Surveying and Mapping

Marine bathymetric surveys are extremely useful in conducting pre- and post-dredging volume calculations; locating aids to navigation; designing docks, boat ramps, moorings, bridges, breakwaters and sub aqueous utility crossings. Hydro EMI also uses the same technology to map environmental features such as oyster beds, grass beds and reefs. As sediment movement and deposition is of concern both environmentally and commercially, repeated measurements over time at the same location can provide clues as to the rate, pattern and causes of the deposition process.

Hydro Engineering & Mapping, Inc. (Hydro EMI) uses Real Time Kinematic Global Positioning System (RTK GPS) electronics to collect and correct three-dimensional position data from orbiting satellites. The RTK GPS system is comprised of two GPS units, a base station and a rover. The base station is established at a point of known elevation and location. The elevation at the base station is usually referenced to the local mean lower low tidal datum, but just about any datum can be used. The base continuously computes and transmits three-dimensional position corrections to the rover unit which is mounted on the mapping boat.

When combined with depth soundings from an on-board echo sounder, the data stream is fed to an on-board computer where it is processed by a sophisticated marine surveying software package known as HYPAK. As the base station is correcting both the horizontal and vertical positions of the rover unit, vertical errors due to the influence of tides and swells on the measured depth are significantly corrected. To obtain the best corrections, the base station is positioned in close proximity to the area being mapped. Since three-dimensional positions and water depth are collected once each second, the potential effect of other errors are minimized by averaging the huge amount of data collected in the depth contouring process. To see a pictorial presentation of this process click here.

Completed data sets are quickly scanned to identify and remove obviously inconsistent data points that occasionally result from propeller turbulence and dislodged bottom sediment. The resulting depth and position information can be displayed by the HYPAK software as individual data points, cross-sections, three-dimensional pictures and contour maps. HYPAK’s triangular irregular network (TIN) terrain modeling feature is utilized to process the position and depth information into depth contours (lines of equal depth) referenced to mean lower low water. The depth contours are typically shown on a geo-referenced aerial photograph background. Depths are typically indicated in feet and positions are referenced to the 1984 World Geodetic System (WGS 84) which is the same datum used by the GPS satellites. However, just about any datum can be used.

When it is desired to collect soundings in areas too shallow to navigate the mapping boat or to include land features, the rover unit is quickly uncoupled from the boat and mounted on a staff for manual measurements on foot.