Progressive LOD terrain page loading

Planning out asynchronous terrain paging/loading scheme across nested hierarchical grids (somewhat like a quadtree) of varying LODs (level-of-details). For further terrain pieces away from the camera position (denoted by mouse position over stage), lower resolutions are used by having them cover a much larger area.

By right, there's supposed to be a global low resolution heightmap that is always displayed at the back for a generic starting LOD which will always be shown before higher LODs are loaded. This will be done in the actual 3d terrain implementation so at least some (low-detail) terrain is shown before building it up gradually at the relevant nearby areas if required (on-demand).

But anyway, I'm testing with Google maps images as a concept proof first to ensure the loading sequence is correct.