Class Name: GM Location 3D

Superclass - Location 3D

Definition

A coordinate within the Geomagnetic (GM) 3D Spatial Reference Frame.

The Geomagnetic Spatial Reference Frame is a right-handed, Object Reference Model/Earth Reference Model (ORM/ERM) mass-centric, rotating coordinate system.

The Z axis defined as parallel to the magnetic-dipole axis. The magnetic-dipole axis is defined as the line through the magnetic north and the magnetic south pole. The geographic coordinates of the north magnetic pole (from the International Geomagnetic Reference Field) are (for epoch 1995) 11.018 deg colatitude (= 90.0 - 11.018 = 78.982 deg N geographic) and -70.905 east longitude. For the purpose of defining the magnetic dipole axis, the magnetic south pole is defined to be diametrically opposed to the magnetic north pole. The magnetic dipole axis is the line defined by the north and south magnetic poles. (NOTE: The location of the true magnetic south pole is not necessarily diametrically opposed to the north magnetic pole.) The location of the north magnetic pole expressed as Vector Vx, Vy, Vz in geocentric coordinates is (0.06252 Re, -0.18060 Re, 0.98157 Re) where Re is Earth radii. The International Association of Geomagnetism and Aeronomy (IAGA) updates the dipole position every 5 years, and 1995 is the epoch for the current location. The U.S. representative to the IAGA working group is John Quinn at the USGS in Golden, CO: 303-273-8475, jquinn@usgs.gov The Air Force's AFRL/VSB maintains the value of the standard radius of the Earth. This value is currently set at 6378145.0 meters.

The Y axis is perpendicular to the geographic poles such that if NM is the north magnetic pole and SG is the south geographic pole, then

Y = NM X SG / |NM X SG|.

This means that the Y axis is perpendicular to the plane defined by the north and south geographic poles and the north magnetic pole. The Y axis has the ORM/ERM mass-center as its origin. By definition, the above cross product produces a right handed coordinate system.

The X axis is defined as perpendicular to the Y-Z plane.

The "prime meridian" of the geomagnetic SRF is defined as the geographic longitude that passes through the magnetic north pole. Therefore, except near the poles, the magnetic longitude will be about 70 degees greater than the geographic longitude (roughly along the east coast of North America).

Transformation:

The following matrix multiplied by the X, Y, Z geographic position vector (expressed in ORM/ERM radii). This yields the X, Y, Z geomagnetic position vector (expressed in ORM/ERM radii).

(0.32110	-0.92756	-0.19112	)		(Vx)		(Vx)
(0.94498	0.32713	0	)	.	(Vy)	=	(Vy)
(0.06252	-0.18060	0.98157	)		(Vz)geo		(Vz)geomag

See the SEDRIS Spatial Reference Model (SRM) for additional details.

Primary Page in DRM Diagram:

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Example

FAQs

Why is the Geomagnetic Coordinate System used?

The positions of magnetic observatories are often expressed in geomagnetic coordinates. Positions of charged particles affected by the earth's magnetic fields as well as the magnetic fields themselves are conveniently expressed in geomagnetic coordinates.

How is the center of earth defined and what is the shape of the surface of the earth that is being assumed?

The references that discuss the geomagnetic coordinate system (ex: Handbook of Geophysics and the Space Environment, Airforce Geophysics Laboratory) do not define the center of the Earth and the shape of the Earth. This is because the applications for ionospheric and near-Earth space analysis that use this coordinate system do not require a formal or precise standard defining the center or the shape of the Earth. If a model were to apply a more precise measure, the output of the model would be essentially unchanged because the inaccuracy inherent in the model is large compared to the differences between the assumption of a spherical earth and another, more accurate ellipsoid. SEDRIS defines the Geomagnetic Coordinate system consistent with the World Geodetic System 1984 (WGS-84) Earth Reference Model definition of the Earth's center and surface ellipsoidal shape. This allows unambiguous interconversion of Geomagnetic and other coordinate system locations (e.g. geodetic).

The matrix transformation listed above assumes that the user has already converted the geographic lat/long/alt to a geocentric vector. This conversion is where the shape of the earth is important. Likewise, once the geomagnetic vector is obtained, the conversion from the geomagnetic vector to a geomagnetic lat/long/radius is where the shape of the earth is important.

Constraints

• Environment Root Spatial Reference Frame
• Locations Under Model

Component of (one-way)(inherited)

• optionally, some Arcs
• optionally, some Base Perimeter Data
• optionally, some Base Reference Vectors
• optionally, some Directional Light Behaviors
• optionally, a Distance Level of Detail Data (notes)
• optionally, some Ellipses
• optionally, some Elliptic Cylinders
• optionally, some Feature Edges
• optionally, a Feature Node
• optionally, some Image Anchors
• optionally, some Labels
• optionally, a Location Table
• optionally, some Morph Points
• optionally, some Point Geometries
• optionally, some Property Grid Hook Points
• optionally, a Reference Origin
• optionally, some Spatial Domains
• optionally, some Spatial Index Related Feature Topologies
• optionally, some Spatial Index Related Features
• optionally, some Spatial Index Related Geometries
• optionally, some Tack Points
• optionally, some Vertices
• optionally, some Vertex with Component Indices
• optionally, some World 3X3s
• optionally, some World Transformations
• optionally, an Attachment Point
• optionally, some Center of Buoyancies
• optionally, some Center of Mass
• optionally, some Center of Pressures
• optionally, a Contact Point
• optionally, some Positional Lights
• optionally, some Separating Planes
• optionally, some Sound Instances
• optionally, some Stamp Behaviors
• optionally, some Volumes
• optionally, some Volume Level of Detail Data
• optionally, some Volume Light Behaviors

Field Elements

Notes

Component of Notes

Distance_Level_of_Detail_Data

 the center point for the LOD test

Fields Notes

gm_latitude

 in degrees

gm_longitude

 in degrees

radius

 distance from the mass-center of the reference object;
 non-negative; expressed in units of ORM/ERM radius