Bathymetry is the measurement of the depths of water bodies from the water surface. It’s the marine equivalent to topography. Bathymetric surveys are generally conducted with a transducer which both transmits a sound pulse from the water surface (usually attached to a boat) and records that same signal when it bounces from the bottom of the water body.
An Echosounder attached to the transducer filters and records the travel time of the pulse. At the same time that the pulse occurs, a GPS unit can record the location of the reading. After many of these readings are taken, corrections are made based on fluctuations in the water surface elevation that may have occurred during the survey. The individual points are then mapped; easily done in a GIS.
Bathymetric surveys are used to characterize the seafloor morphology, generation of bathymetric profiles and in pre & post dredging survey operations and volume calculations.
Bathymetry surveys are carried out with either an over-the-side or hull mounted narrow beam [5°] transducer in a survey vessel. The narrow beam averages over a smaller section of the bottom decreasing error. Prior to the start of survey, the echosounder will be subjected to standard “bar check” calibration for high quality measurements of sound velocity. In addition, a portable sound velocity unit is to measure the water column in case of temperature stratification. GPS is provided by a sub-meter differentially corrected GPS unit mounted over the transducer. Coverage is assured using specialized survey software (Coastal Oceanographics’ HYPACK; QINSY; EIVA etc.) which generates “planned lines” according to the accuracy needed. Water level corrections are obtained using either predicted tidal readings or a portable measuring device surveyed into location with survey grade DGPS.
To minimize the error caused by wave action, the survey boat is equipped with a heave/pitch/roll [compensator] sensor which sends information to the survey software for signal correction.
Once data is collected and edited for obvious errors in the survey software, the soundings are converted to depth or elevation in either feet or meters and then exported to text files for import into AUTOCAD/MICROSTATION/ GIS. Then, the points are interpolated in order to generate a drawing / surface map.