The system consists of an origin and three coordinate planes.  See Figure 2-13.     1.  Origin:  The origin is the intersection point of the three coordinate planes and is located at the center of the reference ellipsoid.  In cases where the origin is also located at the center mass of the earth, the system is considered to be geocentric.    2.  Coordinate Planes:  The three mutually perpendicular coordinate planes intersect in three straight lines called coordinate axes.  The axes intersect at right angles at the origin. a.  Z-Axis:  The Z-Axis corresponds to the rotational axis of the reference ellipsoid (Semi-minor Axis).  It lies perpendicular to the plane containing the X and Y Axes.  The z-axis is positive from the origin to the north pole. b.  X-Axis:  The X-Axis lies on the equatorial plane of the reference ellipsoid at the intersection of the equatorial plane and the plane containing the Prime Meridian.  It is perpendicular to the plane containing the Y and Z Axes.  The x-axis is positive from the origin to the Prime Meridian. c.  Y-Axis:  The Y-Axis lies on the equatorial plane of the reference ellipsoid, perpendicular to the X-Axis.  It is perpendicular to the plane containing the X and Z Axes.  The y-axis is positive east of the Prime Meridian. Figure 2-13  Coordinate Planes b.  The position of a point on the surface of the earth is described in terms of X, Y, and Z coordinates.  These coordinates are distances, usually in meters, from the plane formed by two axes to the point along a line that is perpendicular to the plane and parallel to the third axis.  See Figure 2-14.  The X, Y, and Z coordinates are discussed below:     1.  X coordinate:  an x-coordinate is the length of a line in the x-y plane that is parallel to the x-axis and measured from the y-z plane.     2.  Y coordinate:  a y-coordinate is the length of a line in the x-y plane that is parallel to the y-axis and measured from the x-z plane.     3.  Z coordinate:  a z-coordinate is the length of a line that is parallel to the z-axis and is measured from the intersection of the x-coordinate and the y-coordinate to a point on the surface of the earth.     4.  The coordinates of the origin are (0,0,0).              Figure 2-14  Cartesian Coordinates 2-18  Geographic Coordinates a.  Geographic coordinates are any three dimensional coordinate system which specifies the position of a point on the surface of the earth in terms of Latitude       (   ), Longitude (   ), and ellipsoid height (h).  See f l Figure 2-15.  It is an inclusive term which can describe both geodetic and astronomic positions.     Figure 2-15  Geographic Coordinates b.  Latitude and Longitude are generally represented in degrees or degrees, minutes, and seconds along with a cardinal direction corresponding to a hemisphere on the earth.  A position will never have more than 60 DRAFT 2-8 ROTATIONAL AXIS POINT ON EARTH'S SURFACE l f h REFERENCE ELLIPSOID EQUATOR  0° 20° 40° 60° 80° 90° 90° 80° 60° 40° 20° N S +