Convert Latitude/Longitude to Geofence for Presence

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For reasonable distances (within a city/town/state), the Haversine Formula is pretty accurate, certainly good enough for presence:

distance(lat1, lon1, lat2, lon2):
    p = pi/180
    a = 0.5 - cos((lat2-lat1)*p)/2 + cos(lat1*p) * cos(lat2*p) * (1-cos((lon2-lon1)*p))/2
    return 12742 * asin(sqrt(a))

edit: source:
https://stackoverflow.com/questions/27928/calculate-distance-between-two-latitude-longitude-points-haversine-formula