Tuesday, March 19, 2013

Vertical Datums

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.