You can get the coordinates of a System.Windows.UIElement relative to any other UIElement. Probably the most common scenario is to get the coordinates of a child element so this is our example.
C#
GeneralTransform generalTransform = childElement.TransformToVisual( parentElement );
Point childToParentCoordinates = generalTransform.Transform( new Point( 0, 0 ) );
Note! If the child is in left of the parent element or above it, the coordinates will be negative.
That's it!