8.1 Topographic and As-Built Surveys
Key Takeaways
- A topographic survey describes existing ground and visible features on project control so someone can design; an as-built survey measures what was constructed and reports deltas versus design without rewriting the design file.
- Aerial, UAV, and airborne LiDAR mapping still need photo-control or equivalent GNSS/inertial control plus independent check points that were not used to fit the model.
- Occupy the same CCS zone, datum realization, epoch, and named benchmark the plans use before storing the first topo or as-built shot.
- In the worked invert example, design 142.50 ft NAVD 88 versus as-built 142.37 ft is a 0.13 ft low delta; the as-built number stays 142.37 ft on the comparison sheet.
- Bathymetric sounding is a Domain III.7 hydrographic method previewed here only; tidal datums and sounding reductions belong in the hydrographic chapter.
Why this field work is on the exam
The Board’s January 2025 California PLS test plan puts topographic and/or as-built surveys in Domain III (Field Operations and Investigations), activity 1. Knowledge item O asks for methods and requirements for topographic, aerial, as-built, and bathymetric surveys. Item N asks how basis of bearings and benchmarks on maps and construction plans become the values you occupy in the field. This OpenExamPrep section teaches those field acts. It does not determine property lines or Public Land Survey System corners (later boundary chapter) and it does not run a hydrographic survey (later hydrographic chapter).
Two products get mixed on exam items and on jobs. A topographic survey measures existing ground and features so a designer can draw. An as-built survey measures constructed work and compares it to design. Both sit on the same control skeleton. Neither is a license to invent a new datum in the data collector because the contours look smooth.
Topography is existing; as-built is constructed
Topographic field work collects:
- Breaklines along grade changes: curb face, flow line, top of bank, retaining-wall top and toe, pavement crown.
- Spot elevations on critical civil points: finished floors, grate rims, accessible inverts, sag points, high points.
- Planimetric features the design will hit or miss: buildings, walls, trees that affect grading, visible utility appurtenances.
- Enough density for the stated contour interval and the design question. A 50-foot GNSS grid across an intersection will miss the gutter.
As-built field work occupies the same control and measures the feature the contractor placed: invert, grate, face of curb, bolt pattern, building corner, top of wall. The office product is a comparison: design versus measured, with signed residuals. If the invert is low, the as-built sheet says it is low. It does not silently replace the design elevation with the field shot.
Worked example: as-built versus design
Plans for a city storm manhole MH-4 show:
| Quantity | Design (plans) | As-built (field) | Delta (as-built minus design) |
|---|---|---|---|
| Northing, CCS83 Zone 5, NAD83(2011) epoch 2017.50 | 1,845,220.15 ft | 1,845,220.31 ft | +0.16 ft |
| Easting, same system | 6,432,108.42 ft | 6,432,108.29 ft | −0.13 ft |
| Invert, NAVD 88 | 142.50 ft | 142.37 ft | −0.13 ft |
Horizontal resultant = √(0.16² + 0.13²) = √0.0425 = 0.21 ft. Vertical delta = 0.13 ft low.
Suppose the project specification allows 0.10 ft on invert at this structure. The as-built fails the vertical spec. The field report still lists invert 142.37 ft. Changing the stored elevation to 142.50 ft because “that is what they meant to build” launders a construction miss into a false topographic fact. If a later crew uses 142.50 ft to set the next pipe, the error repeats downstream.
The same numbers also teach control hygiene. If the rover was set to CCS83 Zone 6, or to epoch 2010.00 while the plans say 2017.50, the 0.16 ft / 0.13 ft plan deltas are meaningless. Confirm zone, realization, epoch, and the named benchmark before the first stored shot.
Methods: terrestrial, aerial, as-built, bathymetric preview
| Method | What you occupy in the field | Typical product | Exam trap |
|---|---|---|---|
| Total-station topography | Checked project hubs; backsight and foresight checks | Selected breaklines, spots, inverts | Radial sideshots from an unchecked spur |
| GNSS RTK / RTN topography | Proven control plus open sky; geoid or leveled heights for orthometric values | Open sites and corridors | Treating a single fixed RMS as vertical control for a sewer invert |
| Terrestrial laser scanning | Control targets or registered setups tied to project control | Dense as-built of plants, bridges, facades | Calling the raw cloud a finished map with no classification or checks |
| Aerial photogrammetry / UAV imagery | Photo-control targets and independent check points | Contours and planimetrics over open ground | “The drone has GNSS, so no ground marks” |
| Airborne LiDAR | Project control, boresight checks, surveyed breaklines at curbs and flow lines | Bare-earth model on large tracts | Skipping breaklines so the TIN cuts through a gutter |
| As-built conventional or scan | Same datum as design; occupied control recovered that day | Comparison sheet / model | Overwriting design with measured values |
| Bathymetric (preview only) | Vessel GNSS plus sounder, or equivalent | Depths relative to a stated vertical datum | Mixing a tidal chart datum with NAVD 88 without a stated relationship |
Aerial mapping is still a field-control job
Knowledge item O names aerial surveys. The PLS in the field is not only the pilot. You:
- Recover or set photo-control (paneled points or well-defined photo-identifiable marks) on the project datum.
- Observe those marks with GNSS or total station from project control, not from a random nail.
- Set independent check points that the mapping adjustment is not allowed to eat. After compilation, compare mapped coordinates and elevations to those checks.
- Capture breaklines the imagery cannot see: flow lines under parked cars, invert shots, eaves, dense canopy holes.
A UAV with on-board GNSS is not a substitute for those steps. Uncontrolled imagery is a picture. A topographic survey is a controlled measurement.
Bathymetric preview only
Domain III, activity 7 covers hydrographic surveys (bathymetric, tidal datum). If an item mentions depths in a lake, channel, or harbor, you still need a stated vertical datum, a sensor (sounder, rod, or multibeam), and a horizontal position for each sounding. Reducing those depths to a tidal datum, and relating that datum to NAVD 88, is hydrographic procedure. Do not treat a random GNSS ellipsoid height over water as a finished bathymetric survey. Stop here and move that work to the hydrographic chapter.
Basis of bearings and benchmarks on topo and as-built
Before storing points, read the control sheet the same way a construction crew must:
- Basis of bearings — often CCS83, Zone n, with a datum realization and epoch (Public Resources Code §§ 8801–8819; §8815.1 requires the realization tag and epoch to two decimal places when CCS83 coordinates are shown).
- Benchmark — designation, elevation, and vertical datum (commonly NAVD 88). Occupy that mark or a checked descendant. A fire-hydrant flange used as an assumed elevation is not the named benchmark unless the plans say so.
- Units — California plane coordinates in feet use the U.S. Survey Foot (1 ft = 1200/3937 m).
If the topographic map will later become the existing-ground surface for grading, a wrong benchmark on day one becomes a systematic cut/fill error on every stake.
Field notes that make the survey defensible
Knowledge item M includes field notes. For topo and as-built, notes or the data-collector equivalent should record:
- Date, crew, instrument make/model/serial, antenna or target height, weather and GNSS sky plot if relevant.
- Control occupied, backsight, and an independent check residual (example: check hub 0.02 ft horizontal, 0.01 ft vertical).
- Datum note copied from the plans: zone, realization, epoch, geoid model if GNSS heights were converted.
- Feature codes and which shots are breaklines versus random spots.
- For as-built: design identifier (MH-4, DI-12, FG at 12+50), measured values, and the computed delta—not a rewritten design.
A file named FINAL.crd with no occupied-control log is not a staking or as-built report.
Field sequence you can defend on an item
- Recover the named control and benchmark; close a check.
- Confirm the collector’s coordinate system matches the plans (zone, epoch, geoid, feet).
- Collect topography for design, or as-built for comparison—not a blend that overwrites design.
- If aerial methods are used, set and observe photo-control and withheld checks.
- Reduce, review breaklines, and report as-built deltas as measured.
Exam traps: calling UAV imagery without ground control a topographic survey; using construction blue-tops as the only vertical datum for as-built inverts; mixing NAVD 88 and an assumed hydrant elevation on the same profile; treating bathymetry as ordinary dry-ground GNSS.
A crew measures constructed storm manhole MH-4 at invert 142.37 ft NAVD 88. The design invert is 142.50 ft. The project specification allows 0.10 ft on this invert. What belongs on the as-built comparison sheet?
A UAV will photograph a 40-acre site for 1-ft contours on the project CCS83 control. Which field requirement still applies?
On a California PLS field item, what is the working distinction between a topographic survey and an as-built survey?