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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):

Example of image perspective

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?

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What you see if the effect of the camera's central projection (the image is projected through the camera's focal point). This means that there is a radial distortion where protruding objects in the images appear to lean away from the center. This is direct effect of object height and the distance from image nadir (center).

The production of so-called "true orthos", where this is corrected for, is possible, but demanding. First, you would need a detailed 3D surface model of any building or object as a basis for the orthorectification, instead of a terrain model as is used here. Secondly, you will need aerial images of the area from different angles, so that you will have imagery of the "blind zone" behind the building. If the image georeferencing, the surface model and teh image mosaicing process are good enough, the result will be an orthorectified image without any building distortions.

Can it be done on any existing orthoimages? Not without leaving gaping holes in the blind zones behind the buildings/objects.

  • So, in my example above; which one of the two pair of coordinates corresponds truly with the lower-right corner of the building? the 'ground' one or the 'roof' one or something in the middle? – Web-GIS entrepreneur Feb 1 '16 at 12:12
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    The one on the ground. Since the buildings are not corrected, you could assume that the orthorectification is based on a terrain model (i.e. ground elevation), and all xy coordinates woudl refer to the ground. – HavardMoe Feb 1 '16 at 12:13

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