267

Study Name:

Survey Type:

PDS-1995-1997

Bathymetry

Survey Name:

PDS-1995-1997 BASELINE Bathymetry

Disposal Site:

PDS (1995-pres)

Start Date:

8/7/1995

End Date:

8/11/1995

Collector:

SAIC

Process Description

Bathymetric data were processed using SAIC's Hydrographic Data Analysis System (HDAS), version

1.03. Raw bathymetric data were imported into HDAS, corrected for sound velocity, and

standardized to MLLW using the NOAA observed tides. The bathymetric data were then used to

construct depth models of the surveyed area. The data is narrow, single-beam bathymetry over a

survey grid with lanes spaced 25 m apart. The individual depth soundings were averaged and

gridded within 12.5 m x 25 m cells, yeilding a digital depth matrix. Contour charts and three-

dimensional representations of the bottom were then produced through interpolaiton between

Logical Consistency

The precise navigation and data integration required for all field operations was provided by use of

SAIC's Portable Integrated Navigation and Survey System (PINSS). The PINSS navigation software

was resident on a Toshiba® 3200XT personal computer (PC) capable of providing real-time

navigation, as well as collect position, depth, and time data for later analysis.

Attribute Accuracy

Bathymetric surveys are conducted using an integrated navigation system and an ODOM

HYDROTRAC echo sounder. Accuracy limits for the echo sounder are rated to 0.01-0.05% for the

overall depth. Bathymetric data are corrected for the speed of sound in sea water calculated with an

internally recorded Sea-Bird Electronics, Inc. SBE 19-01 Seacat Profiler CTD. The data are also tide

corrected during post survey processing using near real time NOAA tide data.

Horizontal Accuracy

Vessel position was determined via a Del Norte Trisponder® System to an accuracy of ±3 m. This

system collects position data from two shore stations were established along the Maine coast at the

known benchmarks of Portland Head Light (43° 37.381´N, 70° 12.502´W) and Cape Elizabeth Light

(43° 33.959´N, 70° 12.034´W) to provide 3-meter position accuracy. This position was then

interfaced into PINSS allowing for real-time navigation. Vessel position was displayed on two

monitors, one for the survey navigator and the second for the helmsman, to aid in steering the vessel

toward target locations (i.e. end of survey lane). All positions received were displayed in North

Vertical Accuracy

An ODOM HYDROTRAC echosounder measured depths with a narrow-beam, 208 kHz transducer.

Accuracy limits for the echosounder are rated at 0.01 to 0.05 percent of the overall depth. The

transducer was installed near the on-board GPS receiver antenna to reduce any horizontal offset

between navigation and bathymetry data. In addition, the transducer was set with a 1-meter draft to

avoid surface interference with acoustical signal. The transducer pole was marked and checked

frequently during the survey for any movement. The 1-meter draft was corrected in the ODOM, and

the speed of sound was fixed at 1500 m/s. A Sea-Bird Electronics, Inc. Model 19-01 CTD was used

to acquire vertical profiles of the speed of sound in the water column at the beginning and end of

each survey day. This information was used after the survey to adjust the bathymetric data if the

Depth Datum Name

Mean Lower Low Water

Depth Distance Units

m

Use Constraints

This data was collected and processed for a specific U.S. Army Corps of Engineers activity and

indicates the general existing conditions at the time of collection. As such, the data are only valid for

their intended use, content, time, and accuracy specifications. The user is responsible for the

results of any application of the data for other than the intended purpose.