Hydrographic survey is a branch of surveying that deals with the measurement of water bodies.
It is the science of measurement and description of the features which affect marine navigation including water depth, shorelines, tides, current, submarine level, contours, and undersea obstructions.
The hydrographic survey deals with the configuration at the bottom and adjacent land areas of the ocean, lakes, rivers, harbors and other water forms. It is used in determining the physical and chemical properties of water.
Importance/Use of hydrographic survey
- It is used for controlling and planning engineering projects like bridges, dams, reservoirs, tunnels, etc.
- To measure the discharge in rivers.
- To determine the mean sea level.
- Used for the observation of tides.
- For determination of shorelines.
- For marine navigation and traffic management.
- Marine environment preservation.
- Exploration of marine resources and laying of submarine cables and pipelines.
Vertical and horizontal control on Hydro-graphic survey
The main operation of the hydrographic survey is to determine the depth of water at a certain point. The measurement of the depth of water below the surface is called sounding.
To take sounding vertical control is necessary and to locate sounding horizontal control is necessary.
In a hydrographic survey, the primary horizontal control is established by running the theodolite and tape traverse. It is the determination of the exact position of the point where sounding is taken. Vertical control is based upon the series of benchmarks established near the shoreline by spirit leveling and these serve for setting and checking tide gauge.
Depth of sounding refers as the water level at the time it is taken.
Sounding and equipment used for sounding
Sounding is a process of measurement of depth below the water surface. This corresponds to the spirit leveling and land survey.
Equipment used in sounding are:
- Sounding boat: It should be sufficiently roomy and stable. For quiet water, flat-bottom boats are used whereas for high current water round bottom boats are used. The sounding platform should be extended enough over the side to prevent the line from striking the boat
- Sounding rods: It is a rod or pole of well-seasoned tough timber with length 5-8 m and a diameter of about 5-8 cm. They are suitable for shallow and quiet water. A pole of 6m is used to find depth up to 4 m.
- Lead lines: It is a rope with which sounding lead is attached to the end. It should be soaked on the water for an hour before sounding. For regular sounding, a chain of brass steel or iron is preferred.
- Sounding machine: When deep sounding is to be carried out then a sounding machine is used. It may be automatic or hand driven. It is mounted on a sounding boat and can be used up to 100ft.
Suggested article: Hydrographic Surveying – Methods, Applications and Uses
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