A hydrographic survey is to be conducted in a tidal river to prepare a navigation chart. The reduced level of the water surface at a given time is obtained by
- Aobserving the staff reading at a tide gauge and subtracting it from the chart datumCorrect
- Bmeasuring the depth of water using an echo sounder
- Cmeasuring the angle of the shoreline from a boat
- Dusing GPS to measure the elevation of the boat above mean sea level
Solution
Concept
Tide gauge observation and chart datum in hydrographic survey
Formula
RL of Water Surface = Staff Reading ± Datum Reference
Step-by-step solution
1. Chart datum serves as the reference plane for sounding depths and navigational heights in hydrographic charts. 2. A tide gauge staff is set up and calibrated against an onshore benchmark to measure water fluctuations. 3. Observing the staff reading relative to the chart datum yields the instantaneous water surface reduced level. Answer: A
Explanation
In hydrographic surveying of tidal waters, the instantaneous water level is continuously referenced to a chart datum or permanent benchmark using a tide gauge staff. The reduced level of the water surface is determined directly from the observed tide staff reading correlated with the established datum. Hence, option A is correct.
Why the other options are wrong
B) Echo sounders measure depth from water surface to bed, not water surface elevation. C) Shoreline angles determine boat position via three-point resection. D) Raw GPS boat elevation contains dynamic wave heave errors and lacks direct tidal datum calibration.
Shortcut
Tide gauge = Water surface level; Echo sounder = Bed depth below water.
Common mistake
Confusing water surface elevation (tide gauge) with seabed depth measurement (echo sounder).
Correct answer: A) observing the staff reading at a tide gauge and subtracting it from the chart datum