Hydrographic Survey Standards & Quality Control Flashcards

Which datum is used as the vertical reference for charted depths in U.S. coastal waters to ensure maximum safety for navigators?

Answer: Mean Lower Low Water (MLLW)

U.S. nautical charts use Mean Lower Low Water (MLLW) as the sounding datum because it represents a conservative low-water level, maximizing mariner safety.

In post-processing bathymetric data, what is the purpose of applying a 'refraction correction' derived from a sound velocity profile?

Answer: To correct the curved path of acoustic beams through water layers of varying sound speed

Refraction correction uses Snell's law to correct for the bending of acoustic beams as they pass through water layers with different sound speeds.

A hydrographer notices that shoal depths in a survey area progressively decrease from east to west with no corresponding natural feature. This pattern most likely indicates what?

Answer: A systematic tidal or water level correction error that varies spatially

A spatial trend in depth that is not corroborated by physical features typically indicates a systematic error in the water level (tidal) correction that varies across the survey area.

According to IHO S-44, which parameter defines the 'depth threshold' separating the fixed and percentage components of the TVU formula?

Answer: The depth at which the percentage component equals the fixed component (a/b ratio)

The depth threshold in TVU calculations is where the fixed component 'a' equals the depth-dependent component 'b × d', defining where each dominates.

When performing a patch test for roll bias on a multibeam system, what survey pattern is used and what indicates successful calibration?

Answer: Parallel lines over flat bottom in opposite directions; roll bias corrected when symmetric depth profiles match

Roll bias patch test uses parallel lines run in opposite directions over flat bottom; symmetric port/starboard depths indicate the roll bias is correctly resolved.

What is the primary quality control check performed on raw GNSS data before applying it to hydrographic positioning?

Answer: Verification of PDOP values and elimination of epochs with excessive dilution of precision

PDOP (Position Dilution of Precision) verification ensures that satellite geometry was adequate during data collection; high PDOP epochs produce unreliable positions.

Under NOAA hydrographic survey specifications, a 'significant shoaling' discovery during survey operations requires what immediate action?

Answer: Submission of a Danger to Navigation report and development of a Hydrographic Survey Report item

NOAA requires immediate submission of a Danger to Navigation report and generation of an HSRR item for any significant shoaling that could endanger navigation.