I do something similar to include the British National Grid lines as an overlay on the OSM map.

Rather than creating a gmapsupp file from your tracks, generate img files. Then use another run of mkgmap to combine these with the img files for your main map into one gmapsupp. This works fine for me.

Roger

On 05/08/2012 10:21, Andreas B. Mundt wrote:
Hi,

I am trying to create a transparent garmin gmapsupp.img layer from gpx
tracks.  The idea is to merge this layer (it contains special cycle
routes) in the end with another gmapsupp.img containing the map.

I did the following:

Load all tracks in josm, convert to a data layer and tag the tracks
highway=cycleway.  Then save as paths.osm.

Use the command

    mkgmap --gmapsupp --transparent --remove-short-arcs --style-file=XXXX  paths.osm

to convert the data to a gmapsupp.img file.  As style file I use the
stuff that is used to create the map containing all other data.

However, the gmapsupp.img created from the tracks looks like empty if
loaded onto the GPS device, nothing shows up, as if the map covers no
area at all.

What's wrong with my approach?  How can that be done?

Many thanks in advance,

     Andi
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Roger Calvert