Problems converting to Lat Lng with the xyToLngLat method

Question

Hi!
I'm a newbie to coordinates, so any help with my question is much appreciated. 🙂

I???m having trouble converting X Y coordinates (RT90) to lat and lng (I think it???s called WGS84?). I'm using an API that returns X and Y coordinates and I get quite strange results when calling the esri.geometry.xyToLngLat(long,lat) method.

For example when I pass  X= 6570898 Y = 1630111  (a bit south of Stockholm, Sweden) the method should return lat: 59.239121,  lng: 18,085032. But instead the esri.geometry.xyToLngLat method returns lat: 59.02738103790395 lng: 14.486671705724888.

Which is very very far west of Stockholm Sweden???..

As I said, I???m a newbie. What I???m I doing wrong? 🙂
Any help, much appreciated!

Kind regards
/Erik

ErikEklund1 · Answer

Ken,
I really appreciate your help and effort! 
I understand most of your answer but I'm kind of stuck on a big thing....

How would I pass my X Y coordinates to the function projectToWebMercator?

For instance a real simple example would be, how would I pass my previously mentioned X= 6570898 Y = 1630111 to the function?

Do I somehow push them into the mapPoint object? Sorry but I just cant figure it out.

But I'm really glad there is people like you who help. 
Because one learns alot!

function projectToWebMercator(evt) {
             var point = evt.mapPoint;

Best
Erik

kenbuja · Answer

The xyToLngLat method will only correctly convert input coordinates in Web Mercator to WGS84. To convert coordinates in any other projection, you'll have to use a Geometry Service. Here's a modification of the Project a Point sample that uses the WKID 3021 Project a point Click a location on the map to Project from LatLng -> Web Mercator: