The technical
characteristics of vertical datum’s are generally less complicated than their
horizontal counterparts. Nationally, two vertical datum’s are in widespread
use, National Geodetic Vertical Datum of 1929, or NGVD 29, and the North
American Vertical Datum of 1988, typically referred to as NAVD 88. A number of
geoid models have been developed over the years to allow determination of orthometric
heights, (elevations) using GPS equipment, which have an effect on the accuracy
of the values determined, and which must be properly matched to the horizontal
datum in use for the project. There is
also a widespread misnomer that the difference between NGVD 29 and NAVD 88 is a
fixed amount, which is an absolute falsehood. The difference between the two datum’s
varies between zero (0) and nine (9) meters nationally, and just within the
boundaries of Delaware from 1.2 at the northern end of the state, 1.1 feet in
the Stanton area, makes a very rapid change to only 0.8 feet in Christiana and
eventually is as little 0.66 feet in Lewes and 0.7 feet in western Sussex
County. It is critical to be aware of this variable amount of difference and
model the real difference for each project you undertake. The NGS program
Vertcon is a very useful tool to model the difference based on geodetic
positions, as is the simpler route of pulling the NGS datasheets for benchmarks
around the site and tabulating the differences.
