Hydrographic Surveying
Hydrographic surveying maps the seabed and water column for navigation, dredging, engineering and environmental purposes. The controlling standard is IHO S-44 Edition 6.0.0, which defines survey orders - Special, 1a, 1b and 2 - each with limits on total vertical uncertainty (TVU), total horizontal uncertainty (THU) and feature detection. TVU is TVU = the square root of (a squared + (b x d) squared), where d is depth; for Special order a = 0.25 m and b = 0.0075, for Orders 1a and 1b a = 0.5 m and b = 0.013, and for Order 2 a = 1.0 m and b = 0.023. Orders 1a and 1b share the same uncertainty but differ on feature detection and full coverage.
Vertical reference: soundings are reduced to chart datum, which in New Zealand is Lowest Astronomical Tide (LAT). Depths are below chart datum; drying heights are above it. Land heights use NZVD2016 - never mix the two. Know the reduction chain: tide gauge or GNSS water level, sound velocity from a probe, bar checks and patch tests for multibeam, cross-lines for verification, and a documented uncertainty budget.
Traps
- Confusing depth and drying height, or quoting a sounding without its datum.
- Adding a and b incorrectly - b is multiplied by depth, not added to it.
- Believing an order only sets accuracy; it also sets line spacing, coverage and feature detection.
- Ignoring sound velocity variation in estuaries and around river mouths.
Worked example
Required TVU at 20 m depth for Order 1a: square root of (0.5 squared + (0.013 x 20) squared) = square root of (0.25 + 0.0676) = 0.56 m at 95% confidence. A sounding outside that band fails the order.