CH Hydrographic Survey Standards & Quality Control 2 — Questions and Answers
Question 1: Under IHO S-44 Order 1a, what is the maximum allowable Total Horizontal Uncertainty (THU) at the 95% confidence level for feature detection?
- 2 m + 5% of depth (Correct answer)
- 5 m + 5% of depth
- 2 m fixed
- 0.5 m + 1% of depth
Correct answer: 2 m + 5% of depth
IHO S-44 Order 1a specifies a THU of 2 m + 5% of depth at the 95% confidence level for feature detection.
Question 2: Which statistical measure is used to combine random and systematic error components in Total Propagated Uncertainty (TPU) calculations?
- Root Sum of Squares (RSS) (Correct answer)
- Arithmetic mean
- Standard deviation only
- Variance inflation factor
Correct answer: Root Sum of Squares (RSS)
Total Propagated Uncertainty combines error components using Root Sum of Squares (RSS) to produce a single uncertainty estimate.
Question 3: A bathymetric survey detects a cross-line check depth that differs from the main scheme depth by more than the allowable TVU. What is the correct action?
- Re-survey the area with the main scheme to investigate (Correct answer)
- Accept the data if only one check line fails
- Adjust the main scheme data to match the check line
- Flag the discrepancy and submit without re-survey
Correct answer: Re-survey the area with the main scheme to investigate
When cross-line discrepancies exceed allowable TVU, the area must be re-surveyed with the main scheme to investigate the source of the error.
Question 4: What percentage of the survey area must typically be covered by cross (check) lines in a standard hydrographic survey?
- 5%
- 10% (Correct answer)
- 20%
- 25%
Correct answer: 10%
Approximately 10% of the survey area is typically covered by cross lines to provide independent quality control checks.
Question 5: In NOAA survey specifications, what is the primary purpose of the 'Danger to Navigation' (DTN) protocol?
- To immediately report newly discovered hazards to mariners (Correct answer)
- To defer hazard reporting until the final survey report
- To catalog hazards for future chart editions only
- To document hazards in the raw data archive
Correct answer: To immediately report newly discovered hazards to mariners
The DTN protocol ensures that newly discovered navigation hazards are immediately communicated to mariners via Notices to Mariners or Local Notices.
Question 6: Which S-44 survey order requires 100% bottom coverage using a system capable of detecting cubic features 1 m or larger in depths up to 40 m?
- Special Order (Correct answer)
- Order 1a
- Order 1b
- Order 2
Correct answer: Special Order
S-44 Special Order requires 100% seafloor coverage and the ability to detect cubic features ≥1 m in depths ≤40 m.
Question 7: A hydrographer identifies a systematic depth bias between port and starboard beams of a multibeam sonar after patch test. This is most likely caused by what?
- Residual roll bias not corrected during patch test (Correct answer)
- Incorrect sound velocity profile applied
- GPS antenna offset error
- Incorrect tide correction applied
Correct answer: Residual roll bias not corrected during patch test
A systematic port/starboard depth bias after patch test typically indicates a residual roll offset that was not fully resolved during calibration.
CH Hydrographic Survey Standards & Quality Control 2
Under IHO S-44 Order 1a, what is the maximum allowable Total Horizontal Uncertainty (THU) at the 95% confidence level for feature detection?