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TEC-GIS45-05 · Lesson 8 · 12 clock hours · lab

GNSS-Assisted Layout and Stake-Out

Objective. Students use a GNSS RTK receiver loaded with design coordinates to stake out five design points, achieving horizontal accuracy within the manufacturer-specified RTK tolerance and recording fixed-solution quality flags

⚠ 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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GNSS-Assisted Layout and Stake-Out

What Is RTK and Why Does It Matter for Construction Layout?

Real-Time Kinematic (RTK) GNSS is a differential positioning technique that delivers centimeter-level accuracy in the field. A base station—either a dedicated rover/base pair or a network correction service accessed via NTRIP (Networked Transport of RTCM via Internet Protocol)—broadcasts correction data to the rover in real time. The rover's onboard processor applies these corrections and reports a fixed solution when it has resolved the carrier-phase ambiguities. A fixed solution is the highest-quality output of an RTK system. Manufacturers typically specify horizontal accuracy for a fixed solution in the range of 8–20 mm (plus a parts-per-million component based on baseline length), though you must always consult your specific instrument's manual for the published specification. Float and autonomous solutions carry significantly larger errors and are not acceptable for construction stakeout.

For construction layout, achieving a fixed solution before setting any stake is non-negotiable. The data collector will display a solution quality indicator—commonly labeled Fixed, Float, or Autonomous. Only proceed when the display reads Fixed.

Coordinate System and Geoid Model Setup

Before you touch a hub or tack, verify three things in your data collector:

1. Project coordinate system – The design file was created in a specific horizontal datum and projection (for example, NAD83 / State Plane). Your receiver must be configured to match. A mismatch here will shift every point you stake by potentially hundreds of feet.

2. Geoid model – GNSS antennas measure ellipsoidal heights relative to the GRS80/WGS84 ellipsoid, not orthometric (mean sea level) heights. A geoid model (such as GEOID18 or a later model from NGS) converts ellipsoidal height to orthometric height. If the design uses orthometric elevations—and nearly all civil plans do—the correct geoid model must be loaded and activated.

3. Design coordinate file – Import your CSV file and confirm that point identifiers, northings, eastings, and elevations display correctly. A quick sanity check: open a known benchmark point in the file and verify its coordinates against the plan sheet before field work begins.

The Stake-Out Workflow

Once the system checks are complete and a fixed solution is confirmed, the data collector's stake-out routine guides you to each design point. The software computes the vector (distance and bearing) from your current position to the target point and updates this vector continuously as you walk. The typical workflow is:

1. Select the target point from your imported coordinate list.

2. Walk toward the target, watching the real-time guidance arrows or distance display.

3. When you are within a few centimeters of the target, stop moving and allow the solution to settle for several seconds.

4. Drive a hub (a flat-topped wood stake) flush with the ground surface, then set a tack (a small nail) at the exact position indicated by the rod tip.

5. Place lath (a colored stake) beside the hub with a flag and any cut/fill notation.

6. Record the as-staked coordinates and solution quality flag displayed by the data collector at the moment of acceptance. Most data collectors log this automatically when you save the observation.

Repeat for all five design points. Do not skip the solution-quality check at any point.

Cut and Fill Notation

The data collector computes the vertical difference between the design elevation and the current ground elevation. A cut means the ground must be removed to reach design grade; a fill means material must be added. Write the cut or fill value on the lath in the conventional format: C 0.45 (cut 0.45 ft) or F 0.30 (fill 0.30 ft). Consistent notation prevents costly grading errors.

Computing and Documenting Horizontal Residuals

Back in the office, open your spreadsheet and set up columns for Point ID, Design Northing, Design Easting, As-Staked Northing, As-Staked Easting, Delta-N, Delta-E, and Horizontal Residual. The horizontal residual is computed as:

Horizontal Residual = √(ΔN² + ΔE²)

Compare each residual against the manufacturer's published RTK horizontal accuracy specification. Any point that exceeds the specification must be flagged with a note indicating whether a fixed solution was confirmed at the time of staking. If a fixed solution was confirmed and the point still exceeds tolerance, the point should be re-staked.

Safety in the Field

Key terms9

RTK (Real-Time Kinematic)A differential GNSS technique that transmits carrier-phase corrections from a base station to a rover in real time, enabling centimeter-level positioning accuracy in the field.
Fixed SolutionThe highest-quality RTK output, achieved when the receiver has resolved carrier-phase integer ambiguities; required for construction-grade stakeout.
NTRIPNetworked Transport of RTCM via Internet Protocol; a standard that delivers GNSS correction data from a network of reference stations to a rover via a cellular or internet connection, eliminating the need for a dedicated base station.
Geoid ModelA mathematical model (such as GEOID18) that describes the separation between the ellipsoid and the geoid, allowing conversion of GNSS ellipsoidal heights to orthometric (mean sea level) heights.
Hub and TackA wooden stake driven flush with the ground (hub) with a small nail (tack) set precisely at the staked position; the standard physical monument used in construction layout.
Horizontal ResidualThe two-dimensional distance between a design (intended) point and the as-staked point, computed as the square root of the sum of the squared northing and easting differences.
Cut / FillThe vertical difference between existing ground elevation and design grade; cut indicates material to be removed, fill indicates material to be added.
As-Staked CoordinatesThe actual GNSS-measured position recorded by the data collector at the moment a stake is accepted and saved, used later to verify compliance with design tolerances.
LathA thin, colored wooden stake placed adjacent to a hub to provide a visible reference for grade information, point identification, and cut/fill notation.

Worked examples4

1. Design Point 1 has coordinates N 5000.000, E 5000.000. After staking, your data collector records N 5000.012, E 4999.993. What is the horizontal residual?
  1. Compute ΔN: 5000.012 − 5000.000 = +0.012 ft
  2. Compute ΔE: 4999.993 − 5000.000 = −0.007 ft
  3. Square each difference: ΔN² = 0.012² = 0.000144; ΔE² = (−0.007)² = 0.000049
  4. Sum the squares: 0.000144 + 0.000049 = 0.000193
  5. Take the square root: √0.000193 ≈ 0.0139 ft
Answer: Horizontal residual ≈ 0.014 ft (rounded to three decimal places). This is approximately 4 mm, well within a typical RTK fixed-solution specification of ~0.030 ft (approximately 10 mm). Point passes.
2. Design Point 3 has coordinates N 5150.000, E 5075.000. As-staked coordinates are N 5150.038, E 5075.029. The manufacturer's published RTK horizontal accuracy is 0.033 ft. Does this point pass or fail?
  1. Compute ΔN: 5150.038 − 5150.000 = +0.038 ft
  2. Compute ΔE: 5075.029 − 5075.000 = +0.029 ft
  3. Square each: ΔN² = 0.038² = 0.001444; ΔE² = 0.029² = 0.000841
  4. Sum: 0.001444 + 0.000841 = 0.002285
  5. Square root: √0.002285 ≈ 0.0478 ft
  6. Compare to specification: 0.0478 ft > 0.033 ft
Answer: Horizontal residual ≈ 0.048 ft. This exceeds the 0.033 ft tolerance. The point fails and must be flagged in the spreadsheet with a note on solution quality. If a fixed solution was active, the point should be re-staked.
3. Your design coordinates are in US Survey Feet on NAD83 State Plane. Your GNSS receiver displays an ellipsoidal height of 312.456 ft at a staked point. The geoid height (N) at that location, from the loaded geoid model, is −96.2 ft. What is the orthometric height of the staked point?
  1. Recall the relationship: Orthometric Height (H) = Ellipsoidal Height (h) − Geoid Height (N), where N is the separation value provided by the geoid model.
  2. Note: geoid height is often negative in the contiguous US (the geoid lies below the ellipsoid), meaning h − (−96.2) = h + 96.2.
  3. Wait—re-examine the sign convention. If the geoid model reports N = −96.2 ft (geoid below ellipsoid), then H = h − N = 312.456 − (−96.2) = 312.456 + 96.2.
  4. H = 408.656 ft
Answer: Orthometric height ≈ 408.656 ft. This value is compared to the design elevation shown on the plan to determine cut or fill. Always confirm the sign convention used by your specific geoid model and data collector software.
4. After staking all five points, your horizontal residuals are: P1 = 0.011 ft, P2 = 0.019 ft, P3 = 0.048 ft, P4 = 0.024 ft, P5 = 0.016 ft. The published RTK horizontal accuracy is 0.033 ft. How many points pass? Does the lab check requirement (at least 4 of 5 within tolerance) pass?
  1. Compare each residual to 0.033 ft: P1 0.011 < 0.033 → Pass; P2 0.019 < 0.033 → Pass; P3 0.048 > 0.033 → Fail; P4 0.024 < 0.033 → Pass; P5 0.016 < 0.033 → Pass.
  2. Count passing points: P1, P2, P4, P5 = 4 points pass.
  3. Lab requirement: at least 4 of 5 within tolerance → 4 ≥ 4.
Answer: Four of five points pass. The lab check requirement is met. Point 3 must be documented in the spreadsheet with a note, and the solution quality flag at the time of staking must be recorded.
In class: Import a design coordinate file into the data collector, verify the project coordinate system and geoid model loaded, and confirm a fixed RTK solution before beginning stake-out · Navigate to each of five design points using the data collector's stake-out routine, set a hub-and-tack at the cut-to position indicated by the receiver, and record the as-staked coordinates and solution quality · Compare as-staked coordinates against design using a spreadsheet, compute horizontal residuals, and document any point exceeding tolerance with a note on whether a fixed solution was maintained
Materials: GNSS RTK rover and base (or NTRIP subscription), data collector with stake-out software, design coordinate CSV file, hubs and tacks, lath, field book, laptop with spreadsheet software, GNSS manufacturer reference manual
Check: All five points staked with fixed RTK solution confirmed; horizontal residuals documented; at least four of five points within published RTK horizontal accuracy specification; spreadsheet comparison submitted

Practice check8

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

NSPS CST L1-2: Field Measurement Procedures
1. Before beginning a stake-out with an RTK GNSS receiver, which solution quality must be confirmed on the data collector?
NSPS CST L1-2: Vertical Control and Datums
2. A design coordinate file uses orthometric elevations referenced to NAVD88. To correctly convert GNSS ellipsoidal heights to orthometric heights in the field, the data collector must have loaded:
NSPS CST L1-2: Calculations and Data Reduction
3. Design Point A has coordinates N 6200.000, E 4800.000. You record as-staked coordinates of N 6200.018, E 4800.024. What is the horizontal residual?
FAA Part 107 / General Occupational Safety
4. During stake-out on an active construction site, you need to raise your 2-meter GNSS rover pole to read a point near a structure. What hazard must you check for first?
NSPS CST L1-2: Construction Layout
5. Your data collector shows a cut of 1.35 ft at a staked point. What does this mean for the grading crew?
NSPS CST L1-2: GNSS Technology
6. NTRIP is used in RTK surveying primarily to:
NSPS CST L1-2: Calculations and Data Reduction
7. After staking all five points, residuals are 0.009, 0.021, 0.028, 0.051, and 0.017 ft. The published RTK horizontal accuracy specification is 0.033 ft. Which statement correctly describes the result?
NSPS CST L1-2: Coordinate Systems and Datums
8. When you import a design coordinate CSV file into the data collector and the first point displays a northing that is several hundred feet away from the expected location on the map, the most likely cause is:
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