Thanks, hence why I brought it up. Puzzled me when I first discovered it. I have never been asked for the value so I never really pursued it further.
If you draw lines from the three vertices of a triangle to the midpoints of the opposite edges the intersection of those lines will be the centroid. To get the weighted average you calcualate the sum of:
Elevation at the centroid of each triangle * (Surface area of the triangle / Total area of the Surface)
14*(50/100) + 20*(50/100) = 17
The result can be affected by the orientation of the TIN. If I flip faces then I get a different answer.
i have a surface with 415 points, Civil 3D reports the mean surface elevation as 373.172'. when i extract the points, i get 373.220' as the arithmatic mean. close, but not spot on. i remember seeing here on the forums someone mention how Civil 3D calculates it, but cannot seem to find it at the moment.
*EDIT: realized after inserting the images that they may be barely readable. the images are attached.
also, in surf3.png, it may appear that all the data rows under "Position Z" column were not selected when calculating the mean, which is not the case. the mean reported is for all the Z values (415 values), which can be seen in surf3.png under the "Count", beside the "Average".
i seemed to have found it, Christopher in his blog post mentions the mean grade/slope, and how it is calculated as a weighted average (i assume weighted average over triangle area). not sure if the mean elevation is the same way:
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