Horseshoe Curve PA: 1854 cut and fill, lidar DEM into Civil 3D
The Horseshoe Curve west of Altoona, PA is an 1854 cut and fill that reads as one clean bench in bare-earth lidar, so it's a good existing-ground surface to try. It was built with picks and shovels, horses and drags, and opened February 15, 1854. J. Edgar Thomson designed it for the Pennsylvania Railroad to ease the westbound grade to the Allegheny summit. The crews put an earth fill over Kittanning Run, cut through the mountain point between the two ravines, then filled Glenwhite Run. It's about 2,375 ft long, 1,300 ft across and turns 220 degrees.
In Civil 3D it's Home tab > Create Ground Data panel > Surfaces drop-down > Create Surface From DEM, then pick the .tif. For a grid surface the file's coordinate system has to match the drawing's or it can't be added, so set the drawing first.
The bench is the smooth band looping around the reservoir's west end. A DEM carries no breaklines, so the bench edges are only as sharp as the grid. The reservoir is a flat patch, lidar doesn't see below water.
Numbered pins on the picture:
Pin 1: The smooth graded bench at the curve's apex
Pin 2: The flat reservoir water surface
Lidar: USGS 3D Elevation Program (3DEP), public domain.