at Point Rimouski/Father's Point. And NAVD 88 is realized as Helmert orthometric heights, which are a good approximation to true orthometric heights. The NAVD 88 was affirmed as the official vertical datum for the United States by a notice in the Federal Register (Vol. 58, No. 120, page 34325) on June 24, 1993. For more information on NAVD 88, please check the NAVD 88 report.

The International Great Lakes Datum (IGLD) 1985 is expressed as dynamic height. Informally, this could also be considered as a height equivalent (based on work to raise a unit mass) above mean sea level. IGLD 85 is also based on an adopted elevation at Point Rimouski/Father's Point. And, IGLD 85 is realized as mean water levels at a set of master water level stations on the Great Lakes. Due to various observational, dynamical, and steric effects, there will be slight departures between a dynamic height and an IGLD 85 height. These departures are known as hydraulic correctors, and are part of the NAVD 88/IGLD 85 datum transformation.

Hydraulic correctors are small quantities, seldom exceeding 10 cm. Because of their complex character, hydraulic correctors are rendered as data grids. And, the datum transformation process described above interpolates the hydraulic corrector for each point. The fitting process for the data grids was chosen to honor the master station values. A post-fit test that re-interpolated the hydraulic correctors at the master stations was shown to agree with less than 1 mm error. Of course, instrumental error and spatial variation of the hydraulic correctors will prevent the complete transformation from being more accurate than about a centimeter or two.

The NAVD 88/IGLD 85 datum transformation is a tool to convert coordinates between these two datums. This computation can be done interactively, or in batch mode by submitting a file of data points.

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.

To interactively compute an IGLD 85 height for a NAVD 88 benchmark

For more information regarding the development of the hydraulic corrector model for IGLD 85, please visit the web archive.