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Tool Kit: Dynamic_Ht - Dynamic Height Computation |  |
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DYNAMIC_HT
The DYNAMIC_HT program computes the NAVD 88 dynamic height at a bench
mark using the orthometric height referenced to the NAVD 88 and a gravity
value.
The dynamic height of a benchmark is a height based on the geopotential
number for that station denoting the geopotential surface through the
benchmark. This value is of interest because two stations with different
orthometric heights may have similar geopotential, due to changes in gravity.
The dynamic height is computed from a geopotential number, C, which was
the value derived in the NAVD 88 adjustment. This application prompts
the user for the NAVD 88 orthometric height and for a gravity value. (If
the gravity value is unavailable, the user can enter the latitude and
longitude and the program will compute a gravity value from the model
used in the adjustment of the NAVD 88.) The geopotential number is computed:
Orthometric height
= C / (gravity [gal]+ (4.24E-5 * ortho_ht [m])).
A dynamic height is then obtained by dividing the geopotential number
by the normal gravity value (G) computed on the Geodetic Reference System
of 1980 (GRS 80) ellipsoid at 45 degrees latitude (G = 980.6199 gal).
Dynamic height =
C / (980.6199).
The geopotential number (C) is measured in geopotential units (g.p.u.),
where: 1 g.p.u. = 1 kgal meter = 1000 gal meter. Since local gravity near
sea level is approximately 0.98 kgal, the magnitude of geopotential numbers
(C) are approximately that of orthometric height in meters.
Dynamic heights are relevant near large bodies of water such as reservoirs or very large lakes, so the Great Lakes are an ideal candidate for dynamic heights. However, the Great Lakes are referenced to the International Great Lakes Datum of 1985 (IGLD 85) dynamicheights. The difference between IGLD 85 dynamic heights and NAVD 88 dynamic heights is known as a hydraulic corrector. An application which includes these hydraulic correctors is available for users who wish to compute IGLD 85 heights directly.
For more information on the NAVD 88 Adjustment:
Results of the General Adjustment of the North American Vertical Datum of 1988
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