Compass + Gravity
I recently discovered that my iPad has a compass, which was a bit of a surprise since I thought it would be like the iPhone where only the 3G version had it and I only got the wi-fi one. So now I’ve got a device that can tell which direction gravity pulls in (with the accelerometer) and can point to magnetic north (with the compass).
Neither sensor is terribly accurate but with a bit of filtering, I can get two reasonable vectors. And with two vectors, I can create a coordinate system. And with that, I can create a crude virtual world display. I’ll post a video next time showing what I mean…