Wow! What's up with that? Are the engineers or surveyors afraid to give up the data? The challenge is going to be that we will not be able to reproduce EXACTLY those contours. The thing is C3D is using an algorithm to create contour lines from the data we gave it, this data is NEVER going to be the same as the data that created those contours we have a few spot elevations, however we do not have proper breaklines. In short we do not have enough data to recreate that surface.
At some point you (we) will have to accept what the data we were given produces and go forward with the understanding that our volumes will be off slightly as the model will never be completely accurate. Unless, you can get the DTM, ask the owner to press the surveyor for this information he did pay for it and it will increase accuracy. Or you go get the original building plans and recreate the FG model from their plan using feature lines.