FTP: The first record located at the top of the header used to identify the file format. VER: HYPACK version number. INF: General survey information filled in by the data technician. On this cruise, none of that information was filled in. The three numbers at the end refer to initial tide correction at start-of-line, initial draft correction at start-of-line and sound velocity. FIL: Raw format file. ELL: Ellipsoid information. The name of the ellipsoid followed by the semi-major axis in meters and the flattening ratio. PRO: Project information record where TME indicates Transverse Mercator and the central meridian of -75 indicates UTM, zone 18. DTM: Datum transformation record. GEO: Geoid Model File. Blank if not present. GEO <geo_file> <h_corr> where 'h_corr' is orthometric height correction in meters. HVU: Horizontal and Vertical Units (meters) TND: Survey time and date in UTC. DEV 0: Indicates that the lines with device designation 0 are lines of Lowrance GPS data. OFF 0: device offsets for device 0. Format of the value is OFF dn n1 n2 n3 n4 n5 n6 n7 where dn=device number; n1=starboard, port offsets where positive is starboard; n2=forward, aft offset where positive is forward; n3=height (antenna) or depth (transducer draft) offset - always positive; n4=yaw rotation angle where positive for clockwise rotation; n5=roll rotation angle where port side up is positive; n6=pitch rotation angle where bow up is positive; n7=device latency in seconds. ***No offsets were recorded for device 0 during this data collection. LIN: planned line data follows in the format "LIN nw" where nw=number of waypoints. PTS: planned line waypoints in the format "PTS x y" where x=waypoint easting; y=waypoint northing. ***In this case, easting and northing UTM zone 18N, meters. LBP: planned line begin point in the format "LBP x y" where x=x grid position; y=y grid position. ***In this case easting and northing, UTM Zone 18N, meters. LNN X: planned line name where X is the line name EOL: end of planned line EOH: end of headerThe remaining elements have similar information in the first 3 columns. The first column will indicate the data type, the second column will indicate the device that recorded the information (0 for Lowrance) and the third column is the time tag (seconds past midnight) that is also sometimes referred to as the latency. The remaining information on each line is specific to the data type.
POS: Position of the ship - in this case the antenna location - in the format "POS dn t x y" where dn=device number; t=time tag (seconds past midnight); x=easting; y=northing. During this survey these values are in UTM, Zone 18, WGS84.
QUA: Position quality information in the format "QUA dn t n m h sat mode" where dn=device number; t=time tag (seconds past midnight); n=number of values to follow; m = 10 minus HDOP (horizontal dilution of precision); h=HDOP; sat=number of satellites; mode=GPS mode (NMEA 0183 standard values) where 0=fix no available or invalid, 1=GPS fix, 2=Differential GPS fix, 3=GPS PPS Mode fix, 4=RTK fix, 5=RTK Float.
RAW: Position information in the format "RAW dn t n lat long alt utc" where dn=device number; t=time tag (seconds past midnight); n=number of values to follow; lat=raw latitude X 100; long=raw longitude X 100; alt=antenna altitude above ellipsoid (meters); utc=GPS time in the format HHMM.
EC1: Echo Sounding (single frequency) in the format "EC1 dn t rd" where dn=device number; t=time tag (seconds past midnight); rd=raw depth.
MSG: Message string in the format "MSG dn t message" where dn=device number; t=time tag (seconds past midnight); message is the message sent from the device. During this survey, three different messages were recorded from the GPS: $GPGGA, $GPGLL and $SDDBT. These will be defined later.
FIX: events marked manually by the user in the format "FIX dn t event_number" where dn=device number (typically 99 as there is no device for manual events); t=time tag (seconds past midnight); event_number=event number such as 1,2,3,4. The NMEA strings present in the HYPACK file are the $GPGGA, $GPGLL, $SDDPT and $SDDBT. These are defined as follows.
$GPGGA is GPS fix data in the format "$GPGGA, t, lat, lath, long, longh, q, sat, h, a, M, alt, M, t2, refcheck" where t=time in UTC in the format hhmmss.ss; lat=latitude in the format ddmm.mmmmmm; lath= N or S indicating the latitude hemisphere; long=longitude in the format dddmm.mmmmmm; longh=E or W indicating the longitude hemisphere; q=fix quality where 1 is a GPS fix, 2 is a DGPS fix, 3 is a PPS fix, 4 is a Real Time Kinematic fix, 5 is Float RTK, 6 is estimated (dead reckoning), 7 is manual input mode, 8 is simulation mode; sat=number of satellites; h=Horizontal Dilution of Precision (HDOP); a=Antenna altitude above mean sea level (geoid); M= units of antenna altitude in meters; alt=height of geoid above WGS84 ellipsoid; M=units of geoidal height in meters; t2=time since last DGPS update; refcheck=DGPS reference station id and the checksum.
Example of a general $GPGGA string: $GPGGA,180707,3835.9291,N,07509.0955,W,1,10,1.18,-6,M,,,,*0A UTC Time = 180707 Latitude = 3835.9291 N Longitude = 07509.0955 W Fix Quality = 1 Number of satellites = 10 HDOP = 1.18 relative accuracy of horizontal position Altitude = -6 meters above mean sea level Height of geoid above WGS84 ellipsoid = not recorded Time since last update = not recorded Checksum = *0A.$GPGLL is geographic position date in the format "$GPGLL, lat, lath, long, longh, t, vcheck" where lat=latitude in the format ddmm.mmmmmm; lath=N or S indicating the latitude hemisphere; long=longitude in the format dddmm.mmmmmm; longh=E or W indicating the longitude hemisphere; t=time in UTC in the format hhmmss.ss; vcheck=data valid and checksum.
Example of a general $GPGLL string: >$GPGLL,3835.9286,N,07509.0972,W,180708,A*31 Latitude = 3835.9286 N Longitude = 07509.0972 W UTC time = 180708 Checksum = A*31$SDDPT is water depth in the format "$SDDPT, data_meters, offset *checksum"
Example: $SDDPT,1.6,0.0*50 Depth in meters = 1.6 Offset from transducer:0.0 Positive - distance from transducer to water line, or Negative - distance from transducer to keel$SDDBT is water depth below the transducer in the format "$SDDBT, depth, units (f=feet), depth, units (m=meters), depth, units (F=fathoms), *checksum"
Example: $SDDBT,5.2,f,1.6,M,0.8,F*0E Depth in feet = 5.2 Depth in meters = 1.6 Depth in fathoms = 0.8 Checksum = *0EThe definitions of the HYPACK strings were acquired from the HYPACK software manual available from: <http://www.hypack.com/>. The definitions of the NMEA strings were obtained from: <http://home.mira.net/~gnb/gps/nmea.html> and the SDDPT definition was obtained from: <http://www.eye4software.com/products/gpstoolkit/nmea/>