Update: I received and installed the unit. It appear that the GPSMAP 700 series doe not support multiple .img files. Whatever I put on the SD card, it only load one supplemental map files, i.e. the one named gmapsupp.img so, I haven't messed with merging different mapsets onto one .img file because of the potential drawing priority problems outlined by ddabcd277, and simply put my various mapsets on different SD cards. Work great!