Hydrographic surveying plays a critical role in understanding water bodies, particularly for improving navigability, monitoring silting and scouring, and planning dredging operations. Sounding, a fundamental aspect of hydrographic surveying, involves measuring the depth of water below the surface. Accurate sounding is essential for safe navigation, environmental assessments, and engineering projects. This article explores the methods used to locate soundings, highlighting their techniques, applications, and advantages.
Methods of Locating Soundings
Soundings can be located using observations made from boats, shores, or a combination of both. The choice of method depends on the survey’s objectives, the characteristics of the water body, and the available resources. Broadly, the methods fall into four categories:
- Conning the Survey Vessel
- Observations with Theodolite or Sextant
- Theodolite Angles and EDM Distances from Shore
- Microwave Systems
Each method offers unique advantages suited to different scenarios, as outlined below.
1. Sounding by Conning the Survey Vessel
Conning refers to keeping the survey vessel or boat on a known course, making it an effective technique for rivers and open sea surveys up to 5 km offshore. This method relies on markers fixed on the shore, known as ranges, to guide the vessel. Two sub-methods are commonly used:
Location by Cross Rope
In this method, a wire or rope marked at known intervals is stretched across the water body. The rope’s starting point on the shore serves as a reference. The boat moves along the rope, and soundings are measured at predetermined distances using a weighted pole. This highly accurate method is ideal for narrow rivers, lakes, and harbors. It is also effective for determining the amount of material removed during dredging operations.
Location by Range and Time Interval
Here, the boat is aligned with two fixed shore signals and rowed at a constant speed. The time taken to reach each sounding point is recorded to calculate its distance along the range. While suitable for narrow channels or rivers, this method is less accurate than the cross rope technique.
2. Sounding by Observations with Theodolite or Sextant
Theodolites and sextants are instruments used to measure angles for locating soundings. This approach is highly versatile, with several sub-methods for different conditions:
By Range and One Angle from the Shore
The boat aligns with two shore signals and moves along the range. Angles measured from the shore to the sounding point help fix its position. This method is accurate and easy to use, making it popular for plotting soundings.
By Range and One Angle from the Boat
Similar to the previous method, but angles are measured from the boat to shore signals. This provides better control over operations and allows for cross-checks by measuring additional angles from the boat to other shore points.
By Two Angles from the Shore
Two fixed instrument stations on the shore are used to measure angles to the boat. This eliminates the need for range signals and is particularly useful in strong water currents.
By Two Angles from the Boat
Using three fixed shore points, angles are observed from the boat to determine its position using the three-point problem. This method is useful when prominent shore landmarks like lighthouses or spires are available.
By One Angle from the Shore and One Angle from the Boat
Combining the above methods, this technique involves measuring one angle from the shore and another from the boat. It requires two instruments and is suitable for specific, precise fixes.
By Intersecting Angles
Used for harbors and reservoirs, this method involves erecting multiple shore signals and rowing the boat perpendicular to the shore. Soundings are measured at intersection points of signal lines, making it effective for tracking silting or scouring over time.
By Tachometry
In this method, a tachometer on the shore measures the distance to a staff on the boat. It is suitable for calm waters near the shore and provides rapid results.
3. Sounding by Theodolite Angles and EDM Distances from Shore
This modern method combines theodolites with Electronic Distance Measurement (EDM) devices for precise location fixing. A reflector is placed on the boat, while the EDM and theodolite remain on shore. The system measures distances and angles to locate the sounding point with high accuracy. This method is most effective in still waters and is widely used in contemporary hydrographic surveys.
4. Sounding by Microwave Systems
The microwave system employs a device called a Tellurometer, consisting of three units: a master unit on the boat, and remote and master antennae on the shore. Microwaves transmitted between the boat and shore stations measure distances accurately, even up to 100 km. This technique is particularly useful for large-scale surveys in expansive water bodies.
Conclusion
The choice of a method for locating soundings depends on the water body’s characteristics, survey objectives, and equipment availability. Each technique, from traditional methods like cross rope and theodolite-based measurements to advanced approaches like EDM and microwave systems, offers unique advantages. By selecting the appropriate method, surveyors can ensure accurate, reliable results, contributing to safer navigation, better resource management, and improved waterway engineering. As technology evolves, these methods will continue to adapt, enhancing the precision and efficiency of hydrographic surveying.