How much detail are you looking for? I have 3 different map chips, all 3 have different amounts detail. That is one reason I loaned one out, it had 1 ft contours, WAY to many lines on the screen for me. And they are always off a bit as the water fluctuates, the tides/currents change the bottom over the years so you really only use them for general info. They are quite close, but if your looking for exact detail, they are typically off up to 30 ft ( 15ft radius)or sometimes more, depending so for me real tight detail doesn't work. I know there are a lot of guys who love all those lines on the screen so think about what your requirements are and go from there. I have a couple friends who bought the chips for their HDS units and went back to the pre-loaded maps.