What Does a Hydrographic surveyor Do?

Surveying Under the Sea

Despite the cartoon image of a deep-sea diver peering through a surveyor's transit, with air bubbles rising all around, the ability to accurately survey underwater topography is extremely important. Hydrographic surveyors are used to identify all sorts of underwater features and objects. Energy exploration firms employ them to help find oil and gas deposits. The shipping industry uses them to make sure that channels are free of dangerous obstacles, whether natural or man-made. Treasure hunters hire them to map out the location of sunken wrecks, and environmental agencies need them to keep track of erosion.

Always in Motion

Wherever there is moving water-oceans, rivers, streams or tide-affected lakes-there is movement beneath the waves as well. The hydrographic surveyor is called upon to create underwater maps of a lake- or seabed, and to mark the navigable portions of rivers. These maps must be updated regularly because of that movement. Silt builds up to clog harbors, and storms can cause massive shifts in the topography below the water line.

Hardware of the Hydrographic Surveyor

Naturally, the surveyor does not go underwater to achieve his goals. However, the most primitive tool at the disposal of a hydrographic surveyor is nothing more than a rock at the end of a string. Measure the depth of a channel by dunking the rock into the water; note the place where the water's surface strikes the string when the rock hits bottom; finally, measure the distance between the rock and the string marking once you hoist the rock out of the water. Note the location on a hand-drawn map and repeat-oh, perhaps a million times or so. Naturally there is a better way to do this. Sonar, the same device that helps surface ships locate enemy submarines, is highly useful for mapping the bottom of a body of water. Sound waves are generated and directed straight down. They then bounce off something, whether the bottom or an obstacle, and return to the electronic device for notation. By blanketing the bottom of a body of water with sound waves, it is possible to create an incredibly accurate picture of what lies below.

Bathymetrics

The term bathymetry is also used to describe hydrographic surveying. In addition to determining the depth of water at any given point, plus providing a picture of the bottom, GPS data are used to fix each exact sonar return to a specific point on the globe. This process allows surveyors to tie in land-based monuments with their over-water equivalents, helping engineers and other end-users of this information understand exactly where a particular peak, valley, rift or underwater piling rests on a map. Bathymetric and hydrographic surveying is usually conducted aboard some sort of watercraft. Depending upon the body of water under examination, this could be a sea-going vessel, a smaller craft such as a tugboat, or even an inflatable device like a Zodiac. There are even hydrographic surveyors who use underwater vehicles, but these are generally employed for picture taking since GPS signals cannot penetrate the water's surface. Lacking the ability to collect GPS data, it is difficult to determine an exact x/y point on water through some lesser function.

Using Software to Perfect Hydrographic Surveys

Data collected under one of several processes-sonar is standard, but some companies have employed LiDAR (Light Detection and Ranging) in shallower areas-is post-processed and then used to create something called a digital terrain model. This is similar in method to what is done for land features, except that elevation readings are replaced by depth readings. A number of factors can affect the viability of data, including the motion of the collection platform (boat), tides, and even water temperature. Once corrections have been made, many software programs have the ability to directly create maps and charts to show what has been collected beneath the waves.