Great circle distance in Cocoa

great-circle-cocoa.m

static NSInteger STEarthRadiusKilometers = 6371;

- (NSNumber *)distanceFromLatitude:(NSDecimalNumber *)inLatitude longitude:(NSDecimalNumber *)inLongitude;
{
    CLLocationDegrees inLatitudeDegrees = [inLatitude doubleValue];
    CLLocationDegrees latitudeDegrees = [self.latitude doubleValue];
    CLLocationDegrees inLongitudeDegrees = [inLongitude doubleValue];
    CLLocationDegrees longitudeDegrees = [self.longitude doubleValue];
    
    double distance = acos(sin(inLatitudeDegrees) * sin(latitudeDegrees) + cos(inLatitudeDegrees) * cos(latitudeDegrees) * cos(longitudeDegrees - inLongitudeDegrees)) * STEarthRadiusKilometers;
    
    return [NSNumber numberWithDouble:distance];
}