Short note: Location of sounding points

In a hydrographic survey, a depth measured from the boat is of no use unless the position of the boat (sounding point) at the time of sounding is known. The usual methods of fixing the sounding points are given below.

1. By range and a time/distance interval (one range line and distance)

A line (range) is marked on the shore by two poles. The boat keeps to this line, and soundings are taken at equal intervals of time or at marked distances, the boat's distance from the shore being found from a tag line or a marked rope. It is used for narrow rivers.

2. By two angles from the shore (intersection)

Two theodolites are set up at the ends A and B of a measured base line on the shore. At a signal from the boat, both observers read the angles $\alpha$ and $\beta$ to the boat. The position is plotted by intersection. The accuracy is good, but it needs two observers and good visibility.

        P (boat)
       /  \
      /    \
     A ---- B  (shore base)

3. By one angle and a distance (range and angle)

A theodolite at A sights the boat while the distance AP is measured by a tape/tag line or an EDM. The boat is plotted by polar coordinates (angle and distance).

4. By two sextant angles (resection)

Three known marks on the shore (A, B, C) are used. A sextant in the boat measures the angles $\angle APB$ and $\angle BPC$ at the instant of sounding. The point is plotted by three-point resection with a station pointer (three-arm protractor) or by calculation. No shore observers are needed, which is why it is popular in open water.

5. By a range and a cross angle

The boat moves along a range line marked on the shore, and one angle is observed to it by a theodolite at a known point on the shore; the intersection of the range and the angle ray fixes the point.

6. By GPS (DGPS/RTK) or total station

The receiver on the boat gives the coordinates at each sounding, and the position is logged together with the echo-sounder depth. It is fast and accurate and is the standard modern method.

Plotting

The base line, shore marks and sounding positions are plotted to scale; the depth at each point, reduced to the datum (corrected for the tide level), is written next to it, and isobaths are interpolated.

The method is chosen according to the width of the water, the distance from the shore, the accuracy needed and the equipment available. The position should be fixed at the same instant as the sounding, so that the boat does not move between the sounding and the fix.