Calculating distance between two points, using latitude longitude?

Future readers who stumble upon this SOF article.

Obviously, the question was asked in 2010 and its now 2019. But it comes up early in an internet search. The original question does not discount use of third-party-library (when I wrote this answer).

public double calculateDistanceInMeters(double lat1, double long1, double lat2,
                                     double long2) {


    double dist = org.apache.lucene.util.SloppyMath.haversinMeters(lat1, long1, lat2, long2);
    return dist;
}

and

<dependency>
  <groupId>org.apache.lucene</groupId>
  <artifactId>lucene-spatial</artifactId>
  <version>8.2.0</version>
</dependency>

https://mvnrepository.com/artifact/org.apache.lucene/lucene-spatial/8.2.0

Please read documentation about "SloppyMath" before diving in!

https://lucene.apache.org/core/8_2_0/core/org/apache/lucene/util/SloppyMath.html

Note: this solution only works for short distances.

I tried to use dommer's posted formula for an application and found it did well for long distances but in my data I was using all very short distances, and dommer's post did very poorly. I needed speed, and the more complex geo calcs worked well but were too slow. So, in the case that you need speed and all the calculations you're making are short (maybe < 100m or so). I found this little approximation to work great. it assumes the world is flat mind you, so don't use it for long distances, it works by approximating the distance of a single Latitude and Longitude at the given Latitude and returning the Pythagorean distance in meters.

public class FlatEarthDist {
    //returns distance in meters
    public static double distance(double lat1, double lng1, 
                                      double lat2, double lng2){
     double a = (lat1-lat2)*FlatEarthDist.distPerLat(lat1);
     double b = (lng1-lng2)*FlatEarthDist.distPerLng(lat1);
     return Math.sqrt(a*a+b*b);
    }

    private static double distPerLng(double lat){
      return 0.0003121092*Math.pow(lat, 4)
             +0.0101182384*Math.pow(lat, 3)
                 -17.2385140059*lat*lat
             +5.5485277537*lat+111301.967182595;
    }

    private static double distPerLat(double lat){
            return -0.000000487305676*Math.pow(lat, 4)
                -0.0033668574*Math.pow(lat, 3)
                +0.4601181791*lat*lat
                -1.4558127346*lat+110579.25662316;
    }
}

was a lot of great answers provided however I found some performance shortcomings, so let me offer a version with performance in mind. Every constant is precalculated and x,y variables are introduced to avoid calculating the same value twice. Hope it helps

    private static final double r2d = 180.0D / 3.141592653589793D;
    private static final double d2r = 3.141592653589793D / 180.0D;
    private static final double d2km = 111189.57696D * r2d;
    public static double meters(double lt1, double ln1, double lt2, double ln2) {
        double x = lt1 * d2r;
        double y = lt2 * d2r;
        return Math.acos( Math.sin(x) * Math.sin(y) + Math.cos(x) * Math.cos(y) * Math.cos(d2r * (ln1 - ln2))) * d2km;
    }

Slightly upgraded answer from @David George:

public static double distance(double lat1, double lat2, double lon1,
                              double lon2, double el1, double el2) {

    final int R = 6371; // Radius of the earth

    double latDistance = Math.toRadians(lat2 - lat1);
    double lonDistance = Math.toRadians(lon2 - lon1);
    double a = Math.sin(latDistance / 2) * Math.sin(latDistance / 2)
            + Math.cos(Math.toRadians(lat1)) * Math.cos(Math.toRadians(lat2))
            * Math.sin(lonDistance / 2) * Math.sin(lonDistance / 2);
    double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    double distance = R * c * 1000; // convert to meters

    double height = el1 - el2;

    distance = Math.pow(distance, 2) + Math.pow(height, 2);

    return Math.sqrt(distance);
}

public static double distanceBetweenLocations(Location l1, Location l2) {
    if(l1.hasAltitude() && l2.hasAltitude()) {
        return distance(l1.getLatitude(), l2.getLatitude(), l1.getLongitude(), l2.getLongitude(), l1.getAltitude(), l2.getAltitude());
    }
    return l1.distanceTo(l2);
}

distance function is the same, but I've created I small wrapper function, which takes 2 Location objects. Thanks to this, I only use distance function if both of locations actually have altitude, because sometimes they don't. And it can lead to strange results (if location doesn't know its altitude 0 will be returned). In this case, I fall back to classic distanceTo function.