Vol. 21, No. 4 June 6, 2013

Update on Fairfax County Horizontal and Vertical Datums

During its April 30, 2013 session, the Fairfax County Board of Supervisors clarified the use of horizontal and vertical datums on land development projects in their county by amending the Public Facilities Manual, the Subdivision Ordinance, and the Zoning Ordinance.

The official vertical datum that all land development projects in Fairfax County is NGVD 29 – and will continue to be NGVD 29 for the foreseeable future. As more land surveyors and engineers utilize modern Real-Time Network (RTN) Global Positioning Systems (GPS) that are referenced to the NAVD 88 vertical datum, it is vitally important to ensure that GPS data be converted when establishing topographic survey control so that NGVD 29 is used on all land development projects in Fairfax County. This will ensure that costly errors and delays in the land development engineering process do not occur.

In Field Notes Vol. 17 No.5, we highlighted examples of Northern Virginia jurisdictions that required land development projects to be referenced to one datum - and their GIS data were referenced to something different. Additionally, the article discussed the use of certain terminology, such as "USGS Datum" in Fairfax and Prince William Counties, which have led to confusion regarding the appropriate vertical datum to utilize during the land development process. The Fairfax County Board of Supervisors' recent clarification now clearly specifies that the official vertical datum in Fairfax County is NGVD 29 and eliminated the use of the term "USGS Datum."

The "Horizontal / Vertical Datums of Northern Virginia Jurisdictions" chart, previously shown in Field Notes Vol. 17 No.5, has been updated below to reflect the Fairfax County NGVD 29 clarification (click here for printable version):

Political Jurisdiction

Existing GIS Data
Horizontal / Vertical Datum

 Land Development Submission Requirements Horizontal / Vertical Datum
City of Alexandria VCS NAD 83,
US Survey Foot / NAVD 881		 VCS NAD 83,
US Survey Foot / NAVD 881
Arlington County VCS NAD 83,
US Survey Foot / NAVD 881		 VCS NAD 83,
US Survey Foot / NAVD 881
Fairfax County
(Updated May '13)		 VCS NAD 83,
US Survey Foot / NAVD 881
(You must ask for the data in
NGVD 29)		 VCS NAD 83, North Zone,
US Survey Foot / NGVD 29
(PFM Sect. 2-0107.1B and 2-0212.11)
(Subdivision Ordinance Sect. 101-2-5)
(Zoning Ordinance Sect. 17-106)
City of Fairfax VCS NAD 83 HARN,
US Survey Foot / NGVD 291		 VCS NAD 83 HARN,
US Survey Foot / NGVD 291
Fauquier County VCS NAD 83,
US Survey Foot / NAVD 881		 VCS NAD 83,
US Survey Foot / NAVD 881
Loudoun County VCS NAD 83 HARN,
US Survey Foot / NAVD 881 		VCS NAD 27,
US Survey Foot or meters/NGVD 29
(Loudoun County Facilities Standards Manual Sect. 8.101.A.8 and 8.101.A.17)
Prince William County VCS NAD 83,
US Survey Foot / NAVD 881		 VCS NAD 83,
US Survey Foot / USGS Datum2
(Prince William County Design and Construction Standards Manual Sections 120.01 and 712.02)
Spotsylvania County VCS NAD 83,
US Survey Foot / NAVD 881		 VCS NAD 83,
US Survey Foot / NAVD 881
Stafford County VCS NAD 83 HARN,
US Survey Foot / NAVD 881		 VCS NAD 83,
US Survey Foot / NGVD 29 or NAVD 88 or Assumed Datum
(Stafford County Code Sect. 22-87)
1 Verified by the jurisdiction's Geographic Information Systems (GIS) Department
2 "USGS Datum" is an ambiguous term; according to the NGS, a "USGS Datum" has never existed
 

Another helpful hint regarding mapping products and datums - When purchasing countywide 2' contour data for Fairfax County from the County's GIS Department, the contours are referenced to NAVD 88. The contour data is available in NGVD 29, but you must specifically ask for it. (See Field Notes Vol. 20 No.6).

For more additional information, please contact Yeoanny A. Venetsanos or Chad Laskaris.