Field GNSS Data Collection — Control and Topographic Points

Why Control and Topographic Shots Matter

Every survey deliverable — a plat, a contour map, a drone-flight ground-truth check — rests on a foundation of accurately placed points. Control points anchor your project to a known coordinate system (typically NAD 83 or a state plane system). Topographic break-line shots capture the shape of the ground: the top and toe of a slope, the edge of pavement, the centerline of a road. If either type of point is collected carelessly, error compounds through every product that follows.

In this field session you will use a survey-grade GNSS rover operating in Real-Time Kinematic (RTK) or Network RTK mode to collect both point types on the campus survey range. You will document every observation in your field notebook, perform a closure check, and meet the project accuracy standard before the session ends.

RTK and Network RTK — A Quick Refresher

RTK GNSS works by pairing your rover with a base station that broadcasts real-time corrections over a radio or cellular link. The rover uses those corrections to resolve carrier-phase ambiguities and report a fixed solution — the only solution type acceptable for control and break-line work. A float solution means ambiguities are estimated, not resolved; accuracy degrades significantly and must never be accepted for final shots. A SBAS or autonomous solution is even less precise and is likewise unacceptable for this project.

Network RTK (e.g., a state Continuously Operating Reference Station network) replaces the single base with a modeled correction surface. The principle is the same: you need a fixed solution before you log a point.

Pre-Field Safety Briefing and PPE

Before stepping onto the range, every student must:

1. Review the Job Hazard Analysis (JHA) for the day. Identify traffic zones, overhead utilities, uneven terrain, and weather risks.

2. Don a high-visibility safety vest — mandatory any time you work near vehicle traffic or active roadways, consistent with OSHA requirements for work-zone visibility.

3. Wear a hard hat if the route passes through or near construction activity or overhead hazards.

4. Wear sturdy, closed-toe footwear suitable for uneven ground. Athletic shoes and sandals are not acceptable.

5. Carry water if conditions are warm; heat illness is a real hazard during outdoor fieldwork.

6. Never work alone. If your instructor leaves the area, buddy up with a classmate.

Equipment safety: handle the rover pole carefully near overhead power lines — carbon-fiber poles are non-conductive, but GNSS antennas mounted at 2 m height bring you closer to lines than you may realize. Keep the rover away from vehicles and never lay it flat on asphalt in direct sun.

Setting Up the Rover and Configuring the Job

1. Power on the rover and verify a correction link is active (radio bars or cellular signal on the controller display).

2. Create or open the project job file. Enter the correct coordinate system, geoid model (typically GEOID18 or current NGS model), and units (US survey feet or meters per project specs).

3. Enter your antenna height — the vertical distance from the ground to the bottom of the antenna phase center reference mark, accounting for any offset. An antenna height error of even 0.02 ft propagates to every point you shoot.

4. Confirm the solution type is Fixed before any observation. Most controllers display PDOP and satellite count; record both for every point.

Collecting Points — Field Procedure

Point codes describe what you are shooting. Common codes used on this project:

• IP — Iron Pin (existing monument)
• TBM — Temporary Benchmark
• EP — Edge of Pavement
• CL — Centerline
• TC — Top of Curb

For each point:

1. Hold the pole plumb (use the bubble level). For RTK, most systems require the pole to be still for the observation window — typically 1–3 seconds per shot, but follow the project specification.

2. Verify Fixed solution, PDOP ≤ the project threshold (commonly ≤ 3.0 for topographic work), and satellite count ≥ 5.

3. Log the point with the correct code and a brief description if needed.

4. Record in your field notebook immediately: point ID, code, PDOP, solution type, satellite count, and antenna height. Do not rely on controller memory alone — field notes are a legal record.

If a shot logs as float or the PDOP spikes, flag it in the controller and notebook. Re-initialize and re-shoot before moving on.

Field Closure Check

After collecting all required points, return to the first control monument you observed at the start of the session. Re-observe it using the same procedure. Compute the horizontal and vertical differences between the opening and closing observations:

• Horizontal close = √[(ΔE)² + (ΔN)²]
• Vertical close = |Elev_close − Elev_open|

Compare both values to the project tolerance (instructor will specify; a common standard for RTK topographic work is ≤ 0.05 ft horizontal, ≤ 0.10 ft vertical). Document the result in your field log with a pass/fail notation. A failing closure means you must investigate: check antenna height entry, check for multipath obstruction, and re-observe as directed.

Wrapping Up

Back in the office, back up your raw data file from the controller. Your field notebook and the electronic job file together constitute your complete record. The notebook must be legible, complete, and signed before you leave for the day. Incomplete field notes are grounds for repeating the exercise.