as orthometric height or Mean Sea Level height. Elevation is labeled "H". 2-4  General     An ELLIPSOID is defined as "a surface whose plane sections (cross sections) are ellipses or circles, or the solid enclosed by such a surface."  It can be more easily identified as a sphere which is flattened, or squashed, on the sides or the top and bottom.  In geodesy, we use an ellipsoid that is flattened on the top and bottom; this is referred to as an oblate ellipsoid. The terms ellipsoid and spheroid are considered interchangeable; this text will use the term ellipsoid unless the mathematical figure of a spheroid is necessary. See Figure 2-3. SPHERE ELLIPSOID Figure 2-3    Ellipsoid 2-5  Defining Parameters of an Ellipsoid a.  An ellipsoid is generally defined in terms of three parameters, or dimensions, which provide the size and ellipticity of the ellipsoid.  Tables A-1a and A-1b list the defining parameters for 42 ellipsoids.     1.  Semi-major Axis:  The semi-major axis of an ellipsoid is the distance along the equatorial plane of an ellipsoid from the center of that plane to its edge; or the equatorial radius.  It is referred to as the long radius of an ellipsoid or one-half of the largest diameter.  The semi-major axis is labeled "a".  See Figure 2-4.     2.  Semi-minor Axis:  The semi-minor axis of an ellipsoid is the distance in a meridional plane from the center of the plane to its closest edge; or the polar radius.  It can also be referred to as the short radius of the ellipsoid or one-half of the shortest diameter.  The semi-minor axis is labeled "b".  See Figure 2-4.     3.  Flattening:  The flattening of an ellipsoid is the ratio of the difference between the equatorial and polar radii (semi-major and semi-minor axis) to the equatorial radius (semi-major axis) and is labeled "f". It is more commonly expressed as the inverse of flattening (1/f).  Flattening can also be called ellipticity.     Figure 2-4    Defining Parameters b.  There are other defining parameters for ellipsoids which are discussed in detail in DMA TM 8358.1 and DMA TR 8350.2.  These parameters include earth gravity information, angular velocity, and eccentricity; an understanding of these parameters is not necessary for artillery surveyors and will not be discussed in this text.   c.  Computing Ellipsoid Parameters DRAFT 2-2 Section II ELLIPSOIDS