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Comment by mikepurvis | Hacker News Reader

Vehicle GPS is usually augmented by a lot of additional sensors and assumptions, notably the speedometer, compass, and knowledge the you'll be on one of the roads marked on its map. Not to mention a fast fix because you can assume you haven't changed position since you last powered on.

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None of the inputs you mention work against multipath effects in cities, which means car GPS won't know which lane you're in and in a grid system may think you're on the next street over.

If you have an HD map you can solve for it using building shapes or by looking at the street with cameras. WiFi seems like it would help, but the locations of the WiFi terminals are themselves based on crowdsourced GPS.

Well. Some part of the 101 was moved a bunch of feet sideways after construction. Really hard to correct for, the GPS and the map localization were constantly fighting like an old couple... Had to re-map that stretch quickly...

> assume you haven't changed position since you last powered on

Sounds like a classic case of programmers ignoring corner cases: Towing, ferries, car trains, pushing the car because it broke down...

It's when you find messages in the log like "this should never happen".

You can pretty clearly use it to correct errors up to a point though. If you have a 5km difference from when the GPS was turned off, you've probably hit a corner case. If you have a 25m difference, and it's converging on the last location as you pick up satellites, snapping to the prior location is almost certainly correct.

As well as a fast fix because you know what mobile cell or wifi network you're on.

And yet, sometimes driving down a divided, limited access highway, Apple/Google/whatever maps will suddenly start giving directions from whatever parallel dirt road I happen to be driving next to. As though there's a situation where I left the highway at highway speed, crossing a ditch, crushing a fence, and possibly smashing through a guard rail, and am now traveling 65mph/100kph down a dirt road.

> Not to mention a fast fix because you can assume you haven't changed position since you last powered on.

... until you use a ferry.

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