Distance between two geolocations in java: NaN
I have this code from http://www.codereye.com/2010/10/how-to-calculate-distance-between-2.html that calculates the approximate physical distance between two IP addresses based on their respective latitude and longitude.
public static double distance(double lat1, double lon1, double lat2, double lon2)
{
double theta = lon1 - lon2;
double dist = Math.sin(deg2rad(lat1)) * Math.sin(deg2rad(lat2)) +
Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) * Math.cos(deg2rad(theta));
dist = Math.acos(dist);
dist = rad2deg(dist);
dist = dist * 60 * 1.1515;
return (dist);
}
Lat and long are signs. However, this code sometimes returns dist as NaN.
Am I missing something? Does theta have to be absolute?
UPDATE : The tested values where Nan is returned: -
lat1 = -23.5477, lon1 = -46.6358, lat2 = -23.5477, lon2 = -46.6358
lat1 = 53.3331, lon1 = -6.2489, lat2 = 53.3331, lon2 = -6.2489
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It looks like when the two input points are identical, the following expression returns a result of just over 1 (1.0000000000000002).
double dist = Math.sin(deg2rad(lat1)) * Math.sin(deg2rad(lat2)) +
Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) * Math.cos(deg2rad(theta));
This must be the result of limited precision floating point arithmetic (or limited precision trigonometric functions).
Math.acos(dist)
returns NaN if dist
> 1. You can overcome this problem simply by changing
dist = Math.acos(dist);
to
dist = Math.acos(Math.min(dist,1));
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