Garmin 547xs

[bmaskew04]

I have this GPS in my boat and I need more maps, it come programmed with everything offshore but what I'm wanting is inland rivers lakes and streams. It has a slot for a microchip. Is there a card I can purchase or do I have to buy a software and download it to a computer then transfer to my unit? I'm not real familiar with boating electronics so any help would be greatly appreciated, thanks in advance