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For many centuries, a hydrographic survey required the use of lead lines – ropes or lines with depth markings attached to lead weights to make one end sink to the bottom when lowered over the side of a ship or boat – and sounding poles, which were poles with depth markings which could be thrust over the side until they touched bottom.
Bathymetric charts (not to be confused with hydrographic charts), are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines (called depth contours or isobaths) and selected depths , and typically also provide surface navigational information.
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.
Bathymetric surveys are traditionally done from the surface, by measuring depth (soundings) at measured positions along transect lines and later plotting the data onto a bathymetric chart, on which lines of constant depth (isobaths) may be drawn by interpolation of soundings.
Depth contour lines show the shape of underwater relief. Coloured areas of the sea emphasise shallow water and dangerous underwater obstructions. Depths are measured from the chart datum, which is related to the local sea level. The chart datum varies according to the standard used by each national Hydrographic Office.
HMS Waterwitch, a hydrographic survey vessel. Even though, in places, hydrographic survey data may be collected in sufficient detail to portray bottom topography in some areas, hydrographic charts only show depth information relevant for safe navigation and should not be considered as a product that accurately portrays the actual shape of the ...
A sounding line can still be found on many vessels as a backup to electronic depth sounding in the event of malfunction. GPS has largely replaced the sextant and chronometer to establish one's position at sea, but many mariners still carry a sextant and chronometer as a backup. Many small craft still rely solely on a sounding line.
Technical developments over the years improved surveying methods and the accuracy of the charts. For depth determination, methods of measuring depth from a moving ship were developed, as well as "sweeping", dragging a horizontal line across an area to detect hazards that might be missed by individual soundings. [24]