Figure 2-6  ECEF Ellipsoid 2-7  Geoid Separation     Geoid separation is the distance from the geoid to the reference ellipsoid, measured along a line that is perpendicular to the ellipsoid.  It is considered positive when the geoid lies outside the ellipsoid and negative when the geoid lies inside the ellipsoid.  See Figure 2-7.  Geoid separation is labeled "N" and can also be called Geoidal Height or Undulation of the Geoid.     Figure 2-7    Geoid Separation     2-8  Ellipsoid Height     Ellipsoid height is the distance from a point on the earth's surface to the reference ellipsoid, measured along a line that is perpendicular to the ellipsoid. Ellipsoid height is labeled "h" and can be referred to as geodetic height.  See Figure 2-8. h ELLIPSOID EARTHS TOPOGRAPHY Figure 2-8    Ellipsoid Height   2-9  h, H, and N     The relationship between ellipsoid height (h), elevation (H), and geoid separation is shown in the formula: h    H+N.  See Figure 2-9.   » 2-10  General     A DATUM is defined as "any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities." Two types of datums are generally considered by surveyors:  Vertical and Horizontal.  When the term datum is used by itself, it is usually referring to a horizontal datum. 2-11  Geodetic Systems a.  A geodetic system serves as a framework for determining coordinates on the earth's surface with respect to a reference ellipsoid and the geoid.  It consists of both a horizontal datum and a vertical datum.   b.  Horizontal and vertical datums are generally defined separately from each other.  For example, horizontal positions in the Korean peninsula may be defined by the Tokyo Datum referenced to the Bessel ellipsoid; while vertical positions are defined by the geoid referenced to the Mean Sea Level Datum. DRAFT 2-4 h ELLIPSOID EARTHS TOPOGRAPHY H N GEOID Section III DATUMS