🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Technology & Design › GIS / Surveying / Drone Operations › Construction Layout: Field Stakeout and Grade Control › Lesson 7
TEC-GIS45-05 · Lesson 7 · 12 clock hours · field

Utility Layout: Centerline, Grade, and Invert Elevation

Objective. Students stake centerline hubs for a simulated storm drain run, mark pipe invert elevations on offset stakes, and verify that the design grade between manholes matches the plan within 0.02 ft per 100 ft

⚠ 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.

Utility Layout: Centerline, Grade, and Invert Elevation

Staking a buried utility run — storm drain, sanitary sewer, or water main — is one of the most precision-sensitive tasks in construction layout. A pipe that is even slightly off grade can pond water, back up sewage, or fail inspection. Your job as the survey technician is to give the excavation crew exact centerline positions and exact elevation targets so every pipe section lands where the engineer designed it.

Reading the Utility Plan-and-Profile Sheet

Every underground utility project has a plan-and-profile sheet: the top half shows the horizontal alignment (centerline, stationing, manholes, offsets to property lines); the bottom half is a vertical profile showing existing ground and the design pipe gradient. Before you leave the office, extract these values for each structure:

Convert stationing to field coordinates using your coordinate geometry (COGO) computations or the control traverse before you arrive on site.

Centerline Hubs and Offset Stakes

Because excavation equipment will destroy any hub placed directly on the pipe centerline, surveyors place two sets of marks:

1. Centerline hubs — wood hubs driven flush with the ground, with a tack on top marking the exact centerline point. Set these at each manhole and at 25 ft intervals along the run.

2. Offset stakes (lath) — tall wood lath driven at a fixed offset (commonly 5 ft) from the centerline, clear of the trench walls. The offset stake carries the cut note that tells the operator how far to dig down from the top of the lath to reach the pipe invert.

Calculating the Cut Note

The cut note is written in the form C 4.72 (cut 4.72 ft). Compute it as:

> Cut = HI − Design Invert Elevation

where HI (height of instrument) = Benchmark elevation + backsight rod reading on the benchmark.

If the result is negative, the pipe is above the instrument line and the note becomes a fill (F), which is uncommon for gravity drains.

Offset Notation Format

Write the lath face clearly in crayon using the APWA (American Public Works Association) convention your jurisdiction follows. A typical face reads:

```

STA 3+25

C 5 OFS

INV 98.34

C 4.72

```

Top line: station. Second line: 5 ft offset (so the crew knows how far the trench centerline is). Third line: design pipe invert at that station. Bottom line: cut from the top of the lath to the invert.

Setting Hubs with the Total Station

Occupy a known control point, back-sight a second control point, and set the horizontal angle to zero on the known azimuth. Use design coordinates (converted from stationing) to turn the correct angle and measure the correct distance to each hub location. Drive the hub, then check your position: the total station should read within 0.05 ft of the design coordinate. If not, pull and reset the hub.

After all hubs are set, drive offset lath at 5 ft from each hub, on the side of the trench that will not be disturbed.

Verifying Grade with a Level Loop

Once the offset stakes are in place, run a level loop — a closed differential leveling circuit — along the stakes to verify that the cut notes you wrote will actually produce the design pipe slope.

1. Set up the level at a point where you can see both a benchmark (or turning point of known elevation) and several stakes.

2. Take a backsight on the benchmark; compute HI.

3. Take foresights on each stake top; compute the stake top elevation.

4. Subtract the design invert elevation from the HI to get the required rod reading that would put the line of sight exactly at the invert; compare this to the cut written on the lath.

5. Close the loop back to the starting benchmark. Acceptable closure for this work is typically 0.05 ft or better over the loop.

For each station, compare your measured computed invert against the design invert. The tolerance is 0.02 ft per 100 ft of pipe run. Flag any station that fails with a wire flag and a note in the field book; that stake must be re-driven or the cut corrected before excavation begins.

Field Safety

Field Book Discipline

Record every setup, every HI, every rod reading, and every computed elevation in your bound field book — never on loose paper. Note the date, weather, instrument serial number, and crew members. Corrections should be crossed out with a single line (never erased) and initialed. Your field book is a legal document.

Key terms8

Invert elevationThe elevation of the inside bottom of a pipe at a given point; the critical grade reference for gravity-flow utilities.
Plan-and-profile sheetA two-part engineering drawing showing the horizontal alignment of a utility run on top and the vertical profile (existing ground vs. design grade) on the bottom.
Cut noteA measurement written on an offset stake, in the form 'C X.XX', indicating how far below the top of the lath the pipe invert should be placed.
Height of instrument (HI)The elevation of the level's line of sight, computed by adding the backsight rod reading on a known benchmark to that benchmark's elevation.
Pipe slope (gradient)The rate of fall of the pipe centerline, expressed as a decimal (ft/ft) or percentage; controls flow velocity and self-cleaning capacity of a gravity drain.
Offset stakeA lath driven at a fixed horizontal distance from the pipe centerline, outside the excavation zone, that carries the elevation and cut information for the crew.
Level loopA closed differential leveling circuit that starts and ends at the same benchmark, used to verify elevations set during layout and detect accumulated error.
StationingA system of distances along a project centerline measured from the origin (0+00), written as 1+25 = 125 ft from origin; used to locate structures and grade breaks.

Worked examples4

1. From the plan-and-profile sheet: Manhole A is at STA 2+00 with an invert-out elevation of 101.40 ft. Manhole B is at STA 4+50. The design pipe slope is 0.006 ft/ft. What is the design invert elevation at Manhole B (invert-in)?
  1. Find the pipe length between manholes: STA 4+50 minus STA 2+00 = 250 ft.
  2. Calculate the total fall over 250 ft: 250 ft × 0.006 ft/ft = 1.50 ft.
  3. Subtract the fall from the starting invert: 101.40 ft − 1.50 ft = 99.90 ft.
Answer: Design invert at Manhole B (invert-in) = 99.90 ft.
2. You set up the level and take a backsight of 4.23 ft on a benchmark with elevation 104.61 ft. The rod reading on the top of an offset stake at STA 3+25 is 7.89 ft. The design invert elevation at STA 3+25 is 98.34 ft. What cut note should you write on the lath?
  1. Compute HI: 104.61 ft + 4.23 ft = 108.84 ft.
  2. Compute the stake top elevation: 108.84 ft − 7.89 ft = 100.95 ft.
  3. Compute the cut from stake top to design invert: 100.95 ft − 98.34 ft = 2.61 ft.
  4. Write the cut note on the lath face: C 2.61.
Answer: Cut note = C 2.61 ft.
3. After setting all offset stakes, you run a level loop and record a measured stake-top elevation at STA 2+50 of 102.17 ft. Your HI is 108.84 ft. The design invert at STA 2+50 is 101.10 ft. What is the measured computed invert elevation, and does this station pass the 0.02 ft tolerance?
  1. Measured stake-top elevation = 102.17 ft (given directly from leveling, no HI subtraction needed here — already computed).
  2. Cut written on lath = stake top − design invert = 102.17 − 101.10 = 1.07 ft; lath note reads C 1.07.
  3. To verify, compute what the invert would be if the crew digs exactly 1.07 ft: 102.17 − 1.07 = 101.10 ft — this matches design exactly. Error = 0.00 ft.
  4. 0.00 ft < 0.02 ft tolerance, so the station passes.
Answer: Measured computed invert = 101.10 ft; error = 0.00 ft; station passes the 0.02 ft tolerance.
4. You are checking whether the measured pipe slope between STA 2+00 and STA 3+00 meets the design slope of 0.006 ft/ft. Your field notes show a measured invert elevation of 101.40 ft at STA 2+00 and 100.74 ft at STA 3+00. Does the slope pass, and by how much?
  1. Pipe run length = STA 3+00 − STA 2+00 = 100 ft.
  2. Measured fall = 101.40 − 100.74 = 0.66 ft.
  3. Measured slope = 0.66 ft ÷ 100 ft = 0.0066 ft/ft.
  4. Design slope = 0.006 ft/ft; design fall over 100 ft = 0.006 × 100 = 0.60 ft.
  5. Difference in fall = 0.66 − 0.60 = 0.06 ft over 100 ft. Tolerance is 0.02 ft per 100 ft.
  6. 0.06 ft > 0.02 ft — this station fails the tolerance and must be flagged and corrected.
Answer: The slope fails; the measured fall exceeds the design fall by 0.06 ft over 100 ft, which exceeds the 0.02 ft/100 ft tolerance.

The math in this lesson2

Learners compute 1.50 ft of fall over 250 ft at 0.6% grade and a 99.90 ft downstream invert.
7.RP.3F-LE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Read a utility plan and profile sheet to extract manhole rim, invert-in, invert-out elevations and pipe slope, converting stationing to field coordinates using design data · Set centerline hubs at each manhole location and at 25 ft intervals between manholes using the total station; place offset stakes at 5 ft offset with a cut note to pipe invert · Verify the set grade by running a level loop along the offset stakes, recording measured invert cuts and comparing the computed pipe slope against design; flag any station that fails the 0.02 ft tolerance
Materials: Total station, level, prism pole, Philadelphia rod, field book, utility plan-and-profile sheets, offset stakes and lath, wire flags, crayon, tape, APWA standard utility detail sheets
Check: Centerline hubs within 0.05 ft of design position; pipe invert marks within 0.02 ft of design elevation at all set points; correct offset notation on all lath; instructor verifies two random hubs

Practice check8

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

NSPS CST L1-2: Differential Leveling and Grade Computation
1. On a utility plan-and-profile sheet, the invert-out elevation at a manhole is 98.50 ft and the pipe slope is 0.005 ft/ft. What is the design invert elevation 200 ft downstream?
NSPS CST L1-2: Differential Leveling
2. A benchmark has an elevation of 106.22 ft. The backsight rod reading is 3.41 ft. A foresight rod reading on an offset stake top is 6.88 ft. What is the elevation of the stake top?
NSPS CST L1-2: Construction Stakeout Principles
3. Why are offset stakes placed instead of hubs directly on the pipe centerline?
NSPS CST L1-2: Grade Control and Stakeout
4. The cut note on an offset stake reads C 3.85. The stake top elevation is 101.20 ft. What is the pipe invert elevation at that station?
NSPS CST L1-2: Grade Verification and Tolerance
5. During a level loop verification, the measured invert at STA 5+75 is 95.41 ft and the design invert is 95.22 ft. The pipe run to the next manhole is 100 ft. Does this station pass or fail the 0.02 ft per 100 ft tolerance, and why?
NSPS CST L1-2: Field Safety and Utility Conflict
6. Before driving hubs in a new area, which step is legally required to avoid striking buried utilities?
NSPS CST L1-2: Construction Stakeout Documentation
7. Which piece of information is recorded on the face of the offset lath according to standard APWA practice?
NSPS CST L1-2: Total Station Layout and Positional Tolerance
8. You occupy a control point and set a hub at STA 3+50. The design position is at coordinates N 5,002.50, E 3,110.75. The total station reads N 5,002.58, E 3,110.72. What is the horizontal error, and does the hub meet the 0.05 ft tolerance?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →