Gilly,

Just to update you, took the ML in for service and told the advisor about the problem (off by 500-1000 ft). They did not believe me so he got the technician and did not believe it as well. We took a ride to my house to prove to them and they were amazed. We went back to the dealership and call the 800 number and told them about this issue. They say that "it's as good as it can get!" So now dealership say I can get a full refund if I'm not satisfied with the Nav. Told them I will let them know how to resolve this in a few days.

I guess there's nothing else I can do?