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Data Collection

Home | Abbreviations | Data Collection | Data Processing | Data Access | Metadata | Appendix 1

The data included in this report are single-beam bathymetry datasets collected by USGS-CMGP scientists during field operations spanning the years 1969 to 2000. This publication contains data from 44 cruises located offshore the continental United States, Hawaii Alaska, and the central and south Pacific Ocean (see fig. 1).

Data collected by the Pacific Coastal and Marine Science Center, (InfoBank group) assigns a unique identifier (cruise ID) to each cruise or field activity. Appendix 1 provides a detailed description of the method used to assign the cruise ID.

If the specific navigation or bathymetry system(s) used for collection are known then detailed information are provided in table 1 and also within the MGD77T header record for each cruise.

Table 1. Cruise identifiers, chief scientists, and navigation and bathymetry equipment used to collect the single-beam bathymetry in this archive. Information about cruise identifier naming conventions are provided in appendix 1.

[DGPS, Differential Global Positioning System; GPS, Global Positioning System; kHz, kilohertz; Loran, long range aids to navigation; RHO-RHO, range-range; SINS, shipboard integrated navigation system]

Bathymetry

Researchers within the USGS-CMGP collect bathymetry data using several types of instruments; this report provides only single-beam bathymetry data. In single-beam systems, an acoustic pulse is emitted from a transducer and propagated in a single, narrow cone of energy directed downward toward the sea floor. The transducer(s) then "listens" for the reflected energy from the sea floor. Water depth is calculated by measuring the amount of time the sound wave takes to travel from the transducer to the seafloor and back to the receiver, called two-way travel time. Two-way travel time is multiplied by the speed of sound in water and divided by two. The individual depth values can then be used to generate bathymetric maps (Duesser and others, 2002).

The speed (or velocity) of sound in water is a function of temperature, pressure and salinity (T-P-S). Rarely, these variables were measured directly by means of a bathythermograph (T-P) or more sophisticated sound velocity profiler (T-P-S). Some bathymetric systems employed the tables of sound velocity compiled by Matthews (1939) or Carter (1980). Most commonly, however, a simple constant velocity was used in the conversion, typically 1,500 meters per second (m/s). Particularly, in deep water, the speed of sound used in the depth conversion introduces considerable uncertainty in the depth calculation. Rarely was the speed of sound that was used documented. 

Bathymetry data in this report include digitally collected bathymetry and data that was hand digitized by researchers from analog records. The frequency of the single-beam bathymetry systems that the USGS-CMGP operated during these surveys ranged from 3.5 to 200 kilohertz (kHz).

Navigation

Navigation is an important element for any acoustic marine survey. The navigation data in this archive were collected using a number of different instruments and methods that are described below.

Shipboard Integrated Navigation System (SINS) used transit satellite navigation and a number of subsystems that were used in dead reckoning between satellite fixes. These included doppler sonar, speed-of-sound velocimeter, water-temperature sensor, gyrocompass, electromagnetic speed log, radionavigation and other subsystems. SINS was highly variable and at best there was a 50-meter(m) accuracy (Brune, 1977).

Long range aids to navigation (Loran-C) was a federally provided radionavigation service managed by the U.S. Coast Guard. Low frequency radio signals transmitted by fixed land based radio beacons served the 48 continental states, their coastal areas, Hawaii, and parts of Alaska. The system provided better than 0.25 nautical mile accuracy for users. (U.S. Coast Guard, 1994). Loran-C could be utilized in different modes of operation; one of these was circular mode, also known as, range-range or RHO-RHO. It only needed a fix from two stations to establish a position and an accuracy of 20 to 40 m was achievable (Telford and others, 1991). The Loran-C signal was discontinued in 2012, although Loran-C had been replaced for most oceanographic research vessels decades prior to 2010.

The miniranger system is a range-range system that utilizes radar frequencies in the microwave band. The system is limited to line-of-sight operation between two or more fixed transponders, but an individual reading can be accurate to within 3 m (Brune, 1977).

The Global Positioning System (GPS) is a space-based satellite navigation system maintained by the U.S. Government and is accessible using a GPS receiver. Most GPS manufacturers quote accuracy the of their receivers between about 15 and 17 m. GPS is based upon the earth-centered coordinate system called the World Geodetic System of 1984 (WGS84) for horizontal and vertical positioning.

Differential GPS (DGPS) uses a network of fixed, ground-based reference stations to augment the information available from satellites. This method provides an improved overall positional accuracy of 10 m or better (U.S. Coast Guard, 2012).

The USGS-CMGP used a wide variety of GPS and DGPS receivers. The positional accuracy is based on the type of receiver. If the type of receiver is known, it is listed in the navigation instrumentation column of table 1.

Home | Abbreviations | Data Collection | Data Processing | Data Access | Metadata | Appendix 1