I was going to buy the gamin touring plus, but found a used gamin 810. From what I read it does everything the touring plus does except for inventing round trip rides for you, from where you are and putting in how many miles you want to ride. And it comes preloaded with city maps, the 810 has to use a SD card, at added cost but does far more. Mine unfortunately is without that SD card. My question is if you buy the SD card from garmin, is it updateable? Can I find a used SD card? How does this all work?