Slanted DEM

[CO]

Hi All

I am new to PhotoScan and am using the Standard Edition. I have been working on a couple of trial projects, and am having a problem with a slanted DEM generated from my models. The objects I am modeling are flat with low relief patterns (2mm to 1 cm deep ridges or grooves). One is a small piece of tile with 2-3mm ridges sitting on a level table. The models seem to be working (I get the detail of the patterns), but an issue I am having is that there is an overall slant to the DEM produced (one end higher than the other), when there should not be.  Is there a way in Photoscan Standard Edition that I can set the reference plane to zero so the model does not show as slanted?  I am interested in getting a depth map / DEM to measure the height or depth of the ridges and grooves (I create the DEM in ArcGIS).

Also, wondering why this is occurring. Is it a result of how the photo's are aligning?

Any help is greatly appreciated.
Thanks