There are no grid like coordinates as the layout of the panes can be very complex - panes can be unpinned, floating, within the documentcontenthost, nested within splitpanes with various orientations, etc. If you want to traverse the panes in some specific manner considering the containing elements then you will probably need to traverse the Panes collection of the XamDockManager. When you traverse that you would then enumerate the Panes collection of each of those SplitPanes. Those objects will be either SplitPane instances (in which case you will likely want to recurse into its Panes collection), TabGroupPane instances (in which case you will want to traverse into the Items collection), ContentPane instances or ContentPanePlaceholder instances (which you can probably ignore). Similarly the Items collection of a TabGroupPane will contain either ContentPane or COntentPanePlaceholder instances.
If you just want to enumerate all the ContentPane instances within the xamDockManager then you would use the GetPanes method.