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TEC-GIS45-05 · Lesson 3 · 12 clock hours · field

Control Networks and Benchmark Recovery

Objective. Students locate existing project control monuments, verify their coordinates and elevations against record documents, and flag any discrepancy exceeding allowable tolerance before proceeding with layout

⚠ 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.

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Control Networks and Benchmark Recovery

Before a single stake is driven on a construction site, you must confirm that the survey control you are about to use is correct. Control points and benchmarks are the foundation of every layout measurement. If a control point has been disturbed, mislabeled, or simply recorded with a transcription error, every subsequent measurement will inherit that error. Recovering and verifying control is not a formality — it is a professional and legal obligation.

What Is a Control Network?

A control network is a set of surveyed monuments whose horizontal positions (northing, easting) and/or vertical elevations have been established to a known accuracy and are recorded in official project documents. On a construction project, the design engineer or owner's surveyor typically sets the control network before work begins. The contractor's surveyor or layout technician must verify those monuments before relying on them.

Control monuments are usually described in a control point description sheet, which gives the point designation, coordinates, elevation, monument type (e.g., rebar with cap, concrete nail, mag nail in asphalt), and a written description of how to find it. The description may also include a sketch and a tie to a permanent feature such as a utility box cover or curb line. You use the description sheet the same way a navigator uses a chart — follow it precisely.

Locating Monuments in the Field

Begin by studying the control map in the office. Identify the points you need, note their relative positions, and plan a logical field route. In the field, use the written description to approach each monument. Look for the monument type described; if the description says "3/4-inch rebar with orange plastic cap," do not accept an uncapped piece of rebar as a substitute. Once found, clear debris and confirm the cap markings match the project designation.

Safety note: When searching for control in areas with vehicular traffic, wear a high-visibility vest (ANSI/ISEA 107 Class 2 minimum) and place traffic cones or a spotter if required by the site safety plan. Never set up an instrument in a travel lane without proper traffic control.

Verifying Horizontal Position with a Total Station

Set up your total station over a known control point using a tribrach and optical or laser plumb. Level carefully — an out-of-level instrument introduces systematic error into every angle and distance measurement. Once set, backsight a second known control point to establish your orientation, and record the backsight azimuth or bearing. Then foresight the monument you are verifying, record the horizontal angle and slope distance, and compute the coordinates of the target point using your traverse or polar calculation.

Compare measured coordinates to the record coordinates. The allowable discrepancy depends on the accuracy class specified for the project. For construction layout at the technician level, a common tolerance is ±0.05 ft or better in both northing and easting, but always defer to the project specifications. If the discrepancy exceeds the allowable tolerance, flag the point — do not proceed with layout from it — and report the finding to your supervisor or the engineer of record.

Running a Differential Level Loop

A differential level loop starts at a benchmark of known elevation, runs through a series of turning points (TP), and returns to the starting benchmark. The purpose is to detect any change between the measured closing elevation and the known starting elevation. That difference is the closure error.

Third-Order Allowable Closure

The allowable closure for third-order leveling (the accuracy class typical for construction control) is expressed as:

Maximum closure error = C × √M

where C is a constant (commonly 0.05 ft for third-order work) and M is the total length of the loop in miles. Always confirm the value of C from the applicable project specification or the NSPS CST Study Guide for your exam jurisdiction.

Run the loop by setting up the level at each instrument station, reading the backsight (BS) rod on the previous point, and the foresight (FS) rod on the next turning point. Record BS, FS, Height of Instrument (HI = elevation + BS reading), and computed elevation (elevation = HI − FS) for every setup. Keep the BS and FS distances roughly equal to cancel out curvature, refraction, and instrument errors.

Field-Note Conventions

NSPS standards require field notes to be complete, legible, and able to stand alone as a legal record. For a level loop, each page must include: project name and number, date, weather, instrument type and serial number, observer and recorder names. The note itself shows columns for Station, BS, HI, FS, and Elevation, filled in for every setup. Sketches of the route with point names are added in the right column.

If a measured value exceeds allowable tolerance, write a narrative flag in red or with a distinct notation: state which value, what was measured, what was expected, and that the point should not be used for layout until resolved. Sign and date every page. Your instructor will review and sign off on your notes before the next field session — treat that review the same way you will treat a licensed surveyor's review on a real project.

Safety Summary

Key terms10

Control networkA set of surveyed monuments with known horizontal coordinates and/or elevations that serve as the reference framework for all project measurements.
Control point description sheetA document describing a monument's designation, coordinates, elevation, type, and written directions for locating it in the field.
Differential levelingA field procedure that determines elevation differences between points by reading a leveling rod at backsight and foresight positions from a level instrument.
Height of Instrument (HI)The elevation of the line of sight of a leveling instrument, computed by adding the backsight rod reading to the elevation of the point occupied.
Closure errorThe difference between the computed elevation at the end of a level loop and the known elevation of the starting benchmark; used to assess the accuracy of the leveling run.
Third-order levelingAn accuracy class of differential leveling commonly specified for construction control, with allowable closure typically expressed as a constant multiplied by the square root of the loop length in miles.
Backsight (BS)A rod reading taken on a point of known elevation to establish the height of instrument; adds to elevation to give HI.
Foresight (FS)A rod reading taken on a point whose elevation is to be determined; subtracted from HI to compute the new elevation.
Turning point (TP)A stable, temporary point where both a foresight and a backsight are taken to carry elevation forward through a level loop.
Narrative flagA written note in the field book stating that a measured value exceeds allowable tolerance and that the affected control point must not be used for layout until the discrepancy is resolved.

Worked examples4

1. A level loop begins and ends at Benchmark A (elevation = 312.45 ft). After running through three turning points, the computed closing elevation is 312.52 ft. The total loop length is 0.36 miles. Using C = 0.05 ft for third-order work, determine whether the loop closes within allowable limits.
  1. Step 1 — Compute the closure error: |312.52 − 312.45| = 0.07 ft
  2. Step 2 — Compute allowable closure: C × √M = 0.05 × √0.36 = 0.05 × 0.60 = 0.030 ft
  3. Step 3 — Compare: 0.07 ft > 0.030 ft, so the loop does NOT close within third-order allowable limits.
  4. Step 4 — Write a narrative flag in the field book: note the computed closure error of 0.07 ft, the allowable closure of 0.030 ft, and flag Benchmark A (or the loop) as unverified pending re-run or investigation.
Answer: Closure error = 0.07 ft; allowable = 0.03 ft. The loop fails third-order tolerance and must be re-run before layout proceeds.
2. At instrument setup 2 of a level loop, the elevation of TP1 is 308.72 ft and the backsight rod reading on TP1 is 4.83 ft. The foresight rod reading on TP2 is 6.21 ft. Calculate HI and the elevation of TP2.
  1. Step 1 — Compute HI: HI = Elevation of TP1 + BS = 308.72 + 4.83 = 313.55 ft
  2. Step 2 — Compute elevation of TP2: Elevation TP2 = HI − FS = 313.55 − 6.21 = 307.34 ft
Answer: HI = 313.55 ft; Elevation of TP2 = 307.34 ft
3. A total station occupying Control Point A (N 5000.00, E 5000.00) backsights Control Point B and then foresights an unknown monument. The polar measurement to the monument gives a northing departure of +48.37 ft and an easting departure of +22.15 ft. The record coordinates for the monument are N 5048.30, E 5022.20. Determine whether the monument is within a ±0.05 ft tolerance in each axis.
  1. Step 1 — Compute measured coordinates: N = 5000.00 + 48.37 = 5048.37 ft; E = 5000.00 + 22.15 = 5022.15 ft
  2. Step 2 — Compute northing discrepancy: |5048.37 − 5048.30| = 0.07 ft
  3. Step 3 — Compute easting discrepancy: |5022.15 − 5022.20| = 0.05 ft
  4. Step 4 — Compare to tolerance: Northing discrepancy 0.07 ft exceeds ±0.05 ft; easting discrepancy 0.05 ft is exactly at the limit (borderline — flag for supervisor review). The northing discrepancy requires a narrative flag and must not be used for layout without investigation.
Answer: Northing discrepancy = 0.07 ft (exceeds tolerance; flag point). Easting discrepancy = 0.05 ft (borderline; flag for review).
4. A level loop has a total length of 1.44 miles. Using C = 0.05 ft for third-order work, compute the maximum allowable closure error.
  1. Step 1 — Identify values: C = 0.05 ft, M = 1.44 miles
  2. Step 2 — Compute √M: √1.44 = 1.20
  3. Step 3 — Compute allowable closure: 0.05 × 1.20 = 0.06 ft
Answer: Maximum allowable closure error = 0.06 ft

The math in this lesson2

Learners compare a 0.07 ft level-loop closure to an allowable 0.05 x sqrt(0.36) = 0.030 ft.
N-RN.2N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Using a control map and description, students navigate to three simulated project control points on campus grounds, verify each with a total station and record measured vs. record coordinates · Run a two-wire differential level loop from a known benchmark through three turning points and back, compute the closure error, and compare it against the allowable closure for third-order work · Document discrepancies in a field book following NSPS field-note conventions: station, rod reading, HI, elevation, and a written narrative flag if any value is out of tolerance
Materials: Total station with tripods and tribrachs, automatic level, Philadelphia rods, field book, control point description sheets, flagging tape, plumb bobs, stake lath, NSPS CST Study Guide (control chapters)
Check: Level loop closure within third-order allowable limits; all three control points verified with coordinates recorded to 0.01 ft; field notes reviewed by instructor and signed off before next field session

Practice check8

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

NSPS CST L1-2 — Field procedures and record interpretation
1. Before setting up a total station to verify a control point, a technician should FIRST:
NSPS CST L1-2 — Differential leveling and accuracy standards
2. A level loop begins and ends at a benchmark with elevation 245.00 ft. After completing the loop the computed elevation is 245.08 ft. The loop length is 0.64 miles and the third-order constant C = 0.05 ft. Which statement is correct?
NSPS CST L1-2 — Differential leveling theory
3. When running a differential level loop, keeping backsight and foresight distances roughly equal primarily serves to:
NSPS CST L1-2 — Differential leveling computation
4. The Height of Instrument (HI) at a level setup is best described as:
NSPS CST L1-2 — Control verification and tolerance evaluation
5. A technician measures a control monument at N 6120.15, E 4350.08. The record shows N 6120.10, E 4350.05. The project tolerance is ±0.05 ft in each axis. What is the correct action?
NSPS CST L1-2 — Field note standards and record keeping
6. According to NSPS field-note conventions, which of the following items is REQUIRED on a level-loop note page?
NSPS CST L1-2 — Field note standards and quality control
7. A narrative flag in a field book is written when:
NSPS CST L1-2 — Field safety and personal protective equipment
8. When searching for a buried or obscured control monument near active vehicle traffic, a technician's MOST important safety precaution is to:
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