PS Module 3: Standards & Specifications

What is relative positional accuracy?

Relative positional accuracy describes the accuracy of one survey point relative to another, expressed at the 95% confidence level. It is computed from the error ellipse dimensions of the adjusted coordinates. For ALTA/NSPS surveys, the standard is 2 cm + 50 ppm. Relative accuracy is often more meaningful than absolute accuracy for boundary surveys because the relationship between nearby points matters most.

What is a least squares adjustment?

A statistical method for adjusting redundant survey observations that minimizes the sum of the weighted squares of the residuals. It provides: (1) the most probable values of the unknowns, (2) error estimates for each adjusted value, (3) statistical tests for blunders and systematic errors, and (4) confidence regions (error ellipses) for adjusted positions. It is the preferred method for adjusting survey networks.

What are the three types of survey errors?

(1) Blunders (gross errors) — mistakes such as reading the wrong target, transposing numbers, or misidentifying a point. Eliminated by checking procedures and redundant measurements. (2) Systematic errors — consistent, predictable errors from known causes (temperature, curvature, refraction). Eliminated by calibration and applying corrections. (3) Random errors — small, unpredictable variations that remain after blunders and systematic errors are removed. Reduced by averaging multiple observations.

What is the 95% confidence level and why is it the standard for survey accuracy?

The 95% confidence level means that, based on statistical analysis, the true position lies within the stated accuracy 95 out of 100 times. It corresponds to approximately 2 standard deviations (1.96σ) for a normal distribution. It is the FGCS and ALTA/NSPS standard because it provides high confidence without requiring the extreme accuracy needed for 99% or higher confidence levels.

What is positional tolerance and how does it differ from closure ratio?

Positional tolerance (e.g., 0.02 m + 50 ppm) expresses accuracy as a maximum allowable positional uncertainty between any two points. Closure ratio (e.g., 1:15,000) expresses traverse accuracy as the ratio of misclosure to total distance. Positional tolerance is the modern standard (used in ALTA/NSPS 2021) because it relates directly to point positions; closure ratio is an older metric that only measures internal consistency of a traverse, not actual positional accuracy.

What is the difference between relative and absolute accuracy?

Relative accuracy describes the positional relationship between two nearby survey points. Absolute accuracy describes the position of a point relative to a defined coordinate system (datum). A local boundary survey may have excellent relative accuracy between its corners but lower absolute accuracy relative to the geodetic datum. For most boundary work, relative accuracy is the more important metric.

What is the leveling closure formula?

Allowable closure = C × √(D), where C is a constant based on the order/class of the survey and D is the distance in kilometers (or miles, depending on the standard). For example, Third Order leveling allows 12 mm × √(D km). The square root relationship reflects that random errors accumulate as the square root of the number of measurements.

What are the key elements of a quality assurance/quality control (QA/QC) program for surveys?

Key elements include: (1) documented procedures and standards, (2) instrument calibration schedules, (3) independent checks and redundant measurements, (4) review of computations by a second qualified person, (5) field check procedures (closing the horizon, check angles, repeated observations), (6) comparison of results against expected values, and (7) documentation and record-keeping. QA prevents errors; QC detects them.

What are the FGCS classification standards for geodetic control surveys?

FGCS classifies horizontal control surveys by order and class: First Order (highest accuracy, for national network), Second Order (Class I and II, for regional networks), and Third Order (Class I and II, for local networks). Each classification has specific accuracy requirements, observation procedures, and closure standards. The required accuracy increases with each higher order.

What is the allowable traverse closure for a Second Order, Class I survey?

For a Second Order, Class I traverse: horizontal angular closure of 1.0" per station and positional closure of 1:50,000. These standards come from the FGCS Specifications for Geodetic Control Networks. The requirements ensure the traverse is suitable for densifying the geodetic control network at the regional level.

What is an error budget in survey planning?

An error budget identifies all sources of error in a measurement process and allocates a portion of the total allowable error to each source. The total error is the root sum of squares (RSS) of the individual components. Error budgets help surveyors plan observations to meet accuracy requirements — if one error source dominates, additional effort should focus on reducing that source.

What is the combined scale factor in GNSS surveys?

The combined scale factor is the product of the grid scale factor (relating geodetic distances to state plane or grid distances) and the elevation factor (relating distances at elevation to distances on the ellipsoid). It must be applied when converting between ground distances and grid distances. Failure to apply the combined scale factor is a common source of error when integrating GNSS positions into local surveys.

What is error propagation?

Error propagation is the mathematical process of computing the uncertainty of a derived quantity from the uncertainties of the measured quantities used to calculate it. For independent measurements, the total error is the root sum of squares (RSS): σ_total = √(σ₁² + σ₂² + ... + σₙ²). Understanding error propagation allows surveyors to predict final accuracy from individual measurement uncertainties.

What is PDOP and why does it matter for GNSS surveys?

Position Dilution of Precision (PDOP) is a dimensionless number that indicates the geometric strength of the satellite constellation at the time of observation. Lower PDOP values (under 4) indicate strong geometry and better position accuracy. High PDOP (over 6) indicates poor satellite geometry and degraded accuracy. PDOP is one factor in overall positional accuracy: σ_position ≈ PDOP × σ_range.

What is network accuracy vs. local accuracy?

Network accuracy describes the accuracy of a point relative to the national geodetic reference frame (the NSRS). Local accuracy describes the accuracy of a point relative to nearby control points. A point can have good local accuracy (well-determined relative to nearby stations) but poor network accuracy (not well-connected to the national reference frame), or vice versa. Both are reported by NGS for their published control.

What observation requirements apply to static GNSS surveys for control?

Static GNSS surveys for control typically require: (1) dual-frequency receivers, (2) minimum observation periods (varies by baseline length — typically 1-2 hours for short baselines, longer for extended baselines), (3) minimum number of satellites observed (usually 4+, ideally 5+), (4) low PDOP values (under 4-6), (5) repeat observations on different days for independent checks, and (6) processing and adjustment meeting applicable accuracy standards.

What is the difference between a compass rule adjustment and a least squares adjustment?

The compass rule (Bowditch) distributes traverse misclosure in proportion to distance traveled — it is a simple, deterministic method suitable for well-conditioned traverses. Least squares adjustment uses all redundant observations to find the most probable solution and provides statistical measures of quality (error ellipses, residuals). Least squares is the superior method for complex networks but compass rule remains acceptable for simple traverses meeting closure standards.

What is the purpose of instrument calibration and how often should it be performed?

Calibration ensures that an instrument measures within its specified accuracy by comparing its readings against a known standard and applying corrections. Frequency depends on the instrument type and use: total stations should be checked at the beginning of each project and after any impact or transport damage; levels should be checked daily (peg test); GNSS antennas should have current calibration values from NGS or the manufacturer. Calibration records must be maintained.

What is the NGS OPUS system?

The Online Positioning User Service (OPUS) is a free NGS service that processes static GNSS data to determine accurate coordinates tied to the National Spatial Reference System (NSRS). Users upload RINEX observation files, and OPUS returns NAD 83 and NAVD 88 positions based on processing against CORS stations. OPUS provides both network accuracy and local accuracy estimates at the 95% confidence level.

What is a multipath error in GNSS surveying?

Multipath occurs when GNSS signals reflect off nearby surfaces (buildings, vehicles, fences, ground) before reaching the antenna, causing the receiver to process a signal that has traveled a longer path than the direct signal. This introduces ranging errors. Mitigation: use ground planes and choke-ring antennas, select sites away from reflective surfaces, observe for longer periods to average out the effect, and reject observations with high multipath indicators.