Updating Bench Mark Coordinates for the New U.S. National Datums

English Abstract:

The National Geodetic Survey (NGS) maintains the National Spatial Reference Systems (NSRS) within the United States. The NSRS currently includes the North American Datum of 1988 (NAVD 88) for orthometric heights and the North American Datum of 1983 (NAD 83) for ellipsoidal coordinates. Replacing these two datums with new geometric and geopotential datums in 2022 will require many changes in standards, technique, policy, and practices. The approach envisioned is to use GNSS observations in conjunction with online processing tools to obtain geometric coordinates, which would then be interpolated into a high accuracy gravimetric geoid (geopotential) model to determine orthometric heights.

This would mark a move away from traditionally defined bench mark coordinates that currently provide definitive positional control. The NGS Integrated Database (NGSDB) serves as the electronic repository for bench mark data on nearly 500,000 NAVD 88 leveling sites and 80,000 NAD 83 coordinate sites. The role of this bench mark data would then become secondary, serving as a backup in case of catastrophic failure of GNSS access. To be useful then, bench mark data will need to be made consistent with the new datums requiring an adjustment of the positional coordinates in the NGSIDB. These values must be consistent with what would be obtained using a GNSS receiver and the geoid model in the new datums. Backward compatibility will be maintained through knowledge of the coordinates under the older and newer datums, which will be used to produce a transformation model similar to VERTCON and NADCON software.