There’s an equation I’ve often found useful and have generally used it for calculating the distance between geographical coordinates. Most recently, I used the equation in a program for a 360 interactive video player to find the distance between an area that a user selected and some point of interest. Fundamentally it is an equation measuring distances on a sphere and has many uses.

I was adjusting the source code to be used in an Android application, and thought that the code might be useful to others. I am reposting it here. I tend to work in SI units, but you could use this for miles, yards, inches, or another unit if you have the radius of the sphere of interest. The constants defined in the class provide the radius of the rarth in miles, kilometers, and meters. One of these values (or your own custom value) must be passed to have the results returned to be scaled for those units.

```
class DistanceCalculator {
companion object {
public val EarthRadiusInMiles = 3956.0;
public val EarthRadiusInKilometers = 6367.0;
public val EarthRadiusInMeters = EarthRadiusInKilometers*1000;
}
fun ToRadian(`val`: Double): Double {
return `val` * (Math.PI / 180)
}
fun ToDegree(`val`: Double): Double {
return `val` * 180 / Math.PI
}
fun DiffRadian(val1: Double, val2: Double): Double {
return ToRadian(val2) - ToRadian(val1)
}
public fun CalcDistance(p1: coordinate, p2: coordinate): Double {
return CalcDistance(
p1.latitude,
p1.longitude,
p2.latitude,
p2.longitude,
EarthRadiusInKilometers
)
}
fun Bearing(p1: coordinate, p2: coordinate): Double? {
return Bearing(p1.latitude, p1.longitude, p2.latitude, p2.longitude)
}
fun Bearing(lat1: Double, lng1: Double, lat2: Double, lng2: Double): Double? {
run {
val dLat = lat2 - lat2
var dLon = lng2 - lng1
val dPhi: Double = Math.log( Math.tan(lat2 / 2 + Math.PI / 4) / Math.tan(lat1 / 2 + Math.PI / 4) )
val q: Double =
if (Math.abs(dLat) > 0) dLat / dPhi else Math.cos(lat1)
if (Math.abs(dLon) > Math.PI) {
dLon = if (dLon > 0) -(2 * Math.PI - dLon) else 2 * Math.PI + dLon
}
//var d = Math.Sqrt(dLat * dLat + q * q * dLon * dLon) * R;
return ToDegree(Math.atan2(dLon, dPhi))
}
}
public fun CalcDistance(
lat1: Double,
lng1: Double,
lat2: Double,
lng2: Double,
radius: Double
): Double {
return radius * 2 * Math.asin(
Math.min(
1.0, Math.sqrt(
Math.pow(
Math.sin(
DiffRadian(lat1, lat2) / 2.0
), 2.0
)
+ Math.cos(ToRadian(lat1)) * Math.cos(ToRadian(lat2)) * Math.pow(
Math.sin(
DiffRadian(lng1, lng2) / 2.0
), 2.0
)
)
)
)
}
}
```

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