How to calculate distance between two points with esri.geometry?

Question

The function http://help.arcgis.com/en/webapi/javascript/arcgis/jsapi/#namespace_geometry/esri.geometry.getLength should calculate distances between points.

When I am trying to calculate distance between points having 100 meters between them

p1=new esri.geometry.Point(3997030.6690969253, 7444299.320646087, new esri.SpatialReference({ wkid: 102113 }));
Object
p2=new esri.geometry.Point(3996951.455397143, 7444142.154020177, new esri.SpatialReference({ wkid: 102113 }));
Object
esri.geometry.getLength(p1, p2)
176.00045037719127

I am getting 176 which is wrong. Projection is Web Mercator (WKID 102113).

UPDATE

@Krystian found that getLength uses just Pythagoras theorem to calculate length. So it gives length in projection meters. How to obtain distance in real meters?

Answer 1

Web Mercator Auxiliary Sphere is WKID 102100

http://resources.arcgis.com/en/help/arcgisonline-content/index.html#//011q00000002000000

var polyline = {
    "paths":[[[3997030.6690969253, 7444299.320646087], [3996951.455397143, 7444142.154020177]]],
    "spatialReference":{"wkid":102100}
};
console.log(JSON.stringify(polyline));
polyline = new esri.geometry.Polyline(polyline);
console.log(JSON.stringify(polyline));
polyline = esri.geometry.webMercatorToGeographic(polyline);
console.log(JSON.stringify(polyline));
console.log(esri.geometry.geodesicLengths([polyline], esri.Units.METERS));
//output 99.93917832865446

6.1 cm short, good enough for govment work?

Answer 2

I just checked it.

esri.geometry.getLength(p1, p2) is counting distance like this:

length = sqrt((p2_X - p1_X)^2 + (p2_Y - P1_Y)^2)

so it is distance between two points in straight line. If you want to count distance between two points on geodetic line you should use geometry service as in this example