Thanks Tom,
In the QS help:
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Maximize Surface Operations
Mathematical surface operations function by draping the elements of each surface onto the other, then processing the resulting pairs of Z values. This bi-directional draping is controlled by the Maximize Surface Operations checkbox. By default, this option is enabled, causing the bi-directional drape. It is a good idea to leave it enabled unless you have a specific reason to do otherwise.
For normal use, leave the Maximize surface operations button enabled.
If you deselect the Maximize Surface Operations checkbox, a mathematical surface operation drapes the first surface onto the second surface only. This means the resulting parts and the plan view geometry of the results < . > surface is based solely on the geometry of the first surface. If the second surface only has points, then point to point operations only are performed.
Consider the example of two surfaces overlying one another: one with 1000 points and a TIN, and the other with 100 points and a TIN. With Maximize disabled, the resulting surface will have different numbers of points depending upon which surface is the first surface in the surface operation. If the first surface is the 1000 point surface, the result will have 1000 points (assuming the surfaces overlap perfectly). If the first surface is the 100 point surface, the result will have 100 points (assuming the surfaces overlap perfectly). The plan geometry of the result reflects the first surface only. By contrast, if Maximize is enabled, the resulting surface will have 1100 points (assuming no points are coincident in XY).
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This is what I have experienced in my 20 years of use of QS. I have only noticed this recently with the 2013 release and have worked around the issue with external manipulations to get the surfaces that I want.
Maybe I am just missing something but if I have 2 non-coincident surfaces and I add (+) them in the surface operations dialog I expect to see all of the points in the resultant surface.
Can you verify that you experience the expected result?
Thanks,
Joel