4-m Grid of the Combined Multibeam and LIDAR Bathymetry Generated from National Oceanic and Atmospheric Administration (NOAA) Surveys H11224, H11225, H11250, H11251, H11252, H11361, H11441, H11442, H11445, H11446, H11997, H11999, H12012, and H12013 Offshore in Eastern Long Island Sound and Westernmost Block Island Sound (ELISCOMB_GEO, Geographic, WGS84)

The NOAA Ship Thomas Jefferson, and two 8.5-m aluminum Jensen launches deployed from the ship, were used to acquire the multibeam echosounder (MBES) bathymetric data between October 2003 and May 2009 for surveys H11250, H11251, H11252, H11361, H11441, H11442 H11445, H11446, H11997, H11999, H12012, and H12013. Aboard the launches, the MBES data were collected with hull-mounted RESON SeaBat 240-kHz 8101 and 455-kHz 8125 shallow-water systems. These systems measure two-way sound travel time across a 150-degree swath and 120-degree swath, respectively. The SeaBat 8101 has 101 beams at a 1.5-degree beam spacing. The SeaBat 8125 has 240 beams with a cross-track beam width of 0.5 degrees at nadir. Aboard the Thomas Jefferson, MBES data for surveys H11250, H11252, and H11361 were collected with a SIMRAD EM1002 system; later surveys were collected with a RESON 7125 system. Original horizontal resolution of the MBES data was 0.5 to 2 m, depending on survey and water depth; vertical resolution of the MBES data is about 0.5% of the water depth. The bathymetric data were acquired in XTF (extended Triton data format) and processed by NOAA using CARIS HIPS (Hydrographic Image Processing System) software for quality control, to incorporate sound velocity and tidal corrections, and to produce the continuous digital terrain model. Navigation was by TSS POS/MV 320 differential GPS-assisted inertial navigation systems; the differential GPS beacons used were from Moriches, New York, and Sandy Hook, New Jersey. HYPACK MAX was used for acquisition line navigation. Sound velocity corrections were derived using frequent SEACAT CTD (conductivity-temperature-depth) profiles. Typically, a CTD cast was conducted every four to six hours of MBES acquisition. Tidal zone corrections were calculated from data acquired from National Water Level Observation stations at New London, Connecticut, New Haven, Connecticut, and Montauk, New York. Vertical datum is mean lower low water. Individuals interested in detail descriptions of the MBES acquisition and processing should consult the descriptive reports. Eleven reconnaissance cruises were conducted aboard a leased lobster boat to monitor water clarity prior to commencement of airborne LIDAR operations of surveys H11224 and H11225. These cruises, which utilized Secchi disks to determine water clarity, were conducted between the Connecticut River and Fishers Island during May 9, 2003 and January 11, 2004, when water clarity was deemed adequate. The LIDAR (light detection and ranging) data were acquired by Tenix LADS, Inc. during January 25 - March 3, 2004 with a LADS Mk II system. Airborne survey operations were conducted aboard a Dash 8-202 series aircraft with a Nd:YAG laser at heights between 1,200 and 2,200 feet, at ground speeds between 140 and 210 knots, and at 4x4 and 3x3-m laser-spot spacings. Green laser pulses were scanned beneath the aircraft in a rectilinear pattern. The returned green-laser energy reflected from the sea surface and seafloor were captured and digitally recorded. Received infrared laser energy, supplemented by an Altitude Heading Reference System (AHRS) inertial height reference and kinematic GPS, determined the height of the aircraft. Real-time positioning was provided by WADGPS (wide area differential global positioning system). Acquired data were processed with CARIS HIPS. Individuals interested in detail descriptions of the MBES and LIDAR acquisition and processing should consult the descriptive reports. Vertical datum is mean lower low water; X and Y units are meters; UTM Zone 18, NAD83.

The field sheets from the surveys (average depths, not the shoal based field sheets) were read back into CARIS HIPS/SIPS software version 7.0. Using the Base Surface -> Combine command within HIPS, all of the source CARIS base surfaces were combined into a master BASE surface (eliscomb) having a 4 meter grid resolution.

The combined base surface was exported from CARIS as a Bathymetric Attributed Grid (BAG) file. See http://www.opennavsurf.org/papers/ons_fsd.pdf for a description of the BAG format.

The BAG surface was imported into Fledermaus v7.2 to make equivalent digital terrain model (DTM) files. The floating point format for each sounding depth point is preserved during data import. Additionally, the BAG surface was imported into Fledermaus as "gridded data", therefore re-gridding of data did not occur during the import process and thus preserved the original resolution of the CARIS base surface. The dtm was then exported as an ESRI ASCII raster file.

The ESRI ASCII raster file was then converted into raster within ArcToolbox v9.3 using the ASCII to Raster conversion tool, and reprojected using the data management tool Project Raster (ArcToolbox, Data Management Tools, Projections and Transformations) from UTM Zone 18N, NAD 83 to Geographic, WGS84. The options specified in the Project Raster tool were: 1. Input coordinate system: NAD_1983_UTM_Zone_18N 2. Output coordinate system: GCS_WGS_1984 3. Geographic transformation method: NAD_1983_WGS_1984_5 4. Resampling technique: BILINEAR 5. Output cell size: 0.000043 (decimal degrees, default as calculated by ArcMap for a 4 meter cell size) The geographic transformation selected is the most accurate one provided by ESRI as it ties in WGS84 with ITRF96, thus reflecting the earth centered offset (x-y-z) between WGS84 and NAD83 (from the ESRI Knowledge Base Document Number 24159).

Edits to the metadata were made to fix any errors that MP v 2.9.46 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). The distribution format name was modified in an attempt to be more consistent with other metadata files of the same data format. Reordered the links in the identification section so a landing page link is first. Added the source citation abbreviation and used the title for the value. Attempted to modify http to https where appropriate. The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

Added keywords section with USGS persistent identifier as theme keyword.