A topographic mapping scope can produce half a dozen different files, and the names get used loosely. Getting the deliverables list right up front saves rework when the data reaches design. This guide explains what each product is, how engineers use it, and what to specify.
Start with the basics: datum, units and accuracy
Every deliverable below is only as useful as its spatial reference. Specify:
- Horizontal datum and coordinate system (for example, NAD83 California State Plane, the correct zone for the site, and the epoch or realization if your agency requires one).
- Vertical datum (typically NAVD88) and the benchmark used.
- Units (US survey feet or international feet, which differ slightly and must be stated).
- Accuracy targets expressed as RMSE under the ASPRS Positional Accuracy Standards, with the number of independent checkpoints for testing.
If you are ordering an ALTA/NSPS survey, Table A Item 5 (vertical relief) asks for the source of the information, contour interval, datum and originating benchmark to be stated on the survey. That is good practice for any topographic deliverable.
The point cloud
A point cloud is a set of 3D points, each with X, Y and Z coordinates and often color or intensity. Photogrammetry produces dense point clouds from overlapping images; lidar produces them from laser returns.
What engineers use it for: visual QA, measuring existing structures, extracting features, volume calculations and as the raw source for the terrain model.
What to specify: format (LAS or LAZ is standard), classification, and density. The ASPRS LAS specification defines standard classes such as 2 (ground), 3 to 5 (low, medium and high vegetation), 6 (building), 9 (water), 11 (road surface) and 17 (bridge deck). A ground-classified cloud is what feeds the terrain model; an unclassified cloud includes roofs, cars and trees.
Watch for: a raw point cloud is not a terrain model. Points on vegetation, vehicles and structures must be classified out before you design to it.
The DTM (digital terrain model)
In civil practice, a DTM is a model of the bare-earth surface built from ground points and breaklines. Caltrans describes the DTM as a three-dimensional digital representation of the terrain derived from topographic mapping and frequently combined with field survey data.
Breaklines are 3D lines along abrupt changes in grade: tops and toes of slope, edges of pavement, flowlines, curb faces, ridges and channel bottoms. They keep the surface from cutting across features that should be crisp. A DTM without good breaklines will misrepresent curb and gutter, swales and retaining walls.
What to specify: the DTM limits (which may differ from the mapping limits), breakline requirements and the target CAD platform.
A note on terms: some sources use DEM (digital elevation model) for a gridded bare-earth raster and DSM (digital surface model) for a surface that includes buildings and vegetation. If a proposal lists a DSM, it is not a bare-earth design surface.
The TIN (triangulated irregular network)
A TIN is a way of storing a surface: points and breaklines are connected into non-overlapping triangles. Civil 3D, OpenRoads and similar platforms use TIN surfaces for grading, profiles, cross sections and earthwork.
In practice, the DTM is often delivered as a TIN surface. The difference that matters is quality control. Ask for a TIN built with breaklines enforced as triangle edges, long or sliver triangles at the edges cleaned up, and no triangles bridging across buildings or voids.
What to specify: native surface format (for example, a Civil 3D surface or LandXML), and whether you want the source points and breaklines delivered as well.
Contours
Contours are lines of equal elevation generated from the DTM. They are a presentation product, not the surface itself. Design should be based on the surface; contours are for plan sheets, review and communication.
What to specify: contour interval and labeling. Caltrans' photogrammetry chapter pairs mapping scales with contour intervals (for example, 1-foot contours at 1 inch = 20 feet for structure sites, and 2-foot contours at 1 inch = 50 feet for engineering design). That is a useful reference point, but today the accuracy of the underlying surface should be specified directly as RMSE. A 1-foot contour interval does not by itself guarantee a surface accurate enough for 1-foot contours.
Watch for: contours drawn at an interval finer than the surface accuracy supports. They look precise and are not.
The orthophoto (orthomosaic)
An orthophoto is aerial imagery corrected for camera tilt and terrain relief so it has a uniform scale and can be measured like a map. An orthomosaic is many images stitched into one seamless image.
What engineers use it for: base imagery under design, existing-conditions exhibits, planimetric feature checks, entitlement and public-outreach exhibits.
What to specify: ground sample distance (pixel size), file format (GeoTIFF is common, with tiling for large areas), coordinate system and horizontal accuracy. ASPRS Edition 2 also limits mosaic seamline mismatch relative to the stated horizontal accuracy.
Watch for: elevated structures such as bridges and tall buildings may show lean or distortion in an orthophoto produced from a bare-earth surface. That is normal; ask how it will be handled if it matters.
Planimetrics and the CAD base map
Planimetrics are 2D or 3D line work for visible features: edges of pavement, curbs, buildings, walls, fences, striping, poles, manholes and visible utility features. With the DTM and contours, this forms the topographic base drawing.
What to specify: CAD platform and version, layer and symbology standards (your firm's or the agency's), and the feature list. Remember that aerial methods will not capture underground utilities, inverts or obscured features; those come from field survey.
A sample deliverables list
- Control information (supplied by your surveyor) with control and checkpoint coordinates and the accuracy test results
- Classified point cloud (LAS/LAZ)
- Bare-earth DTM as a TIN surface with breaklines
- Contours at the specified interval
- Planimetric CAD base map to your layer standard
- Orthomosaic (GeoTIFF) at the specified pixel size
- Metadata stating datum, units, collection date and tested accuracy
FAQ
What is the difference between a DTM and a TIN?
A DTM is the bare-earth terrain model; a TIN is a common data structure (connected triangles) used to store and work with that model in CAD.
Can I design directly from a point cloud?
Not from a raw cloud. It must be classified so only ground points and breaklines feed the design surface.
What contour interval should I ask for?
Choose based on the project's design needs, then make sure the surface accuracy, stated as RMSE, actually supports that interval.
Get deliverables your designers can use
Tell us your CAD platform, datum and accuracy needs and we will scope a deliverables package that drops straight into design. Call Arrowhead Mapping at (909) 889-2420 or contact us at /contact.