Hello,
To clarify:
1. I am looking for a iMac-based open-sourced program to replace Garmin's MapSource function.
2. Garmin MapSource is only available for Windows-based PC, not Mac.
3. Garmin stopped supporting MapSource with their last update v.16.3 on October 25, 2010, at which time MapSource was replaced with BaseCamp software.
4. I have two Garmin Zumo 660LM gps devices that were built to be compatible with MapSource software (not BaseCamp). MapSource interfaces with City Navigator North America maps allowing users to create custom routes on a PC that can be transferred to the Zumo 660LM device.
5. MapSource is the only recommended software to be used with Zumo 660LM (not BaseCamp). Zumo 660LM devices are NOT compatible with Garmin's BaseCamp, a fact Garmin has reluctantly confirmed.