function calcCrow(lat1, lon1, lat2, lon2) {
    var R = 6371 // km
    var dLat = toRad(lat2 - lat1)
    var dLon = toRad(lon2 - lon1)
    var lat1 = toRad(lat1)
    var lat2 = toRad(lat2)

    var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
        Math.sin(dLon / 2) * Math.sin(dLon / 2) * Math.cos(lat1) * Math.cos(lat2)
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))
    var d = R * c
    return d
}

// Converts numeric degrees to radians
function toRad(Value) {
    return Value * Math.PI / 180
}