12^ 



photographs were later reconverted to NAD 27 with CORPSCON for DAMOS database 

 entry and reporting. 



2.3 Bathymetric Data Collectioii and Processing 



An ODOM DF3200 Echotrac® Survey Fathometer with a narrow beam, 208 kHz 

 transducer measured individual depths to a resolution of 3.0 cm (0.1 ft) as described in the 

 DAMOS Navigation and Bathymetry Reference Report (Murray and Selvitelli 1996). 

 Depth values transmitted to INDAS were adjusted for transducer depth. The acoustic 

 remms of the fathometer can reliably detect changes in depth of 20 cm or greater due to 

 the accumulation of errors introduced by the positioning system, changes in sound velocity 

 through the water column, the slope of the bottom, vertical motion of the survey vessel, 

 and tidal corrections. 



Observed tidal data were obtained through the National Oceanographic and 

 Atmospheric Administration (NOAA), Ocean and Lake Levels Division's (OLLD) 

 National Water Level Observation Network. This network is composed of 181 water level 

 stations that are located throughout the Great Lakes and coastal regions of United States 

 interest. These stations are equipped with the Next Generation Water Level Measurement 

 System tide gauges and satellite transmitters that have collected and transmitted tide data to 

 the central NOAA facility every six minutes, since 1 January 1994. 



Observed tide data are available 1 to 6 hours from the time of collection in a station 

 datum or referenced to Mean Lower Low Water (MLLW) and based on Coordinated 

 Universal Time (UTC). For the 1996 WLIS survey, data from NOAA tide station 

 8467150 in Bridgeport Harbor, Bridgeport, CT, was used for tidal calculations. The 

 NOAA 6-minute tide data was downloaded in the MLLW datum, corrected to local time, 

 and tidal differences based on Greens Ledge, Sheffield Island, Connecticut, were applied. 



In order to make valid comparisons between present and past bathymetric surveys of 

 the area, the July 1992 and June 1990 bathymetry models were recorrected to observed 

 MLLW. The OLLD database also provides historic NOAA observed tidal data (31 

 December 1993 and earlier) as hourly water heights. Through interpolation, a smooth tidal 

 curve was developed to allow for accurate tidal corrections of historic bathymetric data 

 sets. 



During the bathymetric survey, a Seabird Instruments, Inc. SBE 26-03 Sea Gauge 

 wave and tide recorder was used to collect tidal data on site. The tide gauge, deployed in 

 the survey area, recorded pressure values every six minutes. After conversion, the 

 pressure readings provided a constant record of tidal variations in the survey area. These 



Monitoring Cruise at the Western Long Island Sound Disposal Site, July 1996 



