return new lat/lon after moving east/north
    by the given number of meters

def gps_offset(lat, lon, east, north):
    '''return new lat/lon after moving east/north
    by the given number of meters'''
    bearing = math.degrees(math.atan2(east, north))
    distance = math.sqrt(east**2 + north**2)
    return gps_newpos(lat, lon, bearing, distance)