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TEC-GIS45-05 · Lesson 9 · 12 clock hours · lab

As-Built Surveying and Documentation

Objective. Students conduct an as-built survey of a completed simulated structure, compare measured positions and elevations against design values, and produce a written deviation report identifying any non-conformances

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

What Is an As-Built Survey?

An as-built survey (also called a record survey or as-constructed survey) documents the actual, measured positions and elevations of a structure or utility after construction is complete. It answers one critical question: does what was built match what was designed? Contractors, engineers, and owners rely on as-built surveys to verify code compliance, support final payment, resolve disputes, and update record drawings. For a survey technician, accuracy and completeness in as-built work protect every party involved.

Why As-Builts Matter

Construction tolerances exist because no field operation is perfect. Design drawings carry coordinates and elevations to paper precision, but equipment error, settlement, forming errors, and layout mistakes all introduce real-world deviation. The surveyor's job is not to pass judgment on whether a deviation is acceptable — that is the engineer's or owner's call — but to measure it honestly, document it clearly, and flag it for the right decision-maker.

NSPS CST study materials emphasize that field notes must be a permanent, unambiguous legal record. Every measurement, instrument setup, and backsight must be recorded so that another technician could reconstruct the work without asking you a single question.

Setting Up the Total Station for Corner Checks

Before measuring building pad corners, complete these steps:

1. Occupy a control point whose coordinates are known and verified against the original design coordinate system. Never assume a hub driven during layout is still undisturbed; check it against a second control point before trusting it.

2. Backsight a second control point and confirm the instrument reads the correct azimuth or bearing. Record the backsight check in your field book with date, time, instrument serial number, and the backsight residual.

3. Collect each corner by centering the prism over the as-built point. Record the raw coordinate (Northing, Easting, Elevation) to 0.01 ft or finer. Label each shot clearly: `NE-CORNER PAD-A`, not just `PT-1`.

After collecting all points, export or transcribe coordinates into a comparison table alongside the design values. Compute the horizontal deviation (plan distance between as-built and design position) and the vertical deviation (as-built elevation minus design elevation). A positive vertical deviation means the structure is high; negative means low. Use consistent sign conventions throughout.

Checking Utility Invert Elevations with a Level

A pipe invert is the inside-bottom elevation of a pipe at a structure (manhole, catch basin, cleanout). Invert elevations control drainage slope; a mis-set invert can cause standing water, backflow, or structural damage.

To check an invert:

1. Set up and level the instrument on stable ground with clear sightlines to the rods.

2. Take a backsight on a benchmark of known elevation. Record HI (Height of Instrument): `HI = BM elevation + backsight rod reading`.

3. Lower the rod into the structure and hold it on the invert. Record the foresight rod reading.

4. Compute: `Invert elevation = HI − foresight rod reading`.

5. Compare to the design invert on the plan. Record the cut/fill difference: `Difference = As-built invert − Design invert`. A negative difference means the pipe was set low (more cut needed to drain, or the pipe may surcharge sooner than designed).

Annotate the printed as-built markup sheet using standard symbols: a circled elevation with a delta (Δ) or a cloud bubble around the discrepant location is common practice. Follow whatever symbol legend appears on the project's drawing set.

Writing the Deviation Report

The deviation report is a professional document, not raw field notes. It synthesizes the field data into actionable information. A one-page report should include:

Write in clear, complete sentences. Avoid vague language like "a little high" — use numbers. Example: "Corner NE-PAD-A is 0.08 ft east and 0.03 ft north of design position, for a horizontal deviation of 0.09 ft, exceeding the stated ±0.05 ft tolerance."

Safety on the Lab Site

Even on a simulated construction pad, treat the environment as a real jobsite:

Key terms9

As-built surveyA survey conducted after construction to measure the actual positions and elevations of completed work and compare them to design drawings.
Invert elevationThe elevation of the inside bottom of a pipe at a specific structure such as a manhole or catch basin; used to confirm correct drainage slope.
Height of Instrument (HI)The elevation of the line of sight through a level, calculated by adding the backsight rod reading to the benchmark elevation.
DeviationThe measured difference between an as-built value (horizontal position or elevation) and the corresponding design value; sign indicates direction of error.
Non-conformanceA condition in which a measured as-built value falls outside the tolerance specified in the contract documents or design standard.
Horizontal deviationThe straight-line plan distance between the as-built position of a point and its design position, computed from Northing and Easting differences.
As-built markup sheetA printed copy of the design plan on which field-measured values and deviation annotations are recorded using standard symbols, becoming part of the project record.
ToleranceThe maximum allowable deviation from a design value, specified in project documents; deviations within tolerance are accepted without further action.
Engineer ReviewA disposition assigned to a non-conformance when the deviation exceeds tolerance but the correct remedy is uncertain and requires a licensed engineer's analysis before a decision is made.

Worked examples4

1. Design corner NW-PAD-A has coordinates N 1000.00, E 500.00, Elev 102.50 ft. The total station measures N 999.94, E 500.07, Elev 102.47 ft. Compute the horizontal deviation and the vertical deviation.
  1. Compute the Northing difference: ΔN = 999.94 − 1000.00 = −0.06 ft (south of design).
  2. Compute the Easting difference: ΔE = 500.07 − 500.00 = +0.07 ft (east of design).
  3. Compute the horizontal deviation using the Pythagorean theorem: deviation = √(ΔN² + ΔE²) = √((−0.06)² + (0.07)²) = √(0.0036 + 0.0049) = √0.0085 ≈ 0.09 ft.
  4. Compute the vertical deviation: ΔElev = 102.47 − 102.50 = −0.03 ft (low by 0.03 ft).
Answer: Horizontal deviation = 0.09 ft (corner is 0.09 ft south-east of design); vertical deviation = −0.03 ft (corner is 0.03 ft low).
2. A level is set up and a backsight of 4.82 ft is read on a benchmark at elevation 98.60 ft. A rod is then lowered into a catch basin and reads 7.34 ft at the invert. The design invert elevation is 95.10 ft. Compute the as-built invert elevation and the deviation.
  1. Compute the Height of Instrument: HI = 98.60 + 4.82 = 103.42 ft.
  2. Compute the as-built invert elevation: Invert = HI − foresight = 103.42 − 7.34 = 96.08 ft.
  3. Compute the deviation: Deviation = As-built − Design = 96.08 − 95.10 = +0.98 ft.
  4. Interpret the sign: the invert is 0.98 ft high relative to design, meaning the pipe was set too high.
Answer: As-built invert elevation = 96.08 ft; deviation = +0.98 ft (pipe invert is 0.98 ft higher than design — a significant non-conformance requiring Engineer Review or rework).
3. A project specifies a horizontal tolerance of ±0.05 ft and a vertical tolerance of ±0.04 ft for building pad corners. Corner SE-PAD-A shows a horizontal deviation of 0.04 ft and a vertical deviation of −0.06 ft. Assign a disposition to each component.
  1. Compare horizontal deviation to tolerance: |0.04| = 0.04 ft ≤ 0.05 ft, so the horizontal position is within tolerance.
  2. Compare vertical deviation to tolerance: |−0.06| = 0.06 ft > 0.04 ft, so the elevation is outside tolerance.
  3. Assign disposition for horizontal: Accept (within tolerance).
  4. Assign disposition for vertical: Rework or Engineer Review. Because the pad is 0.06 ft low and regrading is straightforward, the technician recommends Rework; final disposition is the engineer's call.
Answer: Horizontal: Accept. Vertical: Rework recommended (pad is 0.06 ft low, exceeding the ±0.04 ft vertical tolerance).
4. Three utility inverts are checked. Their deviations from design are: Invert A = +0.02 ft, Invert B = −0.15 ft, Invert C = +0.04 ft. The specified vertical tolerance is ±0.05 ft. How many non-conformances exist, and what percentage of the three checked points are non-conforming?
  1. Check Invert A: |+0.02| = 0.02 ft ≤ 0.05 ft → conforming.
  2. Check Invert B: |−0.15| = 0.15 ft > 0.05 ft → non-conforming.
  3. Check Invert C: |+0.04| = 0.04 ft ≤ 0.05 ft → conforming.
  4. Count non-conformances: 1 out of 3.
  5. Compute percentage: (1 ÷ 3) × 100 = 33.3%.
Answer: 1 non-conformance (Invert B at −0.15 ft); 33.3% of checked inverts are non-conforming.
In class: Measure the as-built corners of a previously staked simulated building pad using a total station, recording coordinates and elevations, then compare each to design values in a side-by-side table · Check three utility invert elevations with a level against the design plan and document cut/fill differences on a printed as-built markup sheet using standard annotation symbols · Write a one-page as-built deviation report listing each discrepancy, its magnitude, and a recommendation (accept, rework, or engineer review), using the field-book and office-computation conventions from NSPS CST study materials
Materials: Total station, level, rod, as-built markup sheet, design drawings, field book, laptop, spreadsheet or word-processing software, NSPS CST Study Guide (documentation chapters)
Check: All measured values recorded to 0.01 ft; deviation table complete with no blank cells; report correctly identifies at least 80% of instructor-planted non-conformances; professional writing standard met

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NSPS CST L1-2 — Field Survey Procedures and Documentation
1. What is the primary purpose of an as-built survey?
NSPS CST L1-2 — Leveling and Elevation Determination
2. A level backsight on a benchmark at elevation 101.25 ft reads 3.47 ft. A foresight on a pipe invert reads 9.12 ft. What is the as-built invert elevation?
NSPS CST L1-2 — Computations and Coordinate Geometry
3. A corner's as-built Northing is 2500.03 ft and its design Northing is 2500.00 ft; its as-built Easting is 1000.04 ft and its design Easting is 1000.00 ft. What is the horizontal deviation to the nearest 0.01 ft?
NSPS CST L1-2 — Construction Survey and Documentation
4. An as-built corner elevation is 0.09 ft below the design elevation, and the specified vertical tolerance is ±0.05 ft. Which disposition is most appropriate for the technician to recommend in the deviation report?
NSPS CST L1-2 — Construction Survey and Documentation
5. When checking a pipe invert from the surface with a level and rod, what measurement does the technician rest the rod on?
NSPS CST L1-2 — Field Notes and Professional Standards
6. Which of the following best describes proper field-book documentation for a total station as-built setup?
NSPS CST L1-2 — Drawing Interpretation and Markup
7. On an as-built markup sheet, a technician circles an elevation and draws a cloud bubble around a catch basin symbol. What does the cloud bubble conventionally indicate?
FAA Part 107 / General Occupational Safety — Hazard Recognition
8. A technician needs to check a pipe invert inside a deep manhole by physically descending into it. What is the minimum additional requirement before entering?
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