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[mkgmap-dev] Merge tiles to gmapsupp based on poly files

From Daniel Vogelbacher daniel at vogelbacher.name on Thu Jul 14 19:29:43 BST 2022

Hi all,

the regular workflow for building maps is something like:

1. get osm pbf data for the region
2. split data using splitter.jar into rectangular tiles
3. build each tile
4. merge all map tiles into single gmapsupp.img

But from my understanding, this raises issues on region boundaries when 
multiple regions (multiple gmapsuppXYZ.img files) are used on device.

The data for an extract is usually defined by a .poly file, so 
everything outside of this polygon is not included. But splitter.jar 
splits not at polygon bounds but by rectangles of different sizes. So 
some tiles for a map contains not the full osm data for this rectangle 
because the data was already cut off during osm data extraction.

For example, building the map for Germany and France by two separate osm 
extracts leads to missing data on the polygon bounds for these tiles. 
And I assume this breaks routing across maps.

A better approach would be to split and build from the whole world (or 
at least Europe), then merge all tiles for Germany into gmap_germany.img 
and all tiles from France into gmap_france.img. The tiles on country 
borders must be included in both .img files but because they have the 
same map-id, the device should be able to take care of duplicated map tiles.
The area of tiles can be defined by the same .poly file used in the 
regular workflow, but now as a selector for tiles instead of osm data.

So because I don't see anything in the docs mentioning this workflow, I 
assume this is not how it works :-)

What is the recommended workflow to build multiple maps and providing 
proper routing across multiple gmapsupp.img - or is this just not 
possible with Garmin devices?

Best regards / Mit freundlichen Grüßen
Daniel Vogelbacher

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