Bathymetric Data Acquisition & Processing Flashcards

What is 'Total Propagated Uncertainty' (TPU) in hydrographic surveying?

Answer: The combined statistical uncertainty from all error sources propagated through to the final depth value

TPU represents the statistical combination (usually at 95% confidence) of all individual error contributions including sonar, motion, positioning, and tidal corrections.

A hydrographer performing a bar check on an echosounder would use this procedure to:

Answer: Calibrate the echosounder's depth readings against a physical reference at known depths

A bar check uses a physical plate lowered to known depths to verify or adjust the echosounder's depth scale factor and draft setting.

In MBES operations, 'across-track resolution' at the outer beams is typically poorer than at nadir because:

Answer: The beam footprint increases with angle, spreading acoustic energy over a larger area

At steep angles from nadir, the projected beam footprint on the seafloor becomes larger in the across-track direction, reducing spatial resolution.

When computing a bathymetric grid from raw MBES soundings, 'gridding resolution' should ideally be set to:

Answer: A cell size appropriate to the swath footprint size and density of soundings

Grid resolution should match the sounding density and footprint size — too fine creates data voids, while too coarse loses detail and may miss features.

Which of the following best describes 'along-track resolution' in a multibeam system?

Answer: It is governed by the transmit beamwidth in the fore-aft direction and vessel speed

Along-track resolution depends on the transmit beam's fore-aft angular width projected onto the seafloor, which also varies with depth and vessel speed.

What is the correct procedure when an MBES system detects a possible shoaling feature during acquisition?

Answer: Immediately reduce vessel speed and cross the feature on additional lines to confirm and characterize it

Field verification of potential hazards by running cross lines or investigating lines is standard hydrographic practice to confirm and characterize shoaling features in real time.

Which datum is most commonly used as the sounding datum in US nautical charts?

Answer: Mean Lower Low Water (MLLW)

MLLW is the standard sounding datum for US nautical charts, representing the average of the lower of the two daily low tides over a 19-year tidal epoch.