Bathy-1500 Survey of Castle Hill

    During the Fall months of 1998, engineers from Ocean Data Equipment Corporation were conducting operational tests on the new Bathy-1500 survey echo sounder. As part of the functional test plan, a survey of the Castle Hill area was conducted. The Bathy-1500 was equipped with a 200kHz transducer. A heave sensor was incorporated to allow heave corrections to be performed in real time. Data was recorded in ODEC HEAVE format to the internal Flash memory of the Bathy-1500 and downloaded to a laptop computer after the survey was completed.

    Following planned survey lines (see Figure 1) the Bathy-1500 recorded simultaneous measurement of time, depth, heave and location. The ODEC HEAVE formatted data was directly imported and processed using "Surfer for Windows" by Golden Software. Surfer creates a representative grid of regularly spaced data from a data set containing irregularly spaced (x, y, z) data. In order to increase the accuracy of the gridded data, the coastline of Castle Hill (zero depth or mean sea level) that fell within the grid boundaries, was introduced into the depth grid. The coastline data points were extracted from a digital coastline file of Narragansett Bay.

     Castle Hill escarpment is located in the entrance to the east passage of Narragansett Bay (See map Figure 2). The entrance to the passage is very narrow, being bounded on either side by Newport and Jamestown. The narrow dimensions of the passage cause tidal currents to move very swiftly as they pass in and out of the bay. Over the years the swift currents have carved out the sea floor thereby deepening the waters near Castle Hill. These unique features make it a prime location for conducting a hydrographic survey to examine the features of the escarpment. In addition, the dynamic sea floor features of the Castle Hill escarpment offers an excellent opportunity to test the reliability and functionality of an echo sounder.

     Once a file of regularly spaced data is generated by Surfer, the results may be viewed in a variety of formats in Surfer. Example formats include two-dimensional contour mapping, post and classified-post mapping, shaded relief mapping, and three-dimensional surface contour maps. In addition, data may be viewed as a 'slice' along any user defined transect or two or more display formats may be combined. As an example, a surface contour map may be combined with a two-dimensional contour map to generate a three-dimensional color surface contour map. Figure 3 combines both two- and three-dimensional contour maps and also a post map. The small "+" marks shown in the figure are locations where a depth measurement was taken and are generated from a post map. The "+" are shown only for every 75th data point to avoid cluttering the figure.

     The two-dimensional contour map of Castle Hill escarpment, shown in Figure 4 illustrates some interesting features of Castle Hill. There is a steep gradient surrounding the shores of Castle Hill. From the water line (MSL) the water depth increases rapidly to 40 or 50 meters deep. The closely packed contour lines surrounding Castle Hill detail this feature. The "cliff' bounds one side of a deep trench which runs through the escarpment parallel to the shoreline of Castle Hill. On the northwest side of the trench the sea floor slopes upward at a gentler grade. In the northwest comer of the grid there is a large flat area. This feature is not representative of the actual bathymetry, but is caused by the absence of any measured data in this area. The main feature observed is the deep hole located immediately to the west of the northern tip of Castle Hill. The depth of the hole is approximately 60 meters (195 feet).

   Data collected from the Castle Hill survey has shown that the Bathy-1500 is capable of providing valid hydrographic data in areas of widely varying bathymetry. During the survey the Bathy-1500 continually tracked the sea floor without loss of data. The resulting data set was easily imported into a post-processing package (Surfer for Windows) for further analysis and presentation. Presently the Bathy-1500 Survey Echo Sounder provides data formats that are compatible with Hypack software by Coastal Oceanographics, HYDROpro by Trimble Navigation, and most data spreadsheet/plotting software packages (e.g., Surfer for Windows, Microsoft Excel, Corel Quattro Pro, etc.).

Figure 1
Castle Hill survey lines.

Figure 2
Narragansett Bay, Rhode Island detailing the Castle Hill Survey area and surrounding area

Figure 3
Combined two-dimensional, three-dimensional, and post map of Castle Hill bathymetry.

Figure 4
Two-dimensional contour map of Castle Hill.

Three-dimensional wire frame representations are shown in Figures 5 through 7

Figure 5 Observes the bathymetry from the Northwest

Figure 6 (constant x-y contours)

Figure 7 (constant z contours) view the data set from the Southwest.

Figure 8

Details a depth profile or "slice" of data extracted from the three-dimensional Surfer grid.
Depth profile along line of constant latitude (41.4640).