Thanks for adding that link Caty. I should have included it, however although i do find the 'clash of titans' opinions in it amusing because they are all basically saying the same thing in different ways to describe the outcome thereby probably confusing any layman-reader in the process. All the OP there wanted to know was 'how is a gps tunnel aware' and does it use the light sensor to switch to night mode in the tunnel. If Giomen had simply said that the gps is aware from the map that a tunnel is being entered and then uses simple dead reckoning [linear interpolation] in his answer rather than explain in detail the thread might not have existed beyond two posts, lol! Cnfhbr in Post #4 there hits the nail on the head by explaining that the unit's interpolation mode takes over when activated by the map's IS_TUNNEL attribute.
Dead [also Ded as short for deduced] Reckoning in it's most basic form assumes that an object traveling in a certain direction [heading] at a known constant speed will continue to do so if no corrections are made and therefore we can deduce/predict where it will be in a given time by calculating the distance traveled from the speed. Or in reverse for our 'tunnel aware' nuvi, how long it will take to reach a known point at that speed. Entering an 4km long straight tunnel at 60kph at 8.11 am will predict an exit at 8.15am [60kph being 1 kilometre per minute].
