Order of CLLocation objects in -distanceFromLocation:

Question

This is my first time posting a question here, but I have found a lot of help from other people's questions.

My question refers to the -distanceFromLocation: function in CLLocation. I had assumed that finding the distance between point A and point B would be the same as that between point B and point A, but I have found that it is not. I have tested this in a very simple case:

CLLocation *test1 = [[CLLocation alloc] initWithLatitude:37.529530 longitude:-122.259232];
CLLocation *test2 = [[CLLocation alloc] initWithLatitude:47.900002 longitude:-103.495102];
NSLog(@"%f vs %f",[test2 distanceFromLocation:test1],[test1 distanceFromLocation:test2]);
[test1 release];
[test2 release]; 

The output to this was 1907269.942754 vs 1908105.959114, a difference of almost 900 meters.

Though 900 meters may be a small percentage, I am trying to determine if the user is closer to something than an annotation and 900 meters can change that.

My question is, which one of these values, if any, is correct? Also if which location I test against is important, do I need to always test against the same location (e.g. user to destination and annotation to destination rather than destination to user and annotation to destination)?

Sorry that the sample is a bit out of context, as my project is rather large and this illustrates my problem. Any help would be appreciated as this has caused me quite a bit of confusion and frustration.

Answer 1

The error you're observing is 1 part in 2000. Presumably the algorithm used in this method is optimized for speed, so sorting a list of locations on distance is fast, rather than accurate to the millimeter.

If you need accurate results, don't use these methods but find another way. If you need reproducible results, order the arguments in a defined way, e.g. always start with the lower latitude.