---
title: "Short note: Underwater mapping"
subject: "Surveying II"
chapter: 4
key: "semester_4/surveying_ii:underwater-mapping"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/underwater-mapping
site: BCT Engineering
---

# Short note: Underwater mapping

Surveying II · Chapter 4: Contouring

- Asked in: 2065 Chaitra (old course), 8 marks
- Video: [Underwater 3D Photogrammetry](https://www.youtube.com/watch?v=729VSMdcgl8) — Hydrographic Surveyor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/surveying_ii/underwater-mapping
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-4

## Question

Write a short note on underwater mapping.

## Short answer

Underwater mapping (bathymetric survey) measures water depth and bed relief. Soundings are taken by echo sounder or lead line from a boat, with positions fixed by GPS or shore angle intersections; depths are reduced to a datum using tide gauge readings. Isobaths (depth contours) are plotted to show bed topography, used for dam, harbor, bridge, and navigation planning.

## Answer

**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.

```text
   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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What are isobaths in underwater mapping?
   A. Lines of equal depth, i.e. underwater contours
   B. Lines of equal tidal range
   C. Lines of equal current speed
   D. Lines joining the shore stations
   Answer: A. The relief of the bed is shown by isobaths, lines of equal depth. (Key point: Isobaths)

2. Why is a tide or water level gauge fixed on the shore during soundings?
   A. To measure the speed of the boat
   B. To reduce every sounding to a common datum
   C. To measure the speed of sound in water
   D. To locate the boat position
   Answer: B. The gauge gives the water surface RL at the time of each sounding so that RL of bed = RL of water - depth is on a common datum. (Key point: Reduction to datum)

3. An echo sounder gives depth d =
   A. v t
   B. v t / 2
   C. 2 v t
   D. v / t
   Answer: B. The pulse travels down and back, so depth is half of v x t, with v about 1500 m/s in water. (Key point: Echo sounder formula)

4. Which device is suitable for depths up to about 20 m but is slower than an echo sounder?
   A. Sounding rod
   B. Multibeam sonar
   C. Lead line
   D. Sextant
   Answer: C. A sounding rod is for depths up to about 5 m, a lead line up to about 20 m; echo sounders give fast continuous records. (Key point: Choice of sounding device)

5. The reduced level of the bed at a sounding point is:
   A. RL of water + depth
   B. Depth - RL of water
   C. RL of water - depth
   D. RL of shore station - depth
   Answer: C. RL_bed = RL_water - d, with RL_water read from the gauge at the time of sounding. (Key point: Reduced level of bed)