A basic introduction to hydrographic surveying

There are different hydrographic surveying jobs and that is why a lot of people are interested in having a career in hydrography. Hydrographic surveying is known to be the wet equivalent of topographic surveying. The objective of this survey is to delineate the shape of a portion of the surface of earth concealed by water.

The surface concealed by water cannot be observed directly or be occupied so it is very necessary to take the necessary measurements to know the distance between the surface of the water and the bottom. It is the process of deducing the underwater topography from different discrete observations of depth at different positions throughout the area of the survey. The quality of the product depends on the density and accuracy of these observations. Hydrography operations are undertaken to obtain site details for offshore or alongshore construction, provide basic information for nautical charting, assess the condition of the marina and port facilities, measure quantities in various dredging projects, and determine the extent of siltation and for other reasons.

Regardless of the magnitude, purpose or scale of the survey, the principles remain the same. Topography is inferred and identical measurements are required. As a source of information for nautical charting, hydrography is undertaken by agencies in the maritime nations of the world.

Procedures and standards for these surveys are usually codified in manuals in the NOS hydrographic manual.

Phases of hydrography

Hydrographic surveying is done in different phases. The execution of hydrography can be divided into preliminary office preparations, fieldwork, sounding operations and data preparation. We will look at these phases separately.

Preliminary office preparations

Hydrographic surveyors should gather all the necessary information regarding the area of the survey. It is important to get positions and descriptions of all horizontal control in the vicinity. Other valuable materials include topographic maps, aerial photography, prior hydrographic surveys, meteorological characteristics, and so on. The specifications of the project will dictate the scale of the survey, required positioning frequency, minimum sounding density and horizontal control references system.

The requirements of the survey should be thoroughly examined and understood. It is also important for the area to be surveyed to be defined clearly especially offshore limits. It is time-consuming and expensive to survey the shoreline than to depth contour at which the sounding vessel can navigate safely. If streams enter the area being surveyed then it is important to determine the upstream extent of the project tract.

It is important to get a large-scale map depicting the survey area and mark on it the limits of the survey area. This will be a very valuable reference throughout the planning process.

During the sounding operations, field plotting sheets are prepared at the survey scale using a convenient plane coordinate grid or projection. Before the advent of computer-generated projections, the NOS utilized the polyconic projection because of the ease of manual construction and the low distortion. We have several plotting sheets that are required to depict the survey area. Different people play different roles and that is why there are a lot of hydrographic surveying jobs out there.

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