Single Beam Depth Sounding
For traditional hydrographic surveying, CRA uses a real time data acquisition system based on notebook PC running HYPACK software. Inputs are taken from real time differential GPS equipment (Trimble Navigation AG132, DSM PRO, or the new SPS 461 receivers.). The computer display screen shows the operator and helmsman the boat position relative to the selected pre?programmed survey line (i.e. the distance off and along line, X and Y coordinate, time, speed, depth, etc.), both graphically and in alpha?numeric form. This display also shows the position and water depth history that is being recorded directly to disk so that any spikes or problems can be seen. The software also puts out a fix closure to other sensors, including the fathometer and side scan sonar or sub-bottom profiler recorders at a selectable multiple of the position fix. CRA uses digital single beam depth sounders from Odom Hydrographic Systems (Hydrotrac and Echotrac) with 200 KHz transducers in both 8 degree and 3 degrees beam widths. These units both have an available second, low frequency transducer (24 to 30 KHz) available for areas with a special “fluff” layer problem. In all of our boats, the GPS antenna is mounted directly above the depth sounder transducer to eliminate any offsets. All transducers are built into or shoot directly through the hull of the boat near to the keel to minimize the effects of roll and pitch. To further eliminate these effects, CRA also uses T.S.S. DMS05 motion sensors, especially in more exposed areas. Single-beam fathometers are usually calibrated using a traditional bar-check method, or in deeper areas or those with high wind or currents, a velocimeter probe is used.
Readings of the variations in water surface elevations (e.g. tides), which are used to reduce the hydrographic data to a suitable vertical datum, are made by finding or establishing a benchmark on land via traditional leveling techniques, and setting either a tide staff or installing an electronic recording gauge. Sometimes, horizontal positioning is achieved via RTK GPS techniques, which also give real time elevations with respect to the WGS84 reference ellipsoid. In an area with a reasonably well known geoidal separation, this yields elevations that can be referenced to terrestrial datums such as NAVD88 or even statistical datums of sea level (e.g. Mean Low Water), using relationships published by the National Oceanic and Atmospheric Administration (NOAA).
Data is first edited to remove noise spikes, and then depths corrected for tides to produce a clean x,y elevation data set that is then transferred CAD (Bentley Microstation J v7 and V8, or AutoCAD 2004) for production of final drawings. Plan view charts are typically sent to clients electronically as PDF or CAD files, but may also be produced using a large format plotter (HP 770 at paper sizes up to 42″ in width. Depth contours are generated in “Surfer” from Golden Software which generates both 2?D and 3?D contour plots. As part of our QA/QC program at least one cross?line is run on a project, preferably on a different day, cutting across the primary survey lines. Once plotted, the cross?over points are studied to ensure that there are no miss ties outside acceptable limits. This checks not only the positional accuracy, but also the validity of tidal corrections.
The image to the left shows a typical data presentation for single beam data – bottom elevations printed as text along the vessel’s track. Color contours are also routinely produced.