Did you know that land surveyors are also responsible for surveying water, including the depths of the oceans? These types of surveys are called hydrographic and bathymetric surveys. The data collected during a hydrographic survey includes water depth, contours along the bottom, the direction of the current, high water marks, and any underwater landmarks or even shore landmarks that may aid in navigating the waterway.
A bathymetric survey is a specific type of water survey dealing exclusively with the underwater land and the mapping of the seabed. There are other specific types of surveys dealing with water, like basin or interior hydrography, which is specifically for rivers and bodies of potable (drinkable) water.
These surveys may be conducted for many different reasons, including the planning of hydropower plants, the locating of mineral resources, especially oil or gas, planning navigational routes, or marine exploration. A hydrographic survey may also be necessary when planning buildings or other improvements to the waterway or the land adjacent to the water. These surveys may even be used for paleobathymetry, which is the study of underwater depth changes over time, or by treasure hunters searching for shipwrecks.
Unlike most land surveys conducted on dry land, hydrographic and bathymetric surveys cannot usually be conducted by physically visiting and measuring the area in question. Surveying of bodies of water has been around in at least a rudimentary form for centuries; the practice began long ago as a navigational aid for sailors. The first bathymetric surveys were conducted using heavy rope or cable lowered over the side of a ship to measure the depth at that point. Because this allows the measuring of only one point at a time, it is quite inefficient. Since the boat is constantly moving, this method is also not very accurate.
Today, these high-tech surveys are actually conducted using GPS, sonar, and LIDAR or LADAR systems. These processes measure the time it takes for a sound or light to travel through the water, bounce off the sea floor, and return to the sounding equipment. They can take hundreds of measurements in a single pass, and store the depth, the exact location (using GPS technology), and other survey information. Some of these processes are conducted by airplanes, while other surveying equipment may be carried
by boats. Modern hydrographic and bathymetric surveys use multiple beams arranged to cover a swath of 90 to 170 degrees, producing a very accurate map with much fewer passes over the seafloor than are required with single beam equipment.
Today’s surveying equipment produces several soundings for every square foot of the water’s surface. Once the readings have been received, they are processed by sophisticated computer equipment to account for many variables, such as the bending of sound waves due to water qualities like temperature and pressure, which vary as the beam travels through the depth of the water. The end result includes a computer-generated visual image of the seabed (known as a digital terrain model) along with other details. A bathymetric chart produced in this manner usually shows contour lines (known as depth contours or isobaths) at selected depths, similar to what might be found on a contour map of dry land.
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