2-1  General     The GEOID is defined as "the equipotential surface in the gravity field of the earth which coincides with the undisturbed mean sea level extended through the continents."  In other words, the geoid is the zero reference for elevation; it is a closed surface of equal gravitational force.  The geoid is perpendicular to the direction of gravity and is closely approximated by Mean Sea Level (MSL) and the extension of MSL through the land masses of the earth. 2-2  Shape of the Geoid      The geoid is affected by variances in the density, type,  and amount of land mass that pushes up through the water or lies below it, causing dips and swells.   It could almost be pictured as an ellipsoid with irregular dips and swells over its entire surface; thus conforming to an equal force of gravity over its surface.  The dips and swells are called undulations.  See figure 2-1.         Figure 2-1  Undulation 2-3  Use of the Geoid a.  Gravity pulls perpendicular to the geoid.  This means that a plumbline lies perpendicular to the geoid and establishes a vertical direction of measurement. An adjusted level vial in a theodolite is centered when it lies parallel with the geoid which establishes a horizontal reference at a specific location. b.  The geoid provides a common reference for elevations.   The point at which the surface of the geoid intersects a land mass is generally referred to as Approximate Mean Sea Level (MSL).     Figure 2-2  Elevation c.  Elevation is the distance between a point on the earth's surface and the geoid, measured along a line that is perpendicular to the geoid (plumb line).  See Figure 2-2.  Points lying outside (above) the geoid have a positive elevation, points inside (below) the geoid have a negative elevation.  Elevation can be referred to http://www.tpub.com/content/USMC/mcr3161a/css/mcr3161a_33.htm

 2-1      CHAPTER 2 BASIC GEODESY     The field artillery surveyor is primarily concerned with viewing small portions of the earth's surface as a plane.  However, farther reaching weapon systems, larger theaters of operation, Global Positioning Systems, and the advent of the World Geodetic System 1984 have all increased the need for the field artillery surveyor to have a basic knowledge of Geodesy that will enable him to evaluate and establish survey and mapping data as well as increase his ability to select and identify datum and ellipsoid options.   Section I THE GEOID     GEOID UNDULATION EARTHS TOPOGRAPHY H GEOID EARTHS TOPOGRAPHY