Geofence dwell and boundary jitter
Use one fix to set a local 60 m circular boundary. Three inside fixes spanning at least 2 s confirm entry. No location leaves this browser.
Local geofence
No fix recorded.
Decision rule
Each sample reports distance, accuracy, fix time and age. A fix overlapping the boundary is uncertain. Only clearly inside fixes count toward dwell. A clearly outside fix resets pending entry.
Worked captures use emulated location in mobile Chromium. On a real phone choose a safe stationary centre and walk only where safe; jitter and timing will vary.
Limit: a confirmed local dwell decision does not prove someone’s exact physical presence.