For that, there is the TransformToAncestor method:
Point relativePoint = myVisual.TransformToAncestor(rootVisual)
.Transform(new Point(0, 0));
Where myVisual is the element you want to get the position of, and rootVisual is Application.Current.MainWindow or whatever you want the position relative to.
reference link:
Get Absolute Position of element within the window in wpf