Short note: Underwater mapping

Underwater mapping (hydrographic or bathymetric survey) is the survey that finds the depth and shape of the bed of a river, lake, reservoir, harbour or sea, and shows it on a map together with the shore features. The relief of the bed is shown by lines of equal depth called isobaths (underwater contours).

Objectives and uses

• Planning of dams, reservoirs and barrages: storage capacity and silting (sedimentation) surveys.
• Design and maintenance of harbours, ports and navigation channels; dredging quantities.
• Location of bridge piers and scour studies; river training and flood control.
• Laying of pipelines, cables and intake structures; navigation charts.

Field work

1. Reconnaissance and shore control: A traverse or base line is established on the shore, and the shore line is surveyed with a level, theodolite or total station.
2. Tide or water level gauge: A gauge is fixed on the shore and read at regular intervals, so that every sounding can be reduced to a common datum (RL of the water surface minus depth).
3. Sounding: Depths are measured from a boat along planned lines (parallel or radial).
   • Sounding rod or pole for depths up to about 5 m.
   • Lead line (a weight on a graduated line) for depths up to about 20 m.
   • Echo sounder: a transducer sends a sound pulse, and the time taken for the echo to return gives the depth, $d = \frac{v\,t}{2}$ ($v \approx 1500$ m/s in water). It is fast and gives a continuous record.
   • Multibeam sonar for complete coverage of the bed.
4. Location of the sounding points: The position of the boat at each sounding is fixed by: angles from two shore stations (intersection), one angle and a distance along a range, two sextant angles to three known shore marks (resection), distance by EDM, or GPS (DGPS/RTK), which is the usual method now.
5. Booking: Each depth is recorded with the time, point number and gauge reading.

Reduction and plotting

• The reduced level of the bed at each point is $RL_{bed} = RL_{water} - d$, where $RL_{water}$ is taken from the gauge at the time of sounding (the depth of the transducer below the surface is also corrected for).
• The sounding points are plotted to scale on the shore map, and their reduced depths are written beside them.
• Isobaths are drawn by interpolation, as for ordinary contours, at a suitable interval.

   shore base
   A ---------- B
   | \         /
   |  \  .   . /    . = sounding points
   |   \  . . /       located by angles at A and B
   |    \ . /
   ~~~~~~~~~~~~ water

Accuracy

Soundings are corrected for the tide, the speed of sound, the draught of the boat and rolling. The sounding positions must be fixed with an error smaller than the plotting accuracy (about 0.5 mm on the map).