osm4etrex10

osm4etrex10 is a small framework around mkgmap that allows you to create OSM based maps for the rather limited eTrex 10 GPS receivers made by Garmin. The maps will also work on other Garmin devices such as the 20(x)/30(x) and probably other device series as well.

There are other frameworks and howto documents on this topic:

another mkgmap map style optimized for the eTrex 10 at http://gps.maroufi.net/etrex10map.shtml (in German)

Basic requirements

For compiling small binary map images you will need

OpenStreetMap data: You can use data in either OSM/XML or OSM/PBF formats. There is a handy export functionality on the website at https://www.openstreetmap.org/ that allows retrieving data for relatively small areas directly. Alternatively, you can use OSM dumps and cut the needed data with osmosis (http://wiki.openstreetmap.org/wiki/Osmosis). Instead of using the whole planet file you can use country based pre-cut extracts hosted e. g. at http://download.geofabrik.de/.
mkgmap map compiler: The mkgmap project at http://www.mkgmap.org.uk/ automatically publishes daily builds after a series of technical tests. Generally it is save and advisable to use the latest build. Daily snapshots are named after their revision number (XXXX): mkgmap-rXXXX.zip. Unzip the snapshot file in the osm4etrex10/lib/ lib directory. This will create a fully workable mkgmap installation.

Create a generic symlink within the osm4etrex10/lib/ pointing to the snapshot:

user@host$ cd lib/
user@host$ ln -s mkgmap-rXXXX mkgmap

The wrapper calling the mkgmap binary relies on this symlink. You can install many different mkgmap snapshots under lib/ allowing you to test their impact on map image size.

Compiling the map

To compile a map within the osm4etrex10 framework you call the bin/mkgmap wrapper. It expects the data extract in the OSM/XML or OSM/PBF format as a parameter:

user@host$ ./bin/mkgmap osm-data.pbf
Time started: Fri Oct 16 10:39:34 CEST 2015
Number of MapFailedExceptions: 0
Number of ExitExceptions: 0
Time finished: Fri Oct 16 10:40:15 CEST 2015
Total time taken: 41028ms

The compiled map is stored as img-output/osmmap.img. For use of the map with certain applications (such as QLandkarteGT) there is also a TDB file img-output/osmmap.tdb created.

To use the compiled map on the eTrex device, copy it to your handheld under it's Garmin directory, renaming it to gmapbmap.img (asuming the device is mounted under /media/etrex/):

user@host$ cp img-output/osmmap.img /media/etrex/Garmin/gmapbmap.img

Technical Background

Device limitations on the Etrex 10

eTrex 10 device have quite a few technical characteristic that make it difficult to use maps on those devices, most notably the display resolution, the memory limitations and different constraints by the device's firmware:

display limitations: The device comes equipped with a 2.2" (5.6 mm) greyscale display showing 176×220 pixels. This is considerably coarser then the resolution of other devices from the new eTrex series that have the same physical dimensions but show 240×320 pixels. In the map style (TYP file), this is catered for by drastically restricting the width of lines to typically one pixel, effectively resulting in a wireframe representation of streets.
memory size: The device has built in permanent memory of 10 MB with no possibility for extension, e. g. by (Micro-)SD cards. A part of the memory is used for device configuration files, stored waypoint and track data (if track recording is enabled), or geocache descriptions (if your into geo-games). Effectively you can expect no more than about X MB for maps if you restrict the use of the device to displaying the map.
software specific limitations: (data structures, access modes)

Map size optimizations

There is a range of techniques and tricks that allows for a size reduction in mkgmap created maps:

Whitelisting map features is the general principle of work for mkgmap styles. In the style descriptions, a mapping from OpenStreetMap map features to Garmin data entities (e. g. roads, POIs) is defined where only the explicitly mapped features end up in the compiled map image.
Map images may contain several separate individual maps which are optimized for display at specific zoom levels. Depending on the zoom level, accuracy of the location information varies. On higher zoom levels, location information is stored with increasing precision, which implies a bigger number of bits per location information. This tradeoff between higher precision and smaller storage space per location information can be shifted in favour of the latter by using lower zoom levels for map images. Lower precision of the map's stored coordinates will generally lead to coarser maps and even disconnected streets after zooming into the map.
Abbreviating geographical names also helps to reduze the size of map images, although the impact is less significant than can be achived with the other strategies. In OpenStreetMap, the policy for naming map features in general is to provide the unabriviated name. Regular expressions are exploited in the map style definition to shorten (parts of) geographical names to common abbreviations, such as (German) Straße to Str, (Dutch) Plein to Pl, or (Spanish) Avenida to Av.
Restricting index creation (tbd.)

Little helpers

tools/decodimg/GarminIMG.py is a small tool that parses Garmin image files and prepares an overview of the embedded blocks together with their sizes. This is a useful debug utility that helps assess the impact of configuration changes or different version of mkgmap.