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    Re: Smartphone level horizon
    From: Frank Reed
    Date: 2011 Jul 10, 17:35 -0700

    David,

    I've been playing around with ideas using Smartphone sensors as well. One thing to bear in mind with your level concept is that the glass on the face of the phone and the level provided by the accelerometer sensors inside the phone may not be the same. The glass screen might be tilted by a degree or even more. This should be a stable offset (you would have to test that, of course) so it's just a matter of making some observations from a known location and calculating the offset. There's a separate issue of repeatability and accuracy. I don't think these accelerometers are more accurate than a tenth of a degree. But maybe with averaging, you can do better.

    I have an idea for a sort of Smartphone sextant. You face the sea horizon with the Sun behind you. You hold up the phone and tilt it until the Sun's reflection sits aligned with the horizon. The tilt of the device as determined from the accelerometers is the Sun's altitude divided by two. A simple "app" could record the altitude every second and average over two minutes (like an old aviation bubble sextant) and then immediately calculate a line of position. I tried it with a very simple bit of software and it worked fairly well, but the error was around half a degree. I don't know if that can be improved upon or not. This error could be just the calibration problem that I described above.

    If you want to mess around with some simple programming on your Android phone, I can recommend a little product called "Basic by Mintoris". It's a Basic interpreter that includes wrappers for the device's various sensors. Nice for trying out "basic" ideas without learning the whole architecture of programming the Android implementation of Java and all that. You can write real functional programs in ten minutes. They won't be pretty and they'll be slow, but it's good fun.

    -FER
    PS: a quick technical/admin note. Your message was delayed a few days because you replied directly to a "NavList digest". Those replies go to a "noreply" email address --the name should be suggestive :). If you see something in a digest message that you want to reply to, either post from the message boards at http://fer3.com/arc or write a new email message with the relevant subject and send it off to the main NavList email posting address, which is, just as a reminder NavList[at]fer3.com.

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