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Orthometric height is the distance measured above the ["geoid"]. It is the height obtained by ["differential leveling"] from known starting points, and is the height used for everyday purposes (a.k.a. ["elevation"]).
Since the geoid is not a uniform surface, orthometric height will differ from ["ellipsoidal height"], which is the height computed by GPS measurements. A GPS height measurement needs to be converted using a mathematical model that estimates the difference between the two heights. That difference is called the geoidal separation, geoidal height, or undulation of the geoid.
The mathematical relation between orthometric height, ellipsoidal height, and geoid height is:
The formula does not include the slight correction needed for the deflection of the vertical. BRhttp://www.holoscenes.com/images/bmwiki/height.png
In the following example datasheet, the orthometric height is the NAVD 88 value shown near the top of the CURRENT SURVEY CONTROL section of the datasheet. At the benchmark location, the ["geoid"] lies 32.59 meters below the [:Ellipsoid:ellipsoid]. That difference was computed using the GEOID03 mathematical model. Since GPS observations were taken at this station, the datasheet includes the ellipsoidal height.
LY2920 *CURRENT SURVEY CONTROL LY2920 ___________________________________________________________________ LY2920* NAD 83(1996)- 41 02 35.79535(N) 074 37 59.60887(W) ADJUSTED LY2920* NAVD 88 - 214.384 (meters) 703.36 (feet) ADJUSTED LY2920 ___________________________________________________________________ LY2920 X - 1,276,642.639 (meters) COMP LY2920 Y - -4,645,341.738 (meters) COMP LY2920 Z - 4,166,168.530 (meters) COMP LY2920 LAPLACE CORR- 3.96 (seconds) DEFLEC99 LY2920 ELLIP HEIGHT- 181.77 (meters) (09/24/01) GPS OBS LY2920 GEOID HEIGHT- -32.59 (meters) GEOID03 LY2920 DYNAMIC HT - 214.287 (meters) 703.04 (feet) COMP LY2920 MODELED GRAV- 980,168.5 (mgal) NAVD 88 LY2920 LY2920 HORZ ORDER - B LY2920 VERT ORDER - SECOND CLASS II LY2920 ELLP ORDER - FOURTH CLASS I LY2920 LY2920.The horizontal coordinates were established by GPS observations LY2920.and adjusted by the National Geodetic Survey in August 1998.. LY2920 LY2920.The orthometric height was determined by differential leveling LY2920.and adjusted by the National Geodetic Survey in September 1996.. LY2920 ... LY2920.The Laplace correction was computed from DEFLEC99 derived deflections. LY2920 LY2920.The ellipsoidal height was determined by GPS observations LY2920.and is referenced to NAD 83. LY2920 LY2920.The geoid height was determined by GEOID03.