Differences between revisions 15 and 16
 Deletions are marked like this. Additions are marked like this. Line 1: Line 1: '''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"]). '''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]]). Line 3: Line 3: 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''. 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''. Line 7: Line 7: h = H + N[[BR]] h = ellipsoidal height[[BR]] H = orthometric height[[BR]] h = H + N<
> h = ellipsoidal height<
> H = orthometric height<
> Line 13: Line 13: [[BR]]http://www.holoscenes.com/images/bmwiki/height.png <
>{{http://www.holoscenes.com/images/bmwiki/height.png}} Line 15: Line 15: 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 [http://www.ngs.noaa.gov/GEOID/GEOID03/ GEOID03] mathematical model. Since GPS observations were taken at this station, the datasheet includes the ellipsoidal height. 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 [[http://www.ngs.noaa.gov/GEOID/GEOID03/|GEOID03]] mathematical model. Since GPS observations were taken at this station, the datasheet includes the ellipsoidal height.

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 32.59 meters below the 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* 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.```

orthometric height (last edited 2010-04-20 11:50:13 by Holograph)