--- title: "Short note: Location of sounding points" subject: "Surveying II" chapter: 5 key: "semester_4/surveying_ii:sounding-point-location-hydrographic" source: https://www.bctengineer.com/q/semester_4/surveying_ii/sounding-point-location-hydrographic site: BCT Engineering --- # Short note: Location of sounding points Surveying II · Chapter 5: Orientation - Asked in: 2066 Magh (old course), 8 marks; 2065 Kartik (old course), 8 marks (asked 2 times) - Video: [Methods Of Locating Sounding Hydrographic Survey Civil Engineers](https://www.youtube.com/watch?v=C8pSULX_04E) — Mr Civil Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/sounding-point-location-hydrographic - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-5 ## Question Write a short note on the location of sounding point in hydrographic survey. ## Short answer Sounding point location methods include: keeping the boat on a shore range and measuring time or distance intervals; intersection by two theodolites from a base line; two sextant angles from the boat to three shore marks for resection; GPS/RTK recording coordinates with depth. The position must be fixed at the same instant as the sounding to avoid movement error. ## Answer 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must the position of the boat be fixed at each sounding? A. A depth without a known position cannot be plotted on the map B. To measure the water speed C. To calibrate the echo sounder D. To find the tide level Answer: A. Soundings are useful only when plotted at their positions so isobaths can be drawn. (Key point: Need for positioning) 2. In the two-angle intersection method: A. Two theodolites at ends of a shore base measure angles to the boat at a signal B. A sextant is used on the boat C. A tag line is used from one shore D. GPS gives the position Answer: A. Angles alpha and beta from the ends of a measured base line fix the boat by intersection; it needs two observers and good visibility. (Key point: Intersection method) 3. What is the advantage of fixing by two sextant angles on three known shore marks? A. No shore observers are needed, so it suits open water B. It needs two theodolites C. It gives depth directly D. It requires a tag line Answer: A. The observer on the boat measures angles APB and BPC and plots by three-point resection, so no one is needed on shore. (Key point: Sextant resection) 4. The range-and-distance method with a tag line is suitable for: A. Open sea B. Narrow rivers C. Deep harbours D. Large lakes Answer: B. The boat keeps on a shore range line and its distance is found from a marked rope, practical only across narrow rivers. (Key point: Range and tag line) 5. The standard modern method of fixing sounding points is: A. Estimation by eye B. DGPS/RTK or total station C. A sounding rod D. Plane table intersection Answer: B. A receiver on the boat logs coordinates with each echo-sounder depth, quickly and accurately. (Key point: Modern method) ## Related in Surveying II - [Contour characteristics, guide points A and B](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acontour-characteristics-six-guide-points) — Surveying II, chapter 4 - [Two point resection, point C from A, B, D](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atwo-point-resection-acb-dca-adb) — Surveying II, chapter 5 - [Two point resection, point C (A, B at 7928.474 N)](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atwo-point-resection-c-a-b) — Surveying II, chapter 5 - [Three point analytical resection, average coordinates of O](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Athree-point-resection-p-w-d) — Surveying II, chapter 5 - [Distances OA and OC by resection on A, B, C](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aresection-abc-sides-oa-oc) — Surveying II, chapter 5 ## Related in other subjects - [Final location and detailed survey](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Adetailed-survey) — Transportation Engineering I, chapter 2, asked 3× - [Tunnel alignment survey and ideal bridge location](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-alignment-survey-ideal-bridge-location) — Transportation Engineering II, chapter 5 - [Critical oxygen deficit and location (table)](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acritical-do-effluent-stream-table-78) — Sanitary Engineering, chapter 6 - [Critical oxygen deficit and location (table 2)](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acritical-do-effluent-stream-table-71) — Sanitary Engineering, chapter 6