Introduction

The National Geodetic Survey is implementing the North American Vertical Datum of 1988 (NAVD 88), which will impact a broad spectrum of users of the National Spatial Reference System (NSRS). The Federal Geodetic Control Subcommittee (FGCS) has a special interest and responsibility with regard to this new datum. This Work Group shall be responsible for making recommendations regarding implementation of NAVD 88. A wide range of users are represented in the membership of FGCS, however it is recognized that there are other Federal, state, local, and private-sector interests in the new vertical datum that should also be considered.

The proper integration of Global Positioning System (GPS) data into the vertical portion of NSRS is of utmost importance in order to continue to provide accurate, cost-effective vertical reference data to the Nation. This Work Group shall investigate and report on presently available information and recommend overall strategies for the future role of GPS as it relates to vertical control surveys.

Mission

The first mission of this Work Group is to provide scientific advice and assistance to the National Geodetic Survey (NGS) in implementing the new vertical datum, NAVD 88. The Work Group will provide information utilizing its own professional abilities, responsibilities, and contacts with users of geodetic data. These contacts will include subject-matter specialists within their own departments and agencies. The goal will be to implement NAVD 88 to minimize disruption to users, while meeting requisite NAVD 88 objectives. Detailed approaches to accomplishing this mission will be determined by the Work Group. The second, and equally important mission will be to investigate and devise procedures to meet geodetic vertical requirements using GPS.

Membership

The Chair of the Work Group normally will be from NOAA, National Geodetic Survey and must possess an extensive, practical knowledge of vertical surveys and GPS procedures. Each member agency of FGCS may designate one or two employees to serve on this Work Group. It is preferred that the Work Group members have a practical knowledge of vertical surveys and/or GPS activities. The Work Group Chair may propose additional Work Group members from non-FGCS Federal agencies as deemed appropriate. The Chair shall be appointed by the FGCS Chair, with the concurrence of the agency representatives.

Operating Procedures

The Work Group shall meet as often as necessary, but not less than twice each year, as directed by the Work Group Chair, to consider directives from FGCS.

The Work Group's effort will be directed toward the development of information needed by NGS to complete the implementation of NAVD 88. The Work Group will:

1. Collect information from users of vertical reference systems.

2. Consider and document the impact on users of NAVD 88.

3. Recommend the appropriate course of action for NGS and other Federal agencies to take with regard to NAVD 88.

4. Document and report the findings of the Work Group to the Chairperson, FGCS, for distribution to FGCS members.

After the implementation effort is well underway, the Work Group will develop procedures for the use of GPS to meet vertical surveying requirements. The Work Group will:

1. Review existing vertical reference systems, including datum definition, data types, and estimates of precision and accuracy.

2. Estimate effects of systematic errors in vertical reference systems, as well as realistic accuracy estimates for heights and height differences and realistic relative weighing schemes for network adjustments.

3. Compile and analyze field data to improve estimates of geoid height differences. Investigate geoid modeling techniques to exploit GPS, leveling, and gravity data.

4. Develop overall strategies to incorporate GPS information into vertical reference systems.

5. Document and report the findings of the Work Group to the Chairperson, FGCS, for distribution to FGCS members.

Administration

The FGCS member agencies have supported the activities of these personnel while engaged in coordination functions; such support would be expected for the Work Group members. The FGCS Secretariat will assist in arranging meeting sites, preparing records of meetings, and finalizing reports and plans for public release.