Question Correct Usage of SpatialAnchors

Hey,

I am trying to implement an Hololens 2 App using SpatialAnchors but it seems that I am not quite understanding the concept right. I am able to create, save and restore SpatialAnchors of the Windows.Perception.Spatial namespace, but whenever I restore them they are placed relative to the start position of the headset.

I use this snippet to create an anchor:

Code (CSharp):

1. SpatialAnchor thisAnchor = SpatialAnchor.TryCreateRelativeTo(coord);

And this to get the relative SpatialCoordinateSystem (from this post https://github.com/microsoft/MixedRealityToolkit-Unity/discussions/10266: )

Code (CSharp):

1.  
2. public static bool UseSGIPSceneCoordinateSystem(out SpatialCoordinateSystem sGIPSceneCoordinateSystem)
3. {
4.     // gain access to the scene object
5.     SceneObserverAccessStatus accessStatus = Task.Run(RequestAccess).GetAwaiter().GetResult();
6.  
7.     if (accessStatus == SceneObserverAccessStatus.Allowed)
8.     {  
9.         Scene scene = Task.Run(GetSceneAsync).GetAwaiter().GetResult();
10.         sGIPSceneCoordinateSystem = SpatialGraphInteropPreview.CreateCoordinateSystemForNode(scene.OriginSpatialGraphNodeId);
11.         return true;
12.     }
13.     else
14.     {
15.         sGIPSceneCoordinateSystem = null;
16.         return false;
17.     }
18. }
19.  

I use this method to restore the SpatialAnchor I need:

Code (CSharp):

1.        
2. public bool GetAnchor(string id, out SpatialAnchor anchor)
3. {
4.  
5.     [...]
6.  
7.     _anchors = _anchorStore.GetAllSavedAnchors();
8.    
9.     foreach (var kvp in _anchors)
10.     {
11.         if(kvp.Key == id)
12.         {
13.             anchor = kvp.Value;
14.             return true;
15.         }
16.     }
17.  
18.     anchor = null;              
19.     return false;
20. }

And this to process the restored Anchor and get a place in the scene for it:

Code (CSharp):

1.  public void SomeOtherMethod(SpatialAnchor anchor)
2. {
3.     SpatialCoordinateSystem showcaseCoordinateSystem = anchor.CoordinateSystem;
4.  
5.     //get the reference SpatialCoordinateSystem
6.     if (!UseSGIPSceneCoordinateSystem(out SpatialCoordinateSystem referenceCoordinateSystem))
7.         return;
8.  
9.     _anchorMatrix = showcaseCoordinateSystem.TryGetTransformTo(referenceCoordinateSystem);
10.  
11.     System.Numerics.Matrix4x4 _notNullMatrix = _anchorMatrix.Value;
12.     Matrix4x4 unityAnchorMatrix = _notNullMatrix.ToUnity();
13.     anchorGameObject.transform.FromMatrix(unityAnchorMatrix);
14. }

I think that I am using the wrong SpatialCoordinateSystem but can´t find information on how to get a SpatialCoordinateSystem which the HoloLens2 generates for the physical spatial surrounding of the user that is persistent.

I am using:
Unity 2020.3.13f1

OpenXR Plugin 1.3.1

MRTK 2.7.3

MRTK-OpenXR 1.2.1

MRTK SceneUnderstanding 0.6.0

I am also confused of the amount of different SpatialAnchor-Systems, reference coordinatesystems and the documentation. Every post I find about SpatialAnchors seems to use a different approach. There seems to be SpatialAnchor-Systems for the Unity WLT, UnityEngine.VR.WSA with WorldManager, Azure Spatial Anchors,
UnityEngine.XR.WindowsMR.WindowsMREnvironment, etc.
And unless I havent overseen it, the microsoft documentation is not really clear about which one to use and how to use it right.

I would be really thankful if someone could bring some light into this issue.