Differences between NGVD29 and NAVD88 in Southeastern Wisconsin

The Southeastern Wisconsin Regional Planning Commission (SEWRPC) has, over the past 30 years, established and promoted use of elevations referenced to the National Geodetic Vertical Datum of 1929 (NGVD29). Through international agreement, cooperation, and participation the vertical datums for the entire North American continent were recomputed, readjusted, and published as the North American Vertical Datum of 1988 (NAVD88). This paper summarizes a report prepared to describe differences between those vertical datums throughout the seven-county region served by SEWRPC. The report also includes a brief discussion of the International Great Lakes Datums (IGLD) and describes how IGLD 1985 is related to NAVD88. Conclusions published in the report are that within the seven-county SEWRPC region: (1) third-order datum differences can be interpolated from an isohypsometric map available from SEWRPC; (2) second-order datum differences can be obtained using VERTCON, an interpolation program available from the National Geodetic Survey, Silver Spring, Md; and (3) datum differences at first-order benchmarks should be determined on the basis of field surveys using first-order equipment and observational techniques.