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TEC-GIS45-09 · Lesson 2 · 72 clock hours · lab

Multi-Method Field Data Collection: GNSS, Total Station, and Drone

Objective. Students will occupy the project site over multiple sessions and collect a complete, usable dataset consisting of a closed traverse with sideshots, GNSS-observed ground control points (GCPs), and overlapping drone imagery—all logged with calibrated field notes and meeting the accuracy specifications defined in the project plan.

⚠ 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.

Overview

This lab brings together every major skill in your program: traversing with a total station, occupying ground control points (GCPs) with a GNSS receiver, and flying a drone under FAA Part 107 rules. You will leave the field with three interlocking datasets that feed directly into photogrammetric processing and GIS analysis. A weakness in any one dataset can compromise the whole project, so accuracy and documentation are non-negotiable.

Safety First

Before anyone picks up equipment, review the site hazard assessment. Near a roadway, place traffic cones and wear a high-visibility safety vest and hard hat at all times—these are minimum PPE for roadside survey work. Never stand in a travel lane to set up a tripod. If you are near overhead power lines, keep the prism pole, tripod legs, and drone well clear of energized conductors; there is no safe "close enough."

For drone operations, establish a clear safety briefing with your visual observer (VO) before each flight. The remote pilot in command (RPIC) holds final authority over whether conditions are safe to fly. Check wind speed, cloud ceiling, and airspace restrictions. Keep bystanders outside the operating area. After landing, treat the battery as a fire hazard: do not leave a swollen lithium-polymer pack unattended, and transport batteries in a fire-resistant bag.

Part 1 — Closed Traverse with the Total Station

A closed traverse loop connects a series of instrument stations so that the survey returns to its starting point, allowing you to compute angular and linear closure. Your loop must contain at least four points.

At each station, set up the total station over the point using a tribrach and optical or laser plummet. Record the instrument height (HI) and target height (HR) in your bound field book—every page, every setup. These heights are required to reduce slope distances to horizontal and to propagate elevations.

Measure each interior angle by pointing to the backsight, zeroing the horizontal circle, then turning to the foresight and reading the angle. Record raw readings; do not erase mistakes—cross them out with a single line and initial. Apply the EDM prism constant (typically −30 mm for a standard prism; confirm with the manufacturer's label) before recording slope distances. Reduce each slope distance to horizontal distance using the vertical angle or percent-grade reading.

After completing the loop, compute the angular misclosure (sum of interior angles versus the theoretical value of (n − 2) × 180°, where n is the number of sides). Distribute angular error equally among the angles, then compute latitudes and departures. The linear closure ratio is the ratio of the linear misclosure to the total traverse perimeter. A ratio better than 1:5,000 is your minimum specification. Apply the Compass Rule adjustment to distribute the linear misclosure proportionally to each leg length.

Part 2 — GNSS Ground Control Points

GCPs tie your drone imagery to a real-world coordinate system. You need at least five targets distributed across the project area, including corners and a central point, to support photogrammetric bundle adjustment.

Place GCP targets (pre-marked panels or painted crosses) before the drone flight so they appear clearly in the imagery. Occupy each target with the survey-grade GNSS receiver. In RTK mode, wait for the solution to initialize and check the displayed horizontal precision indicator; your project specification requires 0.05 m (5 cm) or better before you log the point. Record occupation start time, end time, number of satellites, and the dilution-of-precision (DOP) values—both PDOP and HDOP—in your field book.

Compare at least one GCP solution against the published coordinates of a nearby NGS Continuously Operating Reference Station (CORS) benchmark to verify that your receiver and correction source are functioning correctly. If any point fails the precision check, re-observe before you leave the site.

Part 3 — UAS Imagery Collection

The remote pilot in command must hold a valid FAA Part 107 remote pilot certificate. Before takeoff, complete the printed pre-flight checklist: airspace authorization (check LAANC or a waiver if required), battery voltage, propeller condition, camera focus and exposure settings, and VO communication plan.

Fly at an altitude that achieves a ground sampling distance (GSD) of 3 cm/pixel or better. GSD decreases as altitude decreases, so lower flight means finer detail but more flight time. Configure your mission software for at least 75% forward (endlap) overlap and 60% sidelap. These overlap percentages ensure that every point on the ground appears in multiple images, which is required for photogrammetric reconstruction.

The VO scans the airspace for manned aircraft and alerts the RPIC of any conflict or hazard. Maintain visual line of sight with the aircraft at all times. Log actual flight altitude, battery consumption, start and end times, wind conditions, and any anomalies in the flight log immediately after landing. After landing, conduct a post-flight inspection: check for propeller nicks, loose motor mounts, and battery condition.

Bringing It Together

Your three datasets—traverse coordinates, GNSS GCP coordinates, and drone imagery—will be imported into ArcGIS Pro and photogrammetric software in the next lessons. Clean, accurate field notes are the foundation. An undocumented observation cannot be verified; if it cannot be verified, it cannot be used. Complete every column in every table before you leave the field.

Key terms10

Closed traverseA series of survey lines that begins and ends at the same point (or two points of known position), allowing angular and linear misclosure to be computed and corrected.
Compass Rule adjustmentA traverse adjustment method that distributes linear misclosure in latitude and departure proportionally to the length of each traverse leg.
Closure ratioThe ratio of the linear misclosure of a traverse to the total traverse perimeter; expressed as 1:N, where a larger N indicates higher accuracy (e.g., 1:5,000).
Ground control point (GCP)A surveyed point of known coordinates placed within the drone imagery footprint so that photogrammetric software can georeference and scale the 3-D reconstruction.
Ground sampling distance (GSD)The real-world distance represented by one pixel in an aerial image; smaller GSD means finer spatial resolution and is achieved by flying lower or using a higher-resolution sensor.
Forward overlap (endlap)The percentage of an aerial image that is repeated in the next image along the flight path; typically 75% or more is required for photogrammetric processing.
Dilution of precision (DOP)A dimensionless number describing how satellite geometry affects GNSS positional accuracy; lower values indicate better geometry. HDOP applies to horizontal position and PDOP to three-dimensional position.
EDM prism constantA fixed offset (often expressed in millimeters) that accounts for the physical path of light through a retroreflective prism; it must be applied to all electronic distance measurements to obtain the true distance.
CORS (Continuously Operating Reference Station)A fixed, continuously tracking GNSS station whose coordinates are published by the NGS; used as an external check or correction source for survey-grade positioning.
Remote pilot in command (RPIC)The FAA-certificated Part 107 pilot who holds final authority and responsibility for the safety of a small UAS operation.

Worked examples4

1. A four-sided closed traverse has a total perimeter of 684.22 m. After computing latitudes and departures, the closure in latitude is +0.08 m and the closure in departure is +0.06 m. Compute the linear misclosure and the closure ratio.
  1. Step 1 — Compute linear misclosure: misclosure = √(ΔLat² + ΔDep²) = √(0.08² + 0.06²) = √(0.0064 + 0.0036) = √0.0100 = 0.10 m
  2. Step 2 — Compute closure ratio: ratio = perimeter ÷ misclosure = 684.22 ÷ 0.10 = 6,842
  3. Step 3 — Express as 1:N and compare to spec: 1:6,842, which is better than the required 1:5,000 (6,842 > 5,000), so the traverse passes.
Answer: Linear misclosure = 0.10 m; closure ratio = 1:6,842; the traverse meets the 1:5,000 specification.
2. Apply the Compass Rule to distribute the latitude misclosure of +0.10 m across two traverse legs. Leg AB has a horizontal length of 210.00 m; leg BC has a horizontal length of 190.00 m. Total perimeter = 684.22 m. What latitude correction is applied to each leg?
  1. Step 1 — The total latitude misclosure to distribute is −0.10 m (corrections are opposite in sign to the misclosure, so we subtract it).
  2. Step 2 — Correction for leg AB = −(leg length ÷ perimeter) × latitude misclosure = −(210.00 ÷ 684.22) × 0.10 = −0.3069 × 0.10 = −0.031 m (rounded to 3 decimal places).
  3. Step 3 — Correction for leg BC = −(190.00 ÷ 684.22) × 0.10 = −0.2778 × 0.10 = −0.028 m.
  4. Step 4 — These corrections are added algebraically to the computed latitude of each leg before computing adjusted coordinates.
Answer: Leg AB latitude correction = −0.031 m; Leg BC latitude correction = −0.028 m.
3. You plan to fly a drone mission at 80 m AGL with a camera sensor width of 13.2 mm, an image width of 5,472 pixels, and a lens focal length of 8.8 mm. Calculate the ground sampling distance (GSD) in cm/pixel.
  1. Step 1 — Compute the image footprint width on the ground using similar triangles: footprint width = (flight altitude × sensor width) ÷ focal length = (80 × 0.0132) ÷ 0.0088 = 1.056 ÷ 0.0088 = 120 m.
  2. Step 2 — Compute GSD: GSD = footprint width ÷ image width in pixels = 120 m ÷ 5,472 pixels = 0.02193 m/pixel.
  3. Step 3 — Convert to cm/pixel: 0.02193 m × 100 = 2.19 cm/pixel.
  4. Step 4 — Compare to spec: 2.19 cm/pixel is finer (smaller) than the required ≤ 3 cm/pixel, so the planned altitude meets the specification.
Answer: GSD ≈ 2.19 cm/pixel, which satisfies the ≤ 3 cm/pixel project specification.
4. A traverse measured with a standard prism (prism constant = −30 mm) shows a raw EDM slope distance of 143.450 m and a vertical angle of +3°12′. Calculate the corrected horizontal distance.
  1. Step 1 — Apply prism constant to get the true slope distance: 143.450 m + (−0.030 m) = 143.420 m.
  2. Step 2 — Convert vertical angle to decimal degrees: 3°12′ = 3 + 12/60 = 3.200°.
  3. Step 3 — Compute horizontal distance: HD = slope distance × cos(vertical angle) = 143.420 × cos(3.200°) = 143.420 × 0.99844 = 143.196 m.
  4. Step 4 — Record 143.196 m as the horizontal distance for this leg in the field book.
Answer: Corrected horizontal distance = 143.196 m.

The math in this lesson2

Learners compute 0.10 m linear misclosure from latitude and departure errors and a 1:6,842 closure ratio.
6.RP.38.G.8
In class: Establish and occupy a minimum four-point traverse loop using the total station; record raw angle and distance observations in a bound field book, apply instrument corrections (instrument-to-target height, EDM prism constant), and close the traverse; compute closure ratio and perform a Compass Rule adjustment, targeting closure better than 1:5,000. · Occupy at least five GCP targets with a survey-grade GNSS receiver in static or RTK mode (per site conditions), record occupation times and dilution-of-precision values, and post-process or confirm coordinate solutions against a local NGS CORS benchmark; achieve horizontal precision of 0.05 m or better before leaving the field. · Fly the UAS (fixed-wing or multirotor) under FAA Part 107 rules with a second student acting as visual observer; capture nadir and, where required, oblique imagery at a planned ground sampling distance ≤ 3 cm/pixel with ≥ 75% forward lap and ≥ 60% sidelap; complete a pre-flight checklist, log all flight parameters in the flight log, and conduct a post-flight safety inspection.
Materials: Total station with prism and tribrach, GNSS receiver (survey grade) with data collector, UAS (multirotor or fixed-wing with camera payload), GCP targets and stakes, prism pole, field books, plumb bobs, safety vest and hard hat, traffic cones if near roadway, FAA Part 107 remote pilot certificates, pre-flight checklist, flight log, NGS data sheets for local benchmark, Esri ArcGIS Field Maps or equivalent for attribute capture
Check: Instructor spot-checks field books for completeness (station names, backsight/foresight readings, occupation times, weather), verifies traverse closure ratio is documented and meets spec, confirms GNSS coordinate solutions show acceptable precision indicators, and reviews drone flight log for regulatory compliance. Pass = all three data sets accepted as complete and meeting accuracy spec; any dataset failing spec requires a documented re-observation before processing begins.

Practice check8

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

NSPS CST L1-2 — Traverse computation and adjustment
1. A closed traverse loop has an angular misclosure of +36 arcseconds across six interior angles. Using equal distribution, how much correction is applied to each observed angle?
NSPS CST L1-2 — Traverse adjustment
2. In the Compass Rule traverse adjustment, corrections to latitudes and departures are distributed proportionally to which quantity?
FAA Part 107 — Regulations and responsibilities
3. Under FAA Part 107 rules, which person holds final authority and responsibility for the safety of a small UAS operation?
NSPS CST L1-2 — GNSS field procedures
4. A survey-grade GNSS receiver in RTK mode displays a horizontal precision indicator of 0.08 m on a GCP. According to the project specification, what must the field crew do?
FAA Part 107 / UAS operations — Mission planning
5. Which combination of image overlap values satisfies the minimum requirements stated in this project plan for drone photogrammetry?
NSPS CST L1-2 — Instrument corrections and EDM
6. What is the purpose of recording the EDM prism constant and applying it to every slope distance measurement?
Safety — PPE and site hazard control
7. You are flying a drone mission near a public road. Which of the following safety measures is required for the field crew on the ground?
NSPS CST L1-2 / Esri ArcGIS Pro — GNSS reference and geospatial data quality
8. A GIS technician using Esri ArcGIS Field Maps wants to verify that GNSS GCP coordinates are connected to a reliable geodetic reference. Which resource provides published coordinates for a nearby permanent GNSS reference station?
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