Locating and mapping what we can’t see with hydrographic surveys

Posted by on Jun 5, 2012

“We can’t locate what we can’t see.”

My first surveying supervisor made this statement on a daily basis. At one time this was a true statement. But today, with hydrographic surveys, we can locate what we can’t see, such as structures and terrain that are submerged under a body of water.

Hydrographic surveying is a process to determine depths and terrain configurations of the bottoms of bodies of water. They are conducted to generate DTM (Digital Terrain Model) for engineering calculations and GIS mapping purposes.

While it’s not a newly invented type of surveying, today’s increasing technology of GPS and Sonar systems has increased the accuracy and comfort level of processing the data gathered to be used for engineering and design.

Back in the days of “We can’t locate what we can’t see,” hydrographic surveys were conducted in a very inaccurate, dangerous and time consuming way.

Conducting  hydrographic surveys today is not only more accurate and time efficient, but it creates a much safer working environment for field personnel.

The process of completing a hydrographic survey is really interesting. Our field crew first accesses the body of water by boat that’s equipped with GPS, sonar and a data collector that will collect and store the obtained data.

The sonar we use is very similar to a transducer on any boat, but it’s linked to a survey grade GPS system that is mounted on the boat.

During the data collection, the GPS is solving for an X, Y and Z location on the submerged sonar. At the same time the sonar is sounding a depth to the bottom of the water body. Simultaneously, the GPS and the sonar send the collected data to the data collector. Once the data is collected we process the data that will give an X, Y and Z location of structures and terrain along the bottom of the body of water.

This technology can be utilized in various areas of engineering, surveying and the private sector as well. With the processed data we can plot cross sections before and after dredging to determine the volume of material removed during dredging.

A  volume of water contained within a body of water can be determined utilizing this process.

This technology has been used in the private sector to map structures and terrain of managed fishing ponds and lakes. With this type of mapping, anglers can study a mapped lake or pond that shows submerged features, this is helpful in  creating a more efficient fishing trip for the angler.

Sain is currently conducting hydrographic surveys to obtain volumes of material removed during dredging of a 300-acre body of water.

We’re also utilizing this technology to monitor a 4,000-foot earth dam. This dam is failing in areas under the water with no visible signs above the water. We have monitored this dam for almost a year, isolating the most critical areas that need immediate attention as well as monitoring areas that reconstruction has been performed to maintain the dam’s stability. The ability to “locate what we can’t see”  helps our client maintain the stability of this dam more cost efficient compared to waiting for the dam to completely fail before repair.

Sain Associates, Inc., is headquartered in Birmingham, Alabama, with offices in Cullman, Alabama, Pulaski, Tennessee and Mandeville, Louisiana. Sain is a site engineering, traffic/transportation engineering and planning and land surveying firm with experience in more than thirty states.

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