Tool Kit: Dynamic_Ht - Dynamic Height Computation

## 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.

Special Note on IGLD 85 Heights:
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) dynamic heights. 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.

Results of the General Adjustment of the North American Vertical Datum of 1988