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TEC-GIS45-05 · Lesson 5 · 12 clock hours · lab

Vertical Control: Differential Leveling for Grade

Objective. Students establish a temporary benchmark (TBM) on a simulated job site and transfer finish-grade elevation to five grade stakes using differential leveling, with all stakes within 0.02 ft of design elevation

⚠ 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.

Vertical Control: Differential Leveling for Grade

Differential leveling is the backbone of grade control on any construction site. When you set a grade stake, you are telling the equipment operator or laborer exactly how much soil to cut away or add (fill) so the finished surface meets the design elevation. Your numbers must be right the first time — a 0.1 ft error in a road subgrade can cause standing water, failed compaction tests, or costly rework.

The Temporary Benchmark (TBM)

A temporary benchmark is a stable, semi-permanent point whose elevation you have either measured from a published control monument or accepted from the project engineer. On a simulated job site you will be given the TBM elevation. In the field, a TBM is often a nail set in a concrete curb, a rebar cap, or a bolt on a utility structure. Before you start leveling, inspect the TBM for stability — if it moves, every stake you set is wrong.

Record the TBM identifier, its description, and its elevation in your field book before you touch the instrument.

Setting Up the Automatic Level

An automatic level uses a compensator pendulum to keep the line of sight horizontal once the circular bubble is roughly centered. Set the tripod on firm ground with legs spread wide; tighten all leg clamps. Level the instrument using the three-screw footscrews while watching the circular bubble. When the bubble is centered, the compensator takes over. Never lean on or touch the tripod during a reading.

Safety: On an active or simulated job site, wear your hard hat and high-visibility vest whenever you are within the work zone boundary. Set the tripod so its legs do not create a trip hazard for others, and keep the instrument cap on when walking between setups to protect the objective lens.

The HI Method

The height of instrument (HI) method is the standard approach for setting multiple grade stakes from one instrument setup.

1. Read the rod held on the TBM. This is your backsight (BS).

2. HI = TBM elevation + BS

3. For any point where you need to set a stake, calculate the grade rod: Grade Rod = HI − Design Elevation

4. Direct the rodperson to move the rod up or down until the rod reads the grade rod value; that rod bottom is at design elevation. Drive the stake flush to that point.

5. If the rod reads more than the grade rod, the ground is too low — fill needed (mark the stake in red).

6. If the rod reads less than the grade rod, the ground is too high — cut needed (mark the stake in blue).

The cut or fill amount = Actual Ground Rod − Grade Rod (positive = fill; negative = cut — use absolute value and the correct color).

Turning Points and Closure

When you cannot see all five stakes from one setup, you carry the HI forward through a turning point (TP). Read a foresight (FS) on the TP, then move the instrument and read a new BS on the same TP. New HI = Previous HI − FS + new BS. Keep a running elevation on every TP. At the end of a level run, loop back to the TBM or a second known benchmark to check closure. Allowable closure for construction stakeout is typically expressed as a tolerance in feet per distance run — your instructor will specify the limit for this lab.

Recording Field Notes

Use a standard level-book format with columns: Station | BS | HI | FS | Elevation | Grade Elev | Grade Rod | Cut/Fill. Write in pencil, never erase — cross out errors with a single line and initial them. A complete, legible field book is a legal document and part of your deliverable.

Marking the Stakes

Drive the grade stake (wooden hub) to grade and lath (the tall flagging stick) beside it. On the lath, write:

Blue crayon = cut; red crayon = fill. This color convention is universal in US construction surveying.

Independent Check

Have your partner set up the level independently and read two of your stakes. The difference between your grade rod and their reading must be within 0.02 ft for both stakes. If not, re-run from the TBM before calling the instructor. This check catches instrument, math, and reading errors before a contractor acts on your numbers.

Safety Summary

Key terms10

Temporary Benchmark (TBM)A stable point of known elevation established on or near a job site to serve as the starting reference for differential leveling; it is not a permanent control monument.
Height of Instrument (HI)The elevation of the instrument's line of sight, calculated as TBM elevation plus the backsight rod reading; all grade rods for a setup are derived from the HI.
Backsight (BS)A rod reading taken on a point of known elevation (TBM or turning point) to establish or continue the HI.
Foresight (FS)A rod reading taken on a turning point or stake to determine its elevation by subtracting the FS from the current HI.
Grade RodThe rod reading required so the bottom of the rod rests exactly at the design (finish-grade) elevation; calculated as HI minus the design elevation.
CutThe vertical distance the existing ground must be excavated downward to reach design grade; marked in blue on the lath with a 'C' prefix.
FillThe vertical distance that material must be added above existing ground to reach design grade; marked in red on the lath with an 'F' prefix.
Turning Point (TP)A stable intermediate point used to carry the elevation forward when stakes are too far from the instrument for a single setup; it receives both a foresight and a backsight.
ClosureThe difference in elevation when a level loop returns to the starting benchmark; it reveals accumulated instrument and reading errors and must fall within the allowable tolerance.
Philadelphia RodA standard graduated leveling rod, typically reading in feet and hundredths of a foot, used with an automatic or dumpy level to measure vertical distances.

Worked examples4

1. TBM elevation = 452.30 ft. You read a backsight of 4.82 ft on the TBM. The design elevation at Station 0+00 is 450.60 ft. What is the HI, the grade rod, and the cut or fill if the actual ground rod reads 6.54 ft?
  1. HI = TBM + BS = 452.30 + 4.82 = 457.12 ft
  2. Grade Rod = HI − Design Elev = 457.12 − 450.60 = 6.52 ft
  3. Actual ground rod = 6.54 ft (rod reads more than grade rod, so ground is below design grade)
  4. Fill amount = 6.54 − 6.52 = 0.02 ft
Answer: HI = 457.12 ft; Grade Rod = 6.52 ft; Fill = 0.02 ft (mark stake red, F 0.02)
2. Continuing from the same setup (HI = 457.12 ft), the design elevation at Station 0+50 is 451.10 ft. The actual ground rod reads 5.87 ft. Find the grade rod, and determine cut or fill.
  1. Grade Rod = HI − Design Elev = 457.12 − 451.10 = 6.02 ft
  2. Actual ground rod = 5.87 ft (rod reads less than grade rod, so ground is above design grade)
  3. Cut amount = 6.02 − 5.87 = 0.15 ft
Answer: Grade Rod = 6.02 ft; Cut = 0.15 ft (mark stake blue, C 0.15)
3. You need a turning point to reach Station 0+200. From HI = 457.12 ft, you read a foresight of 7.33 ft on the TP. You then read a backsight of 3.61 ft on the same TP from the new instrument position. What is the elevation of the TP and the new HI?
  1. Elevation of TP = HI − FS = 457.12 − 7.33 = 449.79 ft
  2. New HI = Elevation of TP + BS = 449.79 + 3.61 = 453.40 ft
Answer: TP elevation = 449.79 ft; New HI = 453.40 ft
4. After setting all five stakes, you close the loop back to the TBM. The computed elevation of the TBM from your notes is 452.27 ft; the true TBM elevation is 452.30 ft. What is the closure error, and does it pass if the allowable limit is 0.05 ft for this run?
  1. Closure error = |Computed − True| = |452.27 − 452.30| = 0.03 ft
  2. Compare to allowable: 0.03 ft < 0.05 ft
Answer: Closure error = 0.03 ft; the loop passes the allowable tolerance.
In class: Set a TBM on a stable object at a known elevation and compute the required height of instrument and rod readings needed to set stakes at three different finish-grade elevations given on a grading plan · Run levels to five grade stakes placed at 50 ft intervals along a simulated road profile, recording backsight, foresight, HI, and grade-rod cut/fill for each · Color-code stakes with blue crayon for cut and red crayon for fill, marking the amount on the lath in standard notation, then have a partner independently check two stakes with the level
Materials: Automatic level, Philadelphia rod with target, tripod, TBM plate, grade stakes and lath, crayon (blue and red), field book, calculator, grading plan sheet, Schofield 'Engineering Surveying' reference
Check: All five grade stakes within 0.02 ft of design elevation verified by instructor; field notes complete with HI carried correctly through all turning points; level loop closure within allowable limits

Practice check8

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

NSPS CST L1-2: Vertical Control and Leveling
1. The height of instrument (HI) is correctly calculated as:
NSPS CST L1-2: Construction Stakeout
2. A grade rod of 5.40 ft is required at a stake. The rodperson reports an actual ground rod reading of 4.95 ft. The correct annotation on the lath is:
NSPS CST L1-2: Vertical Control and Leveling
3. Which field-book column contains the value that transfers the elevation forward through a turning point?
NSPS CST L1-2: Instruments and Equipment
4. An automatic level's compensator performs which function?
NSPS CST L1-2: Construction Stakeout
5. HI = 461.55 ft. The design elevation at a stake is 458.30 ft. What is the grade rod?
NSPS CST L1-2: Quality Control and Error Checking
6. A partner independently checks two of your grade stakes and finds one stake differs from your reading by 0.03 ft. According to this lab's acceptance criterion, what should you do?
NSPS CST L1-2: Safety and Professional Practice
7. Which PPE combination is required for a survey crew working within an active construction work zone?
NSPS CST L1-2: Safety and Professional Practice
8. A rodperson is walking from one stake to the next and must pass under low-hanging electrical distribution lines. The correct action is to:
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