Does anybody have any input on how accurate the internal GPS speed is at very low speeds on the Humminbird units? I've heard they are not very accurate unless going at least 2 mph? If not very accurate it would defeat the purpose of buying a new unit for that purpose for the front of my boat. I would use to troll and we all know how just .2mph in speed can make a difference in catching fish. Would I be better off to just use a handheld unit to determine my speed over depending on a fishfinder? thanks