The hook
A DTM (Digital Terrain Model) is the surface itself — not a map of it. Civil software grades, computes volumes, and renders 3D from a DTM. Two dominant representations: TIN (Triangulated Irregular Network — the surface as triangular facets) and grid DEM (a regular array of elevations). Each has trade-offs the exam will test.
Memorize these
Concepts that show up on the exam
TIN (Triangulated Irregular Network)
A surface model built by Delaunay-triangulating the survey points. Each triangle has its own slope; breaklines are honored as edges. Memory-efficient for variable terrain density.
Grid DEM (Digital Elevation Model)
A raster where each cell holds an elevation. Easy to process (uniform indexing), simple math (slope = (Δz/Δx) cell-by-cell). Can be huge for large areas.
DTM vs. DEM vs. DSM
DTM = bare-earth terrain. DEM is generic — could be either. DSM (Digital SURFACE Model) includes vegetation + buildings. Lidar produces a DSM first; "ground filtering" extracts the DTM.
Breakline
A line in the surface where slope changes sharply (curb edge, ridgeline, top of bank). TINs honor these as triangle edges; DEMs only honor them if you raster-burn them in.
Contour from DTM
Civil software interpolates contour lines from the DTM at the requested interval. Quality of contours = quality of the underlying DTM.
Cut/fill from DTM
Compute the volume difference between two DTMs (existing vs. proposed). Foundational to earthwork estimation, mass diagrams, grading design.
| TIN | Grid DEM | |
|---|---|---|
| Storage | Vertices + edges (compact for variable terrain) | Every cell stored (huge for large areas) |
| Breaklines | Native — triangle edges follow features | Must be burned into the raster |
| Slope analysis | Per-triangle (variable size) | Per-cell (uniform; faster) |
| Edit-friendliness | Must re-triangulate after edits | Edit one cell value, re-derive products |
| Common uses | Civil 3D, site grading, ALTA topo | Lidar, regional terrain, SRTM |
Don't fall for these
What trips people up
TIN extrapolation outside the data
A TIN only knows about the area enclosed by its outermost triangles. Pulling contours just past the edge invents elevations from nothing. Always restrict the analysis area to the surveyed extent.
Grid DEM at the wrong resolution
A 30 m DEM is great for a watershed; useless for a parking lot. The cell size IS the spatial resolution. Don\'t infer cm-level features from m-level cells.
Mixing DSM and DTM
If you compute a building footprint elevation by sampling a DSM, you get the roof, not the slab. Always confirm whether you\'re working with bare-earth or surface data.
Test yourself
How well did it stick?
A quick 5-question check on Digital Terrain Models. See where you stand and what to review.