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This method revolutionized hydrographic surveying, as it allowed a quicker, less laborious, and far more complete survey of an area than did the use of lead lines and sounding poles. From a navigational safety point of view, a wire-drag survey would not miss a hazard to navigation that projected above the drag wire depth.
The earliest known depth measurements were made about 1800 BCE by Egyptians by probing with a pole. Later a weighted line was used, with depths marked off at intervals. This process was known as sounding. Both these methods were limited by being spot depths, taken at a point, and could easily miss significant variations in the immediate vicinity.
Table of geography, hydrography, and navigation, from a 1728 Cyclopaedia.. Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary purpose of safety of navigation and in support of all other marine activities ...
These types of survey may be done in or of the underwater environment, in which case they may be referred to as underwater surveys, which may include bathymetric, hydrographic, and geological surveys, archaeological surveys, ecological surveys, and structural or vessel safety surveys.
Nautical charts are based on hydrographic surveys and bathymetric surveys. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated and are sometimes unreliable. Depths are measured in a variety of ways. Historically the sounding line was used.
Bathymeric charts showcase depth using a series of lines and points at equal intervals, called depth contours or isobaths (a type of contour line). A closed shape with increasingly smaller shapes inside of it can indicate an ocean trench or a seamount, or underwater mountain, depending on whether the depths increase or decrease going inward.
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SDB methods can provide bathymetric data with varying spatial resolution, depending on the analysis method and its underlying physics. The most common methods for coastal and very high resolution (1 to 30 meters) bathymetric data are based on multispectral satellite sensors and the analytical inversion of the radiative transfer equation - often referred to as physical SDB methods [2] or ...