Hydrographic Surveying
What the exam tests
Hydrography items test sounding reduction, water-level datums, positioning, sonar systems, and volume computation. Expect a tide or draft correction calculation.
Sounding reduction
- Soundings are reduced to a chart datum such as MLLW, not to the ellipsoid.
- Apply transducer draft, water level above datum, and sound-velocity corrections.
- Single-beam sonar gives a profile along the track; multibeam gives full swath coverage; side-scan imagery shows texture and targets but is not a depth measurement.
- IHO S-44 orders set accuracy standards by use.
Positioning and volumes
Horizontal positioning is usually RTK GNSS or a total station; vertical accuracy is the limiting factor for dredge volumes. Volumes come from comparing surveyed surfaces to a design or prior surface.
Worked example
A sounder reads 12.4 ft from the transducer to the bottom. Transducer draft is 0.6 ft, and the tide is 2.1 ft above MLLW. Depth below the surface is 12.4 + 0.6 = 13.0 ft. Depth below MLLW is 13.0 - 2.1 = 10.9 ft. The sounding is reported as 10.9 ft below MLLW.
Traps
- Reporting uncorrected soundings or depths below the transducer.
- Adding tide when it should be subtracted.
- Neglecting transducer draft or sound velocity.
- Using ellipsoid heights as water-surface elevations.
- Computing dredge volumes from sparse single-beam lines without accounting for coverage.