ECEF stands for Earth-Centered, Earth-Fixed, and is a Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system. It represents positions as an X, Y, and Z coordinate. The point (0,0,0) is defined as the center of mass of the earth, hence the name Earth-Centered. Its axes are aligned with the International Reference Pole (IRP) and International Reference Meridian (IRM) that are fixed with respect to the surface of the Earth, hence the name Earth-Fixed.

The z-axis is pointing towards the north but it does not coincide exactly with the instantaneous Earth rotational axis. The slight "wobbling" of the rotational axis is known as polar motion

Polar motion

Polar motion of the earth is the movement of Earth's rotational axis across its surface. This is measured with respect to a reference frame in which the solid Earth is fixed...

. The x-axis intersects the sphere of the earth at 0° latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 (Equator

Equator

An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

) and 0° longitude

Longitude

Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

 (Greenwich

Prime Meridian

The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...

). This means that ECEF rotates with the earth and therefore, coordinates of a point fixed on the surface of the earth do not change. Conversion from a WGS84 Datum to ECEF can be used as an intermediate step in converting velocities to the North East Down

North East Down

North east down , also known as local tangent plane , is a geographical coordinate system for representing state vectors that is commonly used in aviation. It consists of three numbers: one represents the position along the northern axis, one along the eastern axis, and one represents vertical...

 coordinate system.

See also

• Geodetic system
  Geodetic system
  Geodetic systems or geodetic data are used in geodesy, navigation, surveying by cartographers and satellite navigation systems to translate positions indicated on their products to their real position on earth....
  
  
• Earth Centered Inertial
  Earth Centered Inertial
  Earth-centered inertial coordinate frames have their origins at the center of mass of the Earth. ECI frames are called inertial in contrast to the Earth-centered, Earth-fixed frames which rotate in inertial space in order to remain fixed with respect to the surface of the Earth...
  
   coordinate system
• International Terrestrial Reference System
  International Terrestrial Reference System
  The International Terrestrial Reference System describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of...
  
   (ITRS)

External links

• ECEF datum transformation Notes on converting ECEF coordinates to WGS-84 datum
• Datum Transformations of GPS Positions Application Note Clearer notes on converting ECEF coordinates to WGS-84 datum
• Geodetic Datum Overview Orientation of the coordinate system and additional information
• GeographicLib includes a utility CartConvert which converts between geodetic
  Geodetic system
  Geodetic systems or geodetic data are used in geodesy, navigation, surveying by cartographers and satellite navigation systems to translate positions indicated on their products to their real position on earth....
  
  and geocentric (ECEF) or local Cartesian (ENU) coordinates. This provides accurate results for all inputs including points close to the center of the earth.