The Profile That Won't Match the Parking Lot
A 1,100-ft access road connects a public street at station 0+00 to a new parking lot entrance at station 11+00. The vertical profile uses a crest curve at station 4+50 and a sag curve at station 8+00. The parking lot entrance pad is fixed at elevation 642.30 ft by the site plan. The road profile's end elevation reads 642.30 ft in the profile view, but the corridor's computed surface at the entrance shows 641.85 ft. The alignment has a horizontal curve between 6+00 and 9+00. The profile was created by layout with tangents and curves, and the last tangent was snapped to a profile PVT at the pad. The corridor has a lane widening on the inside of the horizontal curve. A colleague says the 0.45-ft gap is just rounding in the surface display.
1. Explain why the profile end elevation and the corridor surface elevation can legitimately differ by 0.45 ft, and why rounding is an unlikely explanation.
Read the worked answer
Rounding in a surface display is typically hundredths of a foot, not 0.45 ft. A gap that size is a modeling difference, not a display artifact.
The profile controls the centerline elevation of the corridor at the alignment. The corridor surface elevation at a given point depends on where that point sits relative to the baseline and the assembly. If the assembly has a cross slope, a crown, or a lane widening, the surface at the pad's actual location (offset from centerline) will differ from the centerline profile elevation by the cross-slope drop. On a 2% cross slope, a 20-ft offset gives 0.40 ft - almost exactly the gap you are seeing.
Also check the horizontal curve: the corridor's lane widening on the inside of the curve changes the assembly's offset geometry, so the point you are sampling may not be where you think it is. And confirm the profile's last PVT is truly at 11+00 and not at a slightly different station. The 0.45 ft is the sum of a cross-slope offset and possibly a station mismatch.
2. Walk through the checks you would run, in order, to pin the discrepancy to a specific cause.
Read the worked answer
- Confirm the sample point. Query the corridor at the pad's exact station and offset, not at the centerline. Compare that to 642.30.
- Check the profile stationing. Verify the final PVT station equals 11+00 and the elevation is 642.30. A PVT at 10+98 with a 2% tangent explains a few hundredths, not 0.45 ft.
- Compute the cross-slope contribution. Multiply the pad's offset from centerline by the assembly's cross slope. If offset × slope ≈ 0.45 ft, you have found it.
- Inspect the widening region. Confirm the assembly's offset targets in the widening zone; a widened lane shifts the edge but should not change the centerline.
- Check for a vertical curve overlap. If the sag curve at 8+00 extends past 11+00, the profile elevation at the pad is on a curve, not a tangent, and the PVT snap may not be where you think.
- Compare surfaces. Build a difference surface between the corridor and a simple design surface from the profile; the divergence pattern will localize the cause.
Run them in that order because each is cheap and eliminates a class of causes before the next.
3. The pad must be exactly 642.30 ft at its actual location. What design change do you recommend, and what does it do to the rest of the profile?
Read the worked answer
If the gap is the cross-slope offset, the cleanest fix is to raise the profile end elevation by the offset amount so the pad lands at 642.30, or to flatten the cross slope near the entrance so the offset contributes less. Raising the profile is simpler but changes the last tangent grade and may push the sag curve at 8+00.
Recommend raising the final PVT to 642.75 (642.30 + 0.45) only if the offset is confirmed and constant. Then re-check the sag curve: the K-value and the curve length must still satisfy sight distance, and the higher PVT steepens the approach grade slightly. Verify the crest at 4+50 is unaffected.
If the widening or horizontal curve is contributing, fix the assembly's offset targeting first, because raising the profile would then overcorrect. Either way, re-run the corridor and confirm the pad reads 642.30 at its actual offset, not at centerline. Document the change and the reason so the site plan and road profile stay coordinated.