Posted on September 23, 2011 by CADS Article Team
It is not just land above sea level that needs to be mapped and measured – but underwater land as well. One of the key methods for mapping surface below water is using a bathymetric survey.
What is it?
The term “bathymetry” refers the study of the terrain and contours of land underwater, such as lakes, rivers or oceans.
Bathymetric surveys are a type of hydrographic survey that measures the depth of water and maps out the shape of the seabed. They can help draw out a detailed map of the different shapes and features of the submerged terrain.
What are they used for?
Bathymetric surveys serve a variety of purposes and are useful on both small and large scales. Understanding the bathymetry of underwater terrain can tell us a lot about what happens above the surface.
For example, the shape of a riverbed can influence the speed, temperature and other characteristics of the water flowing above it. Water reacts in different ways depending on the shape of the land beneath it.
One of the most common uses for this type of survey is to ensure safe surface or sub-surface navigation along waterways. Captains of both small river boats and large commercial ships use bathymetric surveys to determine the path they will take to reach their destination safely, as the bathymetric survey shows where both shallow and deep waters are.
It is important to note that these surveys differ from hydrographic surveys in that they are not as accurate for use in safe navigation. Bathymetric surveys are much more detailed than hydrographic details at outlining the terrain – whereas hydrographic surveys specifically make the charts much simpler to understand for navigators.
How are they drawn?
Over the years, the methods used for creating bathymetric surveys have changed significantly. In the very early stages of bathymetry, simple methods such as dropping long chains or ropes into the water off of a boat were used to measure the depth of the water and create fairly inaccurate bathymetric surveys.
This method later advanced to echosounders that were placed beneath or over the side of boats. The echsounder would ping a beam of sound towards the ocean floor and the amount of time it took for the sound to travel through the water, hit the seafloor and come back again indicates how deep the water is (in a similar way to how pregnancy ultrasounds are made).
These days, echosounders are still one of the most common forms of creating bathymetric surveys – but they have advanced from single-beam sounders to multibeam echosounders (MBES). These have hundreds of very narrow beams that stretch up to 170 degrees across allowing fast, accurate coverage of the terrain.
Echosounders work alongside GPS systems. The GPS notes down the current location using a time tag simultaneously with the reading from the echosounder. These two readings can be put together to create both a bathymetric contour map and 3-D view.
Who makes them?
In the USA, surveys of the majority of navigatable inland waterways are performed by the United States Army Corps of Engineers. Ocean bathymetric surveys are generally commissioned by the National Oceanic and Atmospheric Administration.
Bathymetric surveys can also be conducted using your own sonar equipment (which can be very expensive) or by hiring a professional that can conduct a more specific survey for you.
Where to find
Many surveys are available for the public to access free of charge – particularly of major waterways.
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