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It should be possible to get GPS accuracy across multiple phones to ~20m in open good conditions, with spikes up to 30m in worse conditions (e.g. under tree cover). It is unlikely we can do much better than that.
Therefore our assumption that a “contact” should be determined by a GPS distance of 20m might be based on unreasonable assumptions about GPS accuracy.
Some phones have GPS disabled by default, and do location detection through WiFi only by default. It is posisble for an App to detect this and direct the user to change their settings. We should consider adding this function. We should also experiment with a range of popular phones to determine what the common sets of options & defaults are.
GPS background logging is frequently killed by battery saver options on phones. It is not possible for an App to stop this, but it is possible to detect that it has happened and warn the user. Again, we should consider adding this function, and testing with popular phones to determine what the common sets of options & defaults are.
Part 3 - Stationary Phone
FInal test was a stationary phone, WiFi off. I left it for 35 mins, with GPS pings every 3 seconds.
It never varied more than 2m away from the start point.
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Compared to the wild variatons of 50m we were seeing with Safe Paths, this is a much much bettter performance.