Method of Locating Soundings: The Complete Field Guide
Everything a surveyor needs to know about how soundings are located on rivers, lakes, harbours and reservoirs — definitions, all field types of methods, step-by-step procedure, safety, pros and cons, and answers to common questions.
In hydrographic and river surveying, a depth reading is only as useful as the accuracy of its position. The method of locating soundings is the set of field techniques surveyors use to fix the exact horizontal location of every point where the depth of water has been measured, so that each sounding can be plotted correctly on a bathymetric chart, cross-section, or contour map.
2Why Locating Soundings Matters
A depth value without a reliable position is essentially unusable for engineering design. Accurate sounding location is what turns scattered depth readings into a trustworthy bathymetric map or river cross-section. This directly affects:
- Navigation safety — charting safe channel depths for vessels.
- Dredging & reservoir capacity — calculating silt accumulation and storage volume.
- Bridge, dam & port design — determining scour depth, foundation levels, and berth depths.
- Flood studies — modelling river cross-sections and discharge capacity.
3Types of Methods of Locating Soundings
Surveyors choose from several established types of methods of locating soundings depending on the width of the water body, required accuracy, available instruments, and time constraints.
1. By Cross-Staff / Optical Square narrow rivers
A cross-staff or optical square is set up on the range line on shore. The instrument operator sights the boat and signals it to move until it lies exactly on the perpendicular cross line. The distance along the range line is then chained or taped.
2. Two Angles from the Shore sextant method
Two fixed points of known position are established on the bank. Observers at each point use a sextant to measure the horizontal angle between a reference line and the boat at the exact instant of sounding. The intersection of the two angle lines fixes the position.
3. One Angle and One Distance angle + tape
The distance from the boat to one shore station is measured directly (by tape, chain, or range finder) while a single angle is observed from a second station. Combining the angle and distance gives the fix.
4. Range and Time Method constant speed
The boat travels along a marked range line at a constant, known speed. Soundings are taken at fixed time intervals using a stopwatch, and each position is computed as distance = speed × time along the range.
5. Range and One Angle Method hybrid
The boat is kept on a fixed range line (using shore range poles for alignment) while a single angle is observed from a station off the line, giving a quick cross-check on the boat’s position along the range.
6. Theodolite Intersection Method high accuracy
Two theodolites, set up at known shore stations, simultaneously track and sight the boat. The intersection of the two lines of sight, plotted on the survey sheet, fixes the sounding location with high precision — ideal for wide rivers, estuaries, and harbour surveys.
7. Tacheometric Method stadia readings
A tacheometer (or total station) at a shore station sights a staff held on the boat. Horizontal distance and direction are computed directly from stadia readings, allowing rapid positioning without separate taping.
8. Plane Table Method graphical fix
A plane table with an alidade is set up at a known shore station (or two stations for intersection). Rays are drawn directly on the plane table sheet toward the boat, giving an immediate graphical location of each sounding as work proceeds.
9. GPS / GNSS with Echo Sounder modern & automated
A GPS or GNSS receiver (often in DGPS or RTK mode) mounted on the survey boat is linked to a digital echo sounder. Position and depth are logged together automatically and continuously, removing the need for shore-based angle observations altogether.
4How to Locate a Sounding: Step-by-Step Procedure
While the exact steps vary by method, a typical field procedure for the method of locating soundings using the theodolite intersection or sextant method follows this sequence:
- Establish base stations. Fix two (or more) shore control points of known coordinates along the survey reach.
- Set up instruments. Station theodolites, sextants, or a plane table at the control points; mount the echo sounder or sounding rod in the boat.
- Synchronize signals. Agree on a signal (flag, whistle, or radio call) that marks the exact instant a sounding is taken.
- Take the sounding. Record water depth at the marked instant using the lead line, sounding rod, or echo sounder.
- Observe the position. At the same instant, shore observers record the angle(s), distance, or time reading required by the chosen method.
- Log the data. Record depth, angle/distance/time, water level, and timestamp together in the sounding book or digital logger.
- Plot the fix. Back in the office (or in real time with GNSS), plot each sounding’s position and reduce the depth to a common datum.
- Repeat along cross-sections or survey lines until the required area or reach is fully covered.
Field Tip
Always reduce recorded depths to a fixed reference datum (such as mean sea level or a benchmark) using the water surface level observed at the time of each sounding — this keeps readings comparable across the whole survey.
5Comparison of Methods of Locating Soundings
| Method | Best Suited For | Relative Accuracy | Speed |
|---|---|---|---|
| Cross-staff / optical square | Narrow rivers, canals | Moderate | Slow |
| Two angles (sextant) | Wide rivers, harbours | Good | Moderate |
| One angle + one distance | Medium-width channels | Moderate | Moderate |
| Range and time | Straight, uniform reaches | Moderate | Fast |
| Range and one angle | Rivers with fixed range lines | Good | Fast |
| Theodolite intersection | Wide rivers, estuaries, harbours | High | Moderate |
| Tacheometric method | Reservoirs, lakes | Good–High | Fast |
| Plane table | Quick reconnaissance surveys | Moderate | Fast |
| GPS / GNSS + echo sounder | All water bodies, large-scale surveys | Very High | Very Fast |
6Advantages of Proper Sounding Location
Key Advantages
- Accurate charts — enables reliable bathymetric and contour maps.
- Flexible techniques — a method can be matched to river width, budget, and available instruments.
- Repeatable data — positioned soundings can be revisited for change/erosion studies.
- Supports design — gives engineers dependable input for channels, bridges, and dredging plans.
- Scalable to modern tech — traditional principles extend directly into GNSS-based automated surveys.
Disadvantages & Limitations
- Weather-sensitive — wind, current, and poor visibility reduce accuracy in traditional methods.
- Labour-intensive — angle-based methods need multiple trained observers working in sync.
- Human error — timing mismatches between the depth reading and the position fix cause errors.
- Limited range — cross-staff and one-angle methods lose accuracy over very wide water bodies.
- Instrument cost — theodolites, tacheometers, and GNSS/RTK equipment involve significant investment.
7Is the Method of Locating Soundings Safe?
Yes — sounding surveys are safe when standard field precautions are followed. Because the work involves boats, water currents, and shore-to-boat coordination, safety planning is an essential part of the procedure, not an afterthought.
Safety Precautions
- Crew members wear life jackets at all times on the water.
- Check weather, current, and water-level forecasts before starting work; avoid floods and storms.
- Use a stable, properly ballasted boat suited to the water body and load.
- Maintain clear communication (radio, flags, or whistle signals) between boat and shore stations.
- Avoid surveying near strong currents, whirlpools, shipping lanes, or low-visibility conditions.
- Carry basic rescue and first-aid equipment on every survey boat.
8Applications & Uses of Located Soundings
- River cross-section & discharge studies
- Reservoir capacity & siltation surveys
- Dredging volume calculation
- Port & harbour depth charting
- Bridge pier & scour design
- Navigation channel safety charts
- Flood plain & hydraulic modelling
- Coastal & estuary mapping
9Modern vs Traditional Methods
Traditional methods — cross-staff, sextant, theodolite intersection, and plane table — rely on manual shore observations synchronized with each depth reading. They remain valuable for small budgets, short reaches, and training purposes.
Modern surveys increasingly use GPS/GNSS positioning integrated with a digital echo sounder and multi-beam or single-beam sensors. Position and depth are logged automatically, in real time, at far higher density and accuracy than manual angle methods — particularly when used in DGPS or RTK mode.
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