New datums by the end of the next decade for the United States of America

Abstract

The National Geodetic Survey (NGS) is the lead federal agency on the Federal Geodetic Control Subcommittee (FGCS), which is a part of the broader Federal Geographic Data Committee (FGDC). While the FGDC focuses on broader collaboration involving geospatial data, the FGCS focuses on maintaining the National Spatial Reference System (NSRS) to ensure maximum accuracy and consistency when referencing the geospatial data. NGS is a responsible for maintaining the NSRS and access to it. The NSRS is composed of a number of elements including the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88). Both of these show significant problems when viewed in the context of global observing systems such as GNSS and data from the GRACE gravity satellite mission (Tapley et al. 2005). Meter level biases and slopes are seen in NAD 83, while NAVD 88 shows similar problems with 30–50 cm regional variations. Both of these are problematic given expectation by users for centimeter‐level of accuracy. NGS is moving to adopt new datums to replace NAD 83 and NAVD 88 by the end of the next decade. These changes will impact not only federal agencies but all those who rely upon the products of those agencies. Examples would include map products from the USGS, flood plain maps (FIRMettes), cadastral information at the county level, and numerous GIS applications. This also will impact commercial and engineering operations that rely upon precise positioning and the local gravity field variations including shipping and port operations, pipelines, and surveys. NGS intends to replace NAD 83 with a new ellipsoidal datum more consistent with more recent reference frameworks (e.g., ITRF05). More significantly, NAVD 88 will be replaced by a gravimetric geoid height model. This model will work in conjunction with the new ellipsoidal datum to provide consistent, cm‐level accurate, GNSS‐derived orthometric heights. These heights will be capable of being transformed to express geopotential numbers to provide dynamic or other types of heights. The new vertical datum will be developed through the Gravity for the Redefinition of the American Vertical Datum (GRAV‐D) project (Smith 2007). This project will collect gravity field information from numerous sources and spectrally meld them into a seamless whole.