chunks are compressed with Magellan's custom compression algorithm.

All this was discovered by Henrik Johansson (see his messages #203 and #797 in Mapsend_Format yahoo group). He created Python program to extract/create BLX files (available at MapSend_Format yahoo group). Many thanks to Henrik for his outstanding work! In my program I'm using his algorithms for compression/decompression of chunks and algorithms to create matrices.

I choose the most explicit format of the output files: Terrain Matrix model compatible with free 3DEM viewer. In this format Height matrix is represented by matrix of signed integers, all georefference information is stored as a plain text in a separate file (*.hdr). Terrain Matrix can be converted then to DEM or 3D-GeoTiff with 3DEM viewer. 3DEM viewer available for free at http://www.visualizationsoftware.com/3dem.html. To my opinion 3DEM is one of the best free converter/viewers.


1. Select *.blx file containing area of interest from \MAP\BLX folder. Normally there are several *.blx files for different areas, for example MapSend Topo US has usa.blx, hawaii.blx, puerto.blx, alaska.blx, alaska2.blx. In the example the file selected is usa.blx

2. Enter coordinates for Left-Right Latitude and Top-Bottom longitude. The coordinates are WGS84 Lat/Long. Negative latitudes for West, negative longitudes for South. In the example we enter initially -80, -78 for latitude and 40, 38 for longitude. Press "Check". Latitude and longitude will be adjusted to match closest cells, total numbers of the cells will be shown together with the output file size. Do not select large areas, in the example 2 by 2 degree are results in ~12 Mb of the output file.

3. Select file for Terrain Matrix (*.bin). Press "Extract". In this example 2-by-2 degree area is composed from 361 cells with the file size of ~12 MB, extraction of this area takes 16 sec (Pentium IV, 2 Ghz, 256 Ram).


Below are 3DEM-viewer images of the extracted area: