# Geodetic Parameters and Positioning

The R/V Marcus G. Langseth is equipped with a number of positioning systems used for navigational capabilities as well as survey calculations and data processing.  Knowing where the ship is and the data collected is extremely important for many reasons.

Vessel Positioning Reference Systems
 System Equipment Primary Nav system C-Nav dGPS Secondary Nav system Seapath dGPS Tailbuoy navigation PosNet rGPS Source navigation PosNet rGPS (1 unit per subarray) Acoustics DigiCourse Navigation processing Concept Sprint 4.3.9 Bird Controller DigiCourse Survey-Gyro (Primary) Simrad GC-80 Ships-Gyro (secondary) Sperry MK-27 Speed Log Furuno DS-50 Multibeam Kongsberg EM-122

All survey calculations will use the World Geodetic System 1984 datum (WGS84) UTM Zone (TBD at sea) projection.  The Global Positioning System (GPS) operates on the WGS84 datum. The vessel’s Differential GPS (dGPS) Reference Stations are defined in the WGS84 datum.

In order to obtain optimized navigation, waypoints need to be in decimal degrees (DD) to five (5) decimal points.  Decimal degrees express latitude and longitude geographic coordinates as decimal fractions and are used in many Geographic Information Systems (GIS), web mapping applications such as Google Maps, and GPS devices.  Negative numbers represent latitudes south of the equator and longitudes west of the Prime Meridian.
Example:  38.88972, -77.00888

The geodetic and projection parameters are detailed in the following tables:

Project geodetic parameters
 Global Positioning System Geodetic Parameters Datum World Geodetic System 1984 (WGS84) Reference Ellipsoid WGS84 Semi Major Axis (a) 6378137.0 m Inverse Flattening (1/f) 298.257224 Survey (local) Geodetic Parameters Datum World Geodetic System 1984 (WGS84) Reference Ellipsoid WGS84 Semi Major Axis (a) 6378137.0 m Inverse Flattening (1/f) 298.257224 Datum Transform Parameters: Global to Survey Datum X shift: 0.0 m X-axis Rotation: 0.0 arcsec Scale correction: 0.0 ppm Y shift: 0.0 m Y-axis Rotation: 0.0 arcsec Z shift: 0.0 m Z-axis Rotation: 0.0 arcsec

Project Projection Parameters - Example (different based on the location of the cruise)
 Map Projection (Project Projection Parameters) Grid Universal Transverse Mercator (UTM) Projection Type Universal Transverse Mercator (UTM), Northern Hemisphere Projection Zone 28 Latitude at Origin 0 Longitude at Origin (Central Meridian) -15 False Easting 500000 False Nothing 0 Scale Factor at Central Meridian 0.9996 Grid Units Meters

Gravity Tie Point
Per standard procedure, a gravity tie will be done before and after this cruise.

Positioning Reference Systems
Two independent standard multi-station dGPS systems are required for the survey.

Magnetic Declination
Magnetic declination is the angle between compass north (the direction the north end of a compass needle points) and true north (the axis around which the earth rotates). Magnetic north is determined by the earth's magnetic field and is not the same as true (or geographic) north. Depending on where you are on the earth, the angle of declination will be different - from some locations, the geographic and magnetic poles are aligned so declination is minimal, but from other spots, the angle between the two poles is pretty big.
Declination is calculated using the current International Geomagnetic Reference Field (IGRF) model. While results are typically accurate to 30 minutes of arc, users should be aware of several factors that can cause disturbances in the magnetic field.
• The accuracy of the model will worsen at locations close to the magnetic poles.
• As the time from the epoch of the model increases, uncertainties in the estimate of secular variation will result in an increasingly large difference.
• Magnetic minerals in local geological formations cause magnetic anomalies that can sometimes be very large. These cannot be reproduced by reference field models.
• Large magnetic storms can temporarily cause large changes in the magnetic field, especially at high latitudes.
The IGRF is a series of mathematical models of the Earth's main field and its annual rate of change (secular variation).
For every cruise, magnetic declination is computed based on the date and position using a magnetic field calculator