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I suppose this isn't exactly a new feature but it is new to me since I don't travel through tunnels very often. What exactly is the software using when it decides to switch over to night mode when in a tunnel? GPS Location? A light sensor? A combination of both?

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simply from rich google map data. – yrajabi Sep 30 '12 at 20:39
I don't think the map data on its own explains it--I travel all the time on the roads directly above without it switching. But yes, you couldn't do it with poor map data...just ask Apple! – Jay Oct 1 '12 at 0:34
So, I think it uses also altitude value from GPS to determine whether you are in the tunnel or the road above it! As in its new feature, In-door navigation through some buildings, It uses altitude to determine in which floor you are (I'm not sure!). – yrajabi Oct 1 '12 at 9:00
The last time I used the in-building navigation, you had to manually select the floor. I don't think it is using altitude, either that or I never have a lock on enough sats to derive an altitude value. I park below I-95 and I'm always being routed directly from I-95 instead of the city streets that are underneath it (where I start). I guess thats why I initially assumed altitude wasn't used. – Jay Oct 1 '12 at 12:01

Hard to say without knowing which device and firmware version we're talking about, so what follows is my speculation.

I have a Garmin Nuvi which continues navigating in a tunnel, turns and all, despite losing its satellite reception. Mine doesn't switch to night mode, but it clearly knows it is in a tunnel from the map data. Since loss of reception is a given in a tunnel, the device must be dead-reckoning while it's in there. (Re-broadcasting is out because it would affect the signal timing which is key to calculating a fix). In fact, I have tested this by entering a tunnel fast and decelerating once inside it. The device alerted that it had lost reception and continued to display my entry speed, "passing" turnouts and the tunnel exit before I did.

From that, I inferred that the firmware was using map information, possibly cross-checked with loss of satellite lock, to know when I've entered a tunnel.

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Hadn't thought about the satellite lock. So would it switch to night mode even if the tunnel was lit (I don't have a lighted nearby)? I guess I could put a flashlight on it. – Jay Oct 1 '12 at 0:40

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