Bathymetric Data Acquisition & Processing Flashcards

What is the purpose of applying a 'vessel offset' or 'lever arm' correction in MBES processing?

Answer: To account for the physical separation between the GPS antenna, motion sensor, and sonar transducer

Lever arm corrections account for the fact that the GPS antenna, IMU, and sonar head are not co-located, requiring geometric offsets to be applied when computing sounding positions.

In hydrographic surveying, a 'cross line' or 'tie line' is used primarily to:

Answer: Provide an independent check of depth consistency between adjacent survey lines

Cross lines run perpendicular to main survey lines and allow depth discrepancies at intersections to be detected and evaluated as a quality check.

Which type of positioning system provides the highest absolute accuracy for offshore hydrographic surveys beyond RTK range?

Answer: Precise Point Positioning (PPP) with satellite clock and orbit corrections

PPP uses precise satellite clock and orbit corrections broadcast or downloaded in near-real-time, achieving decimeter-level accuracy anywhere globally without a local reference station.

What does 'seafloor feature detection' capability in IHO S-44 Order 1a require compared to Order 2?

Answer: Order 1a requires 100% seafloor coverage using a system capable of detecting features; Order 2 does not mandate full bottom coverage

Order 1a mandates 100% seafloor search (full coverage) to detect hazardous features, while Order 2 surveys may use systematic line spacing without full coverage.

In tidal zoning for a large survey area, what is the primary reason for dividing the area into multiple tidal zones?

Answer: To account for spatial variation in tidal range and timing across the survey area

Tidal phase and range vary spatially, especially in estuaries and along coasts; zoning applies locally appropriate tidal corrections to each sounding.

A hydrographer observes that soundings at the intersection of two MBES lines differ by 0.8 m in 50 m of water. The IHO S-44 Order 1a TVU allowance at 50 m is approximately 0.5 m. The most appropriate action is to:

Answer: Investigate possible causes such as SVP changes, motion sensor errors, or tidal corrections before accepting or re-acquiring data

Intersection discrepancies exceeding TVU specifications require investigation of root causes before data can be accepted or re-acquisition ordered.

When converting a raw bathymetric surface to a chart sounding product, 'shoal biasing' refers to the practice of:

Answer: Selecting the shallowest sounding within each chart cell to protect mariners from undetected hazards

Shoal biasing selects the least depth (shallowest) value within each chart cell, ensuring the chart does not overstate available depth and thus protects mariner safety.