Originally Posted by willcasp
The 478 was a marine unit. The 640 is the replacement for it, and for marine functionality, it is pretty much a feature match.
Not so for on road use though. IIRC the feature set is reduced quite a bit once you're out of marine mode. I think there's a track feature or two missing as well. The whole line from the 276C and it's predecessors on were chartplotters or marine units.