The iPhone 3GS's compass can be messed up by nearby magnetic fields -- apparently London is built on a lump of lead, because we often find that the phone makes pleas for compass calibration, and Nearest Tube is no different.
The message usually disappears after a few seconds, however, even if we didn't do the figure-eight manoeuvre that it requested, and the app still worked when the error was visible.
One thing to note is that all this GPS and compass orienteering exhausted our iPhone, eating 20 per cent of the battery life during an hour of testing time. It's not an app that needs to run for hours though, since we only needed to check it once or twice to keep us on course to the Tube. For London types, we think it's a useful app that also offers a tantalising whiff of augmented realities to come.
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