MGL1109centerdepth.shp: Multibeam bathymetry data collected by the U.S. Geological Survey in the Gulf of Alaska in 2011 during cruise MGL1109, along-track center-beam depths extracted from 100-meter gridded data in shapefile format, geographic coordinates

Multibeam bathymetry and backscatter data were collected using a 12 kHz Kongsberg Simrad EM-122 multibeam echosounder and SIS (v3.8.2) acquisition software. The system consists of two hull-mounted transducer arrays, a transmit unit (150 deg. x 1 deg.) located along the ship centerline, and a receive unit (1 deg. x 30 deg.) that is abaft and oriented athwart-ships. The transducer pod is tilted 4 degrees towards the bow to reduce noise from cavitation and bubbles. The system forms a swath of 288 beams that produce an across-track profile of 432 soundings. The SIS software processes range/angle data from each ping during acquisition, merging position, inertial-motion, and sound-velocity data to derive a solution for vessel motion and raybending, and calculate sounding depths and positions. Sound velocity information entered into SIS was obtained from the Applied Micro Systems Micro SV mounted on the transducer pod and 52 XBT profiles. XBT profiles were extended to 12 km depth with data generated by the unix program 'mblevitus', a module of the multibeam processing package MB-System (Caress and Chayes, 2006). Average water column salinity of 34.4 ppt was determined by extrapolating data from XCDT casts. Multibeam data were collected nearly continuously throughout the cruise, effectively mapping the seafloor along the course of the ship's track. Beam angle was set at 70 on each side throughout the program and swath widths were generally between 3 and 4 times water depth. SIS logged bathymetry and backscatter solutions to the Simrad '.all' format for archival purposes.

CARIS processing for MGL1109: A new CARIS HIPS project (ver. 7.1) was created with projection information set to Universal Transverse Mercator (UTM) Zone 6N, WGS84. Each Simrad EM122 data file (*.all) was imported to the new CARIS project using the Import/Conversion Wizard. A 100 meter resolution Bathymetric and Statistical Error (BASE) Surface was created and new files were added to the surface each Julian day. With the addition of new files, the BASE surface was reviewed for any inconsistencies or data anomalies. Navigation was edited as needed using the navigation editor tool in CARIS. Filters were applied to each line including beam to beam slopes and across track angle. The CARIS "surface filter" was applied to the data using the two standard deviation option. Swath data were also edited manually to remove obviously outlying soundings.

The 100 meter CARIS HIPS BASE surface for MGL1109 was exported from CARIS as a Bathymetric Attributed Grid (BAG) file. The resulting BAG file was used to import the surface into IVS3D DMagic as gridded data, then the surface was saved as a Fledermaus Digital Terrain Model in the IVS3D SD format. Finally, the Fledermaus ArcGIS module was used to export the surface directly to an ESRI Binary Grid.

Navigation information (Easting,Northing,Longitude,Latitude,UTC Time,Year-Julian Day,SIS Filename) were extracted from the CARIS HIPS database utilizing a Perl script that ran the CARIS command 'printfnav' on all edited EM122 data files. Unix 'cat' was used to concatenate the output nav files to a single file, 'grep' was used to extract only soundings flagged as accepted, then an 'awk' filter was used to extract records with unique navigation fixes, one per second. The resultant text file was used to create an ArcGIS point feature class (ArcMap, create feature class from XY table).

The navigation text file created in the previous step was used to create an ArcGIS point feature class (ArcMap, create feature class from XY table). The navigation point feature class and the ESRI binary grid 'MGL1109_bathy' were used as the point and raster inputs for the ArcGIS Spatial Analyst tool 'Extract values to points', with the interpolate values at point locations option selected. The resultant point shapefile containing "Easting, Northing, Longitude, Latitude, UTC Time, Year-Julian Day, SIS Filename, Depth (meters)" was exported to ASCII text.

A trackline shapefile strung through the original centerdepth file (MGL1109_EM122centerdepth.txt) had a loop that was not present in the 1-minute shipboard navigation fixes. The original file appeared to have been sorted on the FILE attribute, which has Julian day and time embedded, but some entries did not match the YRJD and UTC attributes and were, consequently, not time sequential. The text file was converted to a point shapefile with a new attribute "jd_hhmmss" combining YRJD and UTC that was used to re-sort the centerdepth file. The "sequence" attribute is the record number after the re-sort. The shapefile attributes are EAST,NORTH,LON,LAT,UTC,YRJD,FILE,EM122depth,jd_hhmmss,sequence. (fwong@usgs.gov)

Metadata was modified to bring up to current USGS PCMSC standards. Point of Contact and Metadata Contact information was updated. Keywords were added and refined. Cross Reference citation added. Details about different Network Resource Name links was given in the Access Instructions section. Minor typos were corrected. No data information was changed.