Forget all the below, and use Qlandkarte GT instead
Just note, you are using a rubbish OS (at least as rubbish as Windows, but less people use it, so I won't help). Best make a good present to your Apple hardware and treat it with a good OS like Ubuntu Linux.
Else note, you can use Qlandkarte GT for viewing the maps without converting them. The only drawback is, that Qlandkarte GT does not support autorouting with garmin format maps (it does allow to use autorouting via online routers). Oh and don't forget to keep Qlandkarte GT up to date, it is getting better quickly.
Don't convert but use gmapsupp.img import by Basecamp
The irony of Basecamp for MacOSx not reading in the standard .img format, is topped up, by the fact that it reads the nearly identically structured gmapsupp.img files.
So use gmaptool or mkgmap or sendmap to create a gmapsupp.img – put it on a external harddisk or flash memory (you could also put it onto your GPS, but than it's slower) and import it into Basecamp. This way you also don't have to actually convert the maps.
Convert the maps with gmapibuilder
wiki.openstreetmap.org/wiki/Gmapibuilder
See also the following post by GP4Flo: openmtbmap.org/tutorials/convert-maps-for-mac-os-x-roadtrip/comment-page-1/#comment-2014
Convert the maps yourself under Windows with tools provided by Garmin:
Unfortunately you need an Windows installation for converting the maps. Principally you have to:
1. Install the maps with install.bat under Windows (You have to have Mapsource installed)
2. Downlaod, install and run Garmin Map Converter for Windows
3. Downlaod and install the Mapstall/MapManager package to your Mac to complete the migration Garmin provides detailed instructions how to do this http://www8.garmin.com/support/download_details.jsp?id=3897 See also http://www8.garmin.com/macosx/index.jsp
Alternatively you can use Sendmap to transfer the maps to your GPS (see Tutorial (still to be written))
You could also install Qlandkarte GT – you will have to compile it first though. Builds are only available for Windows (plus easy installation on Ubuntu – but probabley added into repos soon anyhow).
BTW: As all my maps are published under CCBYSA 2.0 / The new Openstreetmap Licence – you may share your mac map files freely – as long as you contribute openstreetmap.org and openmtbmap.org and stick to the same license.
Make maps compatible with Basecamp:
This is not needed anymore as ov Basecamp v3. I strongly recommend to use openmtbmaps with Basecamp v3, and not Roadtrip which is neither stable nor correct in its map rendering.
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