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Terrain (USGS 3DEP surface + compare)

An existing-ground surface from USGS lidar in one pick, and a check of how far a drawing surface is from it. Build samples the USGS 3D Elevation Program DEM on a grid and creates a TIN surface; Compare samples a surface already in the drawing at the same grid and reports the deltas with statistics, a histogram, an HTML report, a CSV and labels at the outliers.

Command VDCTERRAIN · VDC Civil tab › Terrain panel
An existing-ground surface from USGS lidar in one pick, and a check of how far a drawing surface is from it. Build samples the USGS 3D Elevation Program DEM on a grid and creates a TIN surface; Compare samples a surface already in the drawing at the same grid and reports the deltas with statistics, a histogram, an HTML report, a CSV and labels at the outliers.
VDCTERRAIN, USGS 3DEP sampled on a grid → TIN surface, Compare: lidar minus the drawing surface picked window · 10 ft grid (coarsens to the sample cap) EG-3DEP (TIN) +1.3 -1.7 TERRAIN COMPARE, lidar minus EG-SURVEY (ft) Samples compared3,844 Mean / RMS+0.08 / 0.41 Within ±0.5 / ±1.0 / ±2.0 ft81% / 96% / 99% Beyond ±1.0 ft (labeled)142 Outside the surface210 histogram of deltas, 0.5 ft bins · fifteen largest differences listed HTML report + CSV of every sample in VDC Reports; delta labels on C-TOPO-3DEP-DELTA. Bare-earth lidar as flown, planning-level terrain, not a field survey. Build / Compare / Both Build: metres → drawing feet, TIN surface named EG-3DEP (never overwrites; 3D points on plain AutoCAD). Compare: pick any surface in the drawing. A mean near zero with a small RMS = agreement; a consistent offset = datum / geoid; big local deltas under canopy or on structures = the lidar. Window / point + radius / extents; spacing default 10 ft; 1,000 samples per call, 60,000 cap.

What it does

  • Build: USGS 3D Elevation Program seamless DEM (1 m where lidar exists) sampled on a grid over the window in the drawing's grid system, metres converted to the drawing's feet, a TIN surface created (default EG-3DEP; an existing name is suffixed, never overwritten). Plain AutoCAD gets 3D points on a layer.
  • Compare: the chosen drawing surface sampled at the same grid; deltas (lidar − surface) summarized, mean, standard deviation, RMS, median, range, percent within ±0.5 / 1 / 2 ft, a histogram, the fifteen largest differences, as an HTML report, a CSV of every sample, and delta labels at the samples beyond a threshold (default ±1.0 ft).
  • Both builds the surface and runs the compare in one pass.

Modes, seats & caveats

  • Area: Window (default) / Point + radius / Extents; WKID (any State Plane zone); spacing default 10 ft (coarsens automatically to the 60,000-sample cap and says so).
  • Plain AutoCAD: Build lands 3D points; Compare needs a Civil 3D surface and reports itself skipped. Ships in VDC.Acad.bundle.
  • Planning-level terrain: bare earth as flown; survey date, vegetation, structures and processing all show up as deltas. Never a substitute for a field topo, every output path says so.
  • Network required. Settings at %APPDATA%\VDC\Terrain\settings.json (env VDC_TERRAIN_SETTINGS).

Dialog

Terrain (USGS 3DEP surface + compare) dialog
Terrain (USGS 3DEP surface + compare), VDCTERRAIN

When to use it

  • Concept grading before the survey exists: an existing-ground surface for the whole site in a minute.
  • Survey QC: how well does the field topo agree with the lidar? A consistent offset points at a datum or geoid mismatch.
  • Change detection: where has the ground moved since the lidar flight?

How to run it

  1. Run VDCTERRAIN - choose Build, Compare or Both; set the area, WKID, spacing, the new surface name, the compare surface and threshold; click OK.
  2. Pick the window (or the point). The status bar counts the samples; the funnel prints grid points / with elevation / no data.
  3. Build: the surface appears in Prospector (an existing name is suffixed). Compare: read the summary, open the HTML report from VDC Reports, inspect the delta labels.

Options

  • Mode: Build / Compare / Both.
  • Spacing: default 10 ft; coarsens automatically above 60,000 samples.
  • Compare surface + threshold: any surface in the drawing; samples beyond +/- the threshold (default 1.0 ft) are counted and labeled.
  • Outputs: HTML report, sample CSV, delta labels (layer, text height, style).

Watch out for

  • Planning-level terrain - never a substitute for a field topo.
  • Large deltas under canopy or on buildings are the lidar, not the survey; a uniform offset is a datum question, not a ground change.
  • Compare needs a Civil 3D surface; on plain AutoCAD only Build runs (as 3D points).
  • Network required; a big window takes about a minute at the sample cap.

Related: Site Data · Ground Thin · Smooth Surface · Soils