16. Hydrographic and Special-Purpose Surveys
Hydrographic and Special-Purpose Surveys
This domain tests water-depth measurement, tide and datum reduction, positioning and specialised surveys. Expect sounding reductions, chart datum, vertical reference and volume computations.
Core rules
- Soundings are reduced to chart datum; tide height is the water level above chart datum.
- Echo sounder measurements need sound-velocity, draft and tide corrections.
- Horizontal positioning must meet the survey order; vertical accuracy is critical for dredging and navigation.
- Chart datum is a low-water reference, not mean sea level.
- Special-purpose surveys include deformation, utility, mining and dredging surveys, each with its own tolerance.
Worked example
An echo sounder measures 12.4 m from the transducer near the surface to the seabed. The tide is 1.8 m above chart datum, and transducer draft is negligible. Seabed elevation relative to chart datum = 1.8 - 12.4 = -10.6 m, so the charted depth is 10.6 m. If the draft were 0.3 m, the depth from the surface would be 12.7 m and the charted depth 10.9 m.
Traps
Adding the tide when it should be subtracted; confusing chart datum with MSL; ignoring transducer draft; using a single sound speed in stratified water; reporting depths without the vertical reference.