2.1 Records, Field, GNSS, and Computations

Why this section matters

The PS exam treats surveying as a professional process, not a collection of isolated instrument skills. A correct coordinate can still be professionally weak if the record search was incomplete, a field note cannot be interpreted later, or a GNSS height was delivered in the wrong vertical reference system.

NCEES places records research, field procedures, Global Navigation Satellite System (GNSS), computations, maps, reports, Geographic Information System (GIS) datums, and metadata inside Professional Survey Practice. The practical exam question is often: what should a prudent surveyor do next?

Records research: build the evidence file first

Records research is the office work that tells the crew what evidence to look for and tells the surveyor what the field evidence means. Do not start with only the current deed and a parcel polygon. The better starting package includes the subject chain, current adjoiner descriptions, prior plats or surveys, title commitment or title evidence, easements, rights of way, subdivision plats, road records, control sheets, and relevant agency maps.

Adjoiner research matters because a boundary is not solved parcel by parcel in isolation. A line in the subject deed may call for an adjoiner, a senior conveyance, a road, a water boundary, or a monument described more clearly in a neighbor's deed. Prior surveys are not automatically controlling, but they preserve professional opinions, monument calls, occupation evidence, and witness ties that may explain why a corner is where it is.

Records checklist

Field notes, redundancy, and check shots

Field notes should let another competent surveyor understand what was found, what was measured, and why a point was accepted, rejected, or held for later analysis. For boundary work, notes should describe monument type, size, cap markings, condition, depth, relation to occupation, search methods, witness ties, photographs, and any conflicting evidence.

Redundancy means collecting more observations than the minimum needed to compute a position. Redundancy creates checks. A single shot can look clean even when it contains a prism-height error, bad backsight, poor RTK initialization, wrong rod point, or transposed field code. A second independent observation gives the surveyor a way to detect the problem.

Check shots are independent verification observations. They may include reoccupying a point from a different setup, checking into a known control point, repeating a GNSS observation after reinitialization, taking a backsight check after moving the instrument, or measuring a tie with a tape where a total station distance is suspect.

A useful field rule is: critical evidence deserves an independent check before the crew leaves. If a corner, benchmark, flood elevation, construction offset, or control point will drive later decisions, it should be checked while the crew can still inspect the site.

GNSS: coordinates are not self-explaining

GNSS is powerful because it connects field positions to geodetic reference frames. It is risky because the numbers look precise even when the reference information is missing. On the PS exam, a coordinate deliverable without the reference system should make you suspicious.

Key terms:

• Datum: the reference frame for positions on the earth.
• Realization and epoch: the version and date of a dynamic frame, important where coordinates change through time.
• Projection: the mathematical conversion from curved-earth coordinates to a flat grid.
• Geoid model: the model used to convert GNSS ellipsoid heights to approximate elevations above the geoid.
• Orthometric height: the elevation normally used for flood, construction, and engineering work; GNSS does not directly observe it.
• Scale factor or combined factor: the relationship between grid distances and ground distances.

For vertical work, remember the relationship in concept: orthometric height is derived by applying a geoid separation to the ellipsoid height. The exam usually tests the professional consequence, not a memorized formula. If a rover reports ellipsoid heights but the deliverable requires North American Vertical Datum of 1988 (NAVD 88) elevations, the surveyor must apply the appropriate geoid model and document the vertical datum.

RTK versus static GNSS

Real-Time Kinematic (RTK) GNSS gives rapid corrected positions in the field. It is common for topo pickup, stakeout, and production observations when site conditions and project tolerances allow it. Its weaknesses are multipath, poor satellite geometry, tree canopy, weak initialization, radio or network correction loss, and overconfidence in a fixed solution.

Static GNSS uses longer simultaneous observations on fixed receivers, usually with post-processing. It is slower, but it is better suited to primary control, long baselines, and networks that must be adjusted and defended. For a high-value control network, static observations with redundant sessions and independent checks are usually stronger than a single RTK visit.

Use this comparison for exam scenarios:

Computations, adjustments, residuals, and misclosure

Misclosure is the failure of a measured figure to close on its expected endpoint or angular sum. It is an early warning, not something to hide with an adjustment. If a traverse misclosure is outside tolerance, first look for blunders: wrong point, bad backsight, prism constant, rod height, transposed angle, incorrect control, or unit mismatch.

Least squares adjustment distributes random observation error through a redundant network according to observation weights. It produces adjusted coordinates, residuals, and precision estimates. A properly weighted least-squares adjustment is stronger than a purely mechanical balancing rule because it can show whether observations fit the network and where the weak observations may be.

A residual is the difference between an observed value and the adjusted value. Large residuals, patterned residuals, or residuals clustered by instrument, crew, time, or method can indicate a blunder, poor weight assignment, unmodeled systematic error, or weak geometry. The professional response is to investigate, not to accept the result because software produced a report.

Exam workflow for computations

1. Check raw observations and field notes before adjusting.
2. Confirm datums, units, scale factors, control values, instrument constants, and point IDs.
3. Evaluate misclosure against project tolerance and applicable standards.
4. Apply an appropriate adjustment method only after blunders are addressed.
5. Review residuals, weights, confidence estimates, and whether the result supports the deliverable.
6. Document assumptions in the report, plat notes, metadata, or project file.

Professional judgment point

Computations can prove internal consistency, but they do not decide legal boundaries by themselves. A closed traverse around the wrong monuments is still wrong. A precise GNSS coordinate on an unverified point is still weak evidence. The PS exam rewards the surveyor who ties measurements back to records, field evidence, standards, and the purpose of the survey.