May I say, if you care about 100 meters you should look for GPS accurate lock only. Issue being, even with "20 meters", subsequent positions jump around by 50 meters under normal circumstances.

Using the positioning desktop applet I can see it wiggle. Not to mention GPS has about 10 meters under ideal circumstances, add real life and it goes dancing for 20-50 meters.

If you want this kind of accuracy, use GPS perfect lock, but what if it isn't available? Smallest radius? For how long? Is it worth having GPS working a minute for a relative improvement of basically within tolerance of a lock?

IMO, lock mode should be Tower, GPS, Tower+GPS and GPS+tower.

In Tower+GPS, it tries tower. If tower is good, hurray.
In GPS+Tower, it tries tower. Then GPS. If GPS fails or is worse than tower, use tower.

Idea being that one does high precision or bust, the other is best try. Then there's battery-saving tower and battery-eating GPS.

Alternately, have Tower and GPS ckeckboxes, and another chack to "use best guess".

If tower, try tower. If GPS, try GPS. If use best guess, it uses the best accuracy it could. Not if it's completely off, though, with a limit like it is now. Ideally, GPS could have a manual timeout setting for each. Me, e.g., I'd keep GPS on, 10 seconds (either I'm out or not), tower and use best guess if under 1000 meters.

So, check box GPS, check Tower, Lock time limit and accuracy limit edit boxes. Should cover all situations thus far?