On a Mac.... The transparent files seem interesting. Downloaded one and now I am trying to figure how to get it to overlay another TOPO map. No success. Must be a way to do this.
This only works on your GPSr, not in BaseCamp.
https://forums.garmin.com/showthread.php?t=17533&page=3&highlight=transparent
This thread is requesting basecamp be modified to allow more than one map displayed at a time. Its about Windows, but if they do it on Windows they should also do it on MAC
The more people who ask for this, the more likely they will provide it.