Chapter 29: Hydrography
Charts exist because of hydrographic survey. The quality and age of the survey beneath a chart determines how much water the navigator can actually trust.
Survey methods
Lead lines and sounding poles gave spot depths with wide gaps between lines of soundings. Single-beam echo sounders measure the two-way travel time of an acoustic pulse, assuming a sound velocity near 1500 m/s; errors arise from temperature and salinity layering, transducer draft and vessel motion. Multibeam systems ensonify a swath giving full-coverage bathymetry, while sidescan sonar images the seabed for obstructions between soundings. Airborne lidar bathymetry covers clear shallow water efficiently.
- Echo sounder depth is below the transducer — apply transducer draft
- Full-coverage multibeam vastly reduces the risk of an uncharted pinnacle
- Sidescan finds objects; multibeam measures depths
Reliability and zones of confidence
ENCs carry CATZOC (category zone of confidence) values from A1 (best) through D and U (unassessed), each with stated position and depth accuracy and seafloor coverage. A paper chart's source diagram gives the equivalent narrative. Treat a ZOC C or D area as requiring a substantially larger under-keel margin, and never plan a deep-draft passage across an unassessed area on the basis of the printed soundings alone.
Formula cards
Echo sounding depth
depth = (c × t) / 2, c ≈ 1500 m/s
Add transducer draft to get depth below the waterline.
Key takeaways
- • The echo sounder measures from the transducer, not the waterline.
- • CATZOC A1/A2 supports tight margins; C, D and U demand generous ones.
- • Sparse historic surveys can hide dangers between lines of soundings.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 29 quiz
0 of 3 answered
1.A CATZOC rating of D indicates:
2.An echo sounder set to read depth below the transducer will understate water depth by:
3.Sidescan sonar is primarily used to: