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:
h = H + N
h = ellipsoidal height
H = orthometric height
N = geoid height
The formula does not include the slight correction needed for the deflection of the vertical.
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 32.56 meters below the ellipsoid. That difference was computed using the GEOID09 mathematical model. Since GPS observations were taken at this station, the datasheet includes the ellipsoidal height.
LY2920 *CURRENT SURVEY CONTROL LY2920 ___________________________________________________________________ LY2920* NAD 83(2007)- 41 02 35.79544(N) 074 37 59.60885(W) ADJUSTED LY2920* NAVD 88 - 214.384 (meters) 703.36 (feet) ADJUSTED LY2920 ___________________________________________________________________ LY2920 EPOCH DATE - 2002.00 LY2920 X - 1,276,642.643 (meters) COMP LY2920 Y - -4,645,341.750 (meters) COMP LY2920 Z - 4,166,168.545 (meters) COMP LY2920 LAPLACE CORR- 3.95 (seconds) DEFLEC09 LY2920 ELLIP HEIGHT- 181.790 (meters) (02/10/07) ADJUSTED LY2920 GEOID HEIGHT- -32.56 (meters) GEOID09 LY2920 DYNAMIC HT - 214.287 (meters) 703.04 (feet) COMP LY2920 LY2920 ------- Accuracy Estimates (at 95% Confidence Level in cm) -------- LY2920 Type PID Designation North East Ellip LY2920 ------------------------------------------------------------------- LY2920 NETWORK LY2920 19 R 1 0.25 0.20 0.69 LY2920 ------------------------------------------------------------------- LY2920 MODELED GRAV- 980,168.5 (mgal) NAVD 88 LY2920 LY2920 VERT ORDER - SECOND CLASS II LY2920 LY2920.The horizontal coordinates were established by GPS observations LY2920.and adjusted by the National Geodetic Survey in February 2007. LY2920 LY2920.The datum tag of NAD 83(2007) is equivalent to NAD 83(NSRS2007). LY2920.See National Readjustment for more information. LY2920.The horizontal coordinates are valid at the epoch date displayed above. LY2920.The epoch date for horizontal control is a decimal equivalence LY2920.of Year/Month/Day. LY2920 LY2920.The orthometric height was determined by differential leveling and LY2920.adjusted in September 1996. ... LY2920.The Laplace correction was computed from DEFLEC09 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 GEOID09. LY2920 LY2920.The dynamic height is computed by dividing the NAVD 88 LY2920.geopotential number by the normal gravity value computed on the LY2920.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45 LY2920.degrees latitude (g = 980.6199 gals.).