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The Site Zoning Vignette. Should I Interpolate the Grade Profile?

The short answer is, no. More specifically, you won’t have to deal with it on the exam. However, here’s the full explanation.


On NCARB’s base drawing for the Site Zoning vignette in ARE 3.1, contour lines 125’ and 120’ intersected section cut line A-A in a way that caused confusion. We know, in reality, that the grade profile in detail 1 rises above 125’ somewhat and the grade profile in detail 2 dips below 120’ somewhat (below left). The exact amount that the grade profile should be drawn above and below the contour elevation is not known. So, where does NCARB want the profile to be drawn? Professor Dorf felt that NCARB wanted to know if candidates really understood the ridge and valley in the profile. Therefore, he felt grade interpolation was appropriate in this situation.

Site Zoning

Apparently NCARB found that there was enough uncertainty regarding this condition that in ARE 4.0 they changed how the contour lines intersect the section cut line (above right). Contour lines 125’ and 120’ now just touch section line A-A at the apex of their curves. This change effectively eliminated the issue of interpolation because each point on the grade profile is known exactly.


In the unlikely event that on the exam you are presented with a base drawing in which grade interpolation is an issue, our position is that the grade profile should be drawn horizontally between the two points, with no interpolation.


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