obtain z value from xy location in 3d model

hi all,

i have a 3d model (any model) (any format - lets say obj).
what i want to do is find the z value at particular xy value in the 3d model.
my ultimate goal is to establish a relation between 3d model and its 2d projection image.

so my question is how to find the depth value/ z value.
( i am only interested on surface of 3d model so there won't be multiple z values and in front of the camera/view)

can anybdy help me or give me a link to something related.

my problem is i don't know what i am searching for so google doesn't helps

thx guys