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Type: Question
Resolution: Done
Priority: Major - P3
Fix Version/s: None
Affects Version/s: None
Component/s: Geo
Labels:
- geoNear

we can't seem to figure out the calculation mongodb uses when it returns the distance ("dis") key for a geoNear command

we need this distance, but we want to use the $nearSphere find query instead of the geoNear command
since the find query doesn't return the calculated distance, we have to calculate it on our own
the problem is that any formula we use to calculate the distance doesn't match the distances calculated by mongodb
the manually calculated distance is very close to the one returned by geoNear, but never exactly the same (they're not even all linearly offset, so it would mean a different order of documents)

these are the formulas we tried so far, which all return the same results (among themselves), but none match mongodb's geoNear distance numbers:

private static $cf_earth_km = 40074;
$theta = $lng1 - $lng2;
$dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
$dist = acos($dist);
$dist = rad2deg($dist);
$km = self::degree2km($dist);

public static function degree2km($degree, $precision = false) {
	$km = $degree * self::$cf_earth_km / 360;
	return $precision ? floatval(sprintf('%.' . intval($precision) . 'f', $km)) : $km;
}

private static $radius_earth_km = 6378;

$pi80 = M_PI / 180;
$lat1 *= $pi80;
$lng1 *= $pi80;
$lat2 *= $pi80;
$lng2 *= $pi80;

$dlat = $lat2 - $lat1;
$dlng = $lng2 - $lng1;
$a = sin($dlat / 2) * sin($dlat / 2) + cos($lat1) * cos($lat2) * sin($dlng / 2) * sin($dlng / 2);
$c = 2 * atan2(sqrt($a), sqrt(1 - $a));
$km = self::$radius_earth_km * $c;

so the question is, how does mongodb calculate the distance returned by geoNear?

looking at mongodb source code, i think i successfully traced the actual calculation to a S2LatLng::GetDistance function (http://code.google.com/p/s2-geometry-library/source/browse/geometry/s2latlng.cc?r=c872048da5d12db6058022e73f9c5dcbfdd93079), but it's basically the same as the first custom calculation function we tried

Assignee:: hari.khalsa@10gen.com

Reporter:: Nemanja Dubravac

Participants:: hari.khalsa@10gen.com, Nemanja Dubravac

Votes:: 0 Vote for this issue

Watchers:: 4 Start watching this issue

Created:: Nov 30 2012 10:49:03 AM UTC

Updated:: Jul 11 2016 05:59:25 PM UTC

Resolved:: Jan 07 2013 03:18:01 PM UTC

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