All available background tiles I know about on the internet (OpenLayers, Bing, Google) are tilted. See a random example below from OpenLayers (you can see the east side of the buildings):
In essence, when you move the mouse from left to right along the same building, the values of the XY coordinates change, how is that?, should not the coordinates remain the same while you are on the same 2-D position?. Obviously OpenLayers does not correct that, neither the others, am I right?
Is there any way of taking this effect into account? Basically what I want to do is overlapping a true vertical digital terrain model (DEM) with aerial tiles on top. Should I worry about this distorsion?