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Comment by thrownaway2424 | Hacker News Reader

What's neat here is how clean their elevation data seems to be. Having spent a long time trying to work with various dataset like the NASA SRTM and USGS files, I quickly came to realize how hard it is to clean up this data. Looking at this feature I tried out a couple of things that would have tripped up a naive implementation and Google Maps seems to have dodged the most obvious pitfalls. For example a bike route that crosses a freeway in a trench correctly shows the route without discontinuities. It also seems like Google are getting around some of the problems just by being intentionally vague and blurry. If you make a route short enough it will just say "mostly flat" instead of trying to draw a misleadingly detailed profile.

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They're almost certainly using barometer data from Android devices to help know elevations and differences between roads. The barometers can measure altitude differences as small as ~0.5m - it's pretty amazing.

That's interesting. What kind of datastream do you get from that sensor? Something that has a useful ∆Z over short intervals but essentially no absolute accuracy?

You can check it out yourself if you like - I provide a live feed of barometric data from Android sensors at http://pressurenet.io/. The data I collect is polled every 10 minutes, and while the exact pressure is often nearly useless due to noise, trends in the data are very clear. I'm looking specifically at the weather, so over periods of ~6 hours we get really really clear trend lines.

When a device moves rapidly in a short period of time, you know that the sensor data will be reflecting the altitude changes and not the weather changes. Moving the device from my feet to my head shows a clear change of ~.5mb typically.

Didn't you also have issues with pressure differences inside, outside and between buildings in dense metro areas?

it looks clean, but from the few routes i checked it might be clean because it's been fuzzed beyond the point of usefulness. it's got no scale on it so i can't be sure what exactly is being showed, but there's a couple major hills near my house (and i mean major, i live in the mountains) that don't even register.

It also doesn't work for some of the major bridges that I ride over.

Google already has lots of people working on Maps to make the data better, correct mistakes, etc. They almost certainly have data about bridges and tunnels they can use here to make sure that you get a straight line for them instead of following the terrain profile.

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