DEM with trend evident from centre of image

[ingrambr]

Hi,

I've been working with the DEM generation part of photoscan and I always seem to have the same problem.  There seems to be a circular trend.  In the image that I've attached, I show 4 images of the same scene but with every advancing image (Top-Left -> Top-Right -> Bottom Left -> Bottom Right) I increase the colourbar range.  As you can see each image is circular in nature.  This doesn't reflect the underlying terrain.  It's almost like the DEM is being wrapped around a sphere.

Does anybody have any intuition as to why this happens?  Is there anyway to avoid this or to remove the effect? I've tried UTM projections, more densely sampled imagery, using ground control points... I constantly have this same pattern.

many thanks.