Error in depth perception using Asus Xtion

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Hi,

I'm trying to understand why there is such a large error in depth perception from my Asus Xtion sensor when a flat surface is being observed. When I point the sensor at a flat wall 2m away, the pointcloud visualized in RViz and viewed from the top will be a curve instead of a line (the problem is quite noticeable). Why would that be the case? What could be the solution?

Btw. calibration/rectification does not seem to make much of a difference.

Thanks!