The Fairfax County GIS and Mapping Services currently offers for purchase a GIS Digital Data product titled "Countywide Property & Topography." Included in the data is topography of the southeast and northeast quadrant of the county showing a 2' contour interval based on the vertical datum, North American Vertical Datum of 1988 (NAVD 88). Please note that NAVD 88 is not approved or permitted for land development projects submitted to Fairfax County. The use of the 2' contours based on NAVD 88 datum for planning or design could have significant impacts to project costs and schedules. There is also countywide topography showing a 5' contour interval based on the county's required vertical datum, National Geodetic Vertical Datum of 1929 (NGVD 29).

As reported in Field Notes Vol. 17 No. 5, Fairfax County has an historic abundance of vertical data referenced to NGVD 29. Although a more recent and accurate adjustment performed by the National Geodetic Survey has been available since 1991 (NAVD 88), Fairfax County has elected to retain all of its vertical datum requirements in NGVD 29. This is due to conversion costs and the anticipation of a new datum to be initiated in 2020. Some confusion regarding this requirement has been caused by the reference to "USGS Datum" in the Public Facilities Manual. To reaffirm and clarify the requirement for NGVD 29, proposed amendments to the Subdivision Ordinance, Zoning Ordinance, and Public Facilities Manual have been approved by the Engineering Standards Review Committee and will likely go to the Board of Supervisors for approval later this year.

For additional information, contact Richard Hudson or Chad Laskaris.