Survey Datums
Vol. 17 No. 5

September 9, 2009

The purpose of this article is to alert landowners, public works and geographic information systems (GIS) departments, engineers, and surveyors about an increasing number of problems we have noticed with the usage and identification of horizontal and vertical datums in the survey data we receive. The misuse and incorrect identification of the datum presented can and has caused extremely expensive problems if not recognized and corrected in a timely manner.

Due to the tough economic climate the development industry is facing, we are dealing with numerous projects where clients are relying on older survey data or a mix of old and new data. In the course of working with multiple engineering and surveying firms in Northern Virginia, WSSI has maintained a GIS inventory of every wetland, stream, and RPA determination we have undertaken since our formation in 1991. Reviewing this accumulated data provides us the ability to observe the problems created by the misuse or omission of horizontal and vertical datums. Problems we have observed include using the incorrect datum per municipal requirement, misidentifying the datum used, and combining multiple and conflicting datums in the same plan. It is not uncommon to see adjacent projects on different datums. The consequences of this datum confusion could include the construction of structures in floodplains and the failure of infrastructure to "tie in" correctly, resulting in economic damages.

Our intent in addressing this issue is to outline basic information regarding vertical and horizontal datums, provide the resources to acquire additional information, and most importantly, to inspire and stimulate an awareness that establishing the correct vertical datum should occur at the earliest stages of the planning and development process.

VERTICAL DATUMS

The two most common vertical datums used in the Northern Virginia area are the National Geodetic Vertical Datum of 1929 (NGVD 29) and the North American Vertical Datum of 1988 (NAVD 88). In general, the difference between these two vertical datums in Northern Virginia is a range of 0.6' to 0.8' (8" to 10") with NAVD 88 always lower in elevation than NGVD 29 (see sample topographic cross section exhibit).

NGVD 29, initially named the "Sea Level Datum of 1929" and changed to its current name in 1973, attempted to incorporate sea level data averaged over time from 26 tide stations (21 in the U.S. and 5 in Canada) to serve as a zero elevation reference for all other leveling. As it turned out, the tide stations did not produce the expected results. A zero elevation at one tide station did not always match the zero elevation of another tide station because sea level varies across the globe due to the earth’s gravitational forces. "USGS Datum" is a term that has been used instead of NGVD 29, but it is incorrect, as the USGS does not have its own vertical datum. NGVD 29 is no longer supported by the federal government.

In the 1970s, the National Geodetic Survey (NGS), a branch of the National Oceanic and Atmospheric Administration (NOAA) responsible for geodetic functions, initiated a new adjustment project that was completed in 1991. The new adjustment is the North American Vertical Datum of 1988 (NAVD 88). It can be unequivocally stated that NAVD 88 is the more accurate datum. In addition, NAVD 88 is the datum of necessity for GPS surveying.

The existence of two datums presents obvious problems. Any benchmark or vertical reference set prior to 1991 was established on the NGVD 29 datum. A number of existing NGVD 29 benchmarks were included in the NAVD 88 adjustment or converted later.

Elevations can be converted by the surveyor from one datum to the other utilizing the free programs VERTCON (NGS) or CORPSCON (U.S. Army Corps of Engineers). CORPSCON also converts between horizontal coordinate systems.

HORIZONTAL DATUMS

There are essentially two horizontal datums in use in Northern Virginia. They are the Virginia Coordinate System of 1927 (based on The North American Datum of 1927 - NAD 27) and The Virginia Coordinate System of 1983 (based on the North American Datum of 1983 – NAD 83). The adjustment of approximately 250,000 points, using baselines established with increasingly accurate techniques and equipment, was conducted between 1975 and 1986 and resulted in The North American Datum of 1983. The High Accuracy Reference Network (HARN) upgrades, conducted statewide between 1986 and 1997, added an additional 16,000 stations. (see Frequently Asked Questions for a short, concise history).

The designation of these coordinate systems is in the Code of Virginia, Title 55, Chapter 17 (55-287).

The Commonwealth of Virginia is divided into a "North Zone" and a "South Zone." The counties comprising each zone are designated in the above referenced Code along with definitions and limitations of the two coordinate systems. 

Section 55-290 declares that the coordinates in the determined system "shall be expressed in U.S. Survey Feet and decimals of a foot." However, "This requirement does not preclude the continued use of the International Foot conversion factor … in those counties and cities where this factor was in use prior to July 1, 1992." Most importantly, "The plat or plan shall contain a statement of the conversion factor used." This section is of particular importance to projects in Prince William County. Prior to February 1, 2003, Prince William County required the use of the International Foot. Plans that were submitted prior to that date and were in the review process were allowed to proceed. Unfortunately, surveys performed prior to February 2003 using the International Foot are still being used for projects that have been dormant due to economic and other factors. It is crucial that the identification of the foot definition used for pre-2003 surveys be determined and labeled.

DATUM TRANSITION AND IMPLEMENTATION PROBLEMS

Most jurisdictions in Northern Virginia have accepted VCS NAD 83 as the horizontal datum and NAVD 88 as the vertical datum of record for their GIS data and submission requirements (see table).

Several counties are in transition from NGVD 29 to NAVD 88. There is one exception. Fairfax County has an historical abundance of data referenced to NGVD 29 and has elected to retain all its datum requirements in NGVD 29. It does not appear that Fairfax County will change datums in the foreseeable future.

Prince William County has adopted NAVD 88 for all GIS data. The current Prince William County cover sheet includes in the "Survey and Topographic Information" block, the following statement: "All elevations must be referenced to that National Geodetic Vertical Datum of 1929 (NGVD 29)."

Loudoun County has also recently adopted NAVD 88 for all GIS data. The current Facilities Standards Manual (Chapter 8, Section 8.101, Paragraph 17, Adoption Date: 12/04/2007) requires "topography related to the National Geodetic Vertical Datum of 1929.

An issue of concern in Loudoun County is the availability of floodplain cross sections furnished by the Office of Mapping and Geographic Information showing floodplain elevations expressed in NGVD 29 values superimposed over topographic contours with NAVD 88 values. On the sample topographic cross section exhibit, flood plain elevations (expressed in NGVD 29 values) labeled on cross sections depicted on legacy NGVD 29 topography are correct. However, those same cross sections with the same NGVD 29 elevations are shown on the NAVD 88 topography. Since the NAVD 88 topography was compiled using a datum of lower elevation, the corresponding flood plain elevation should be lower. Obviously, using the flawed information included in the NAVD 88 topography drawing could have disastrous consequences if used in the planning process.

The federal government has adopted NAVD 88 officially and NGVD 29 is no longer supported. However, the Federal Emergency Management Agency’s (FEMA) Flood Insurance Rate Maps (FIRM) are all referenced to NGVD 29. As part of the Flood Map Modernization program, FEMA is upgrading all of the elevations to NAVD 88. The United States Geological Survey (USGS) is not planning to update the 7.5 minute quadrangle map series from NGVD 29 to NAVD 88, although conversion values are given on some quad sheets. The USGS's National Map incorporates the National Elevation Dataset (NED), which is referenced to NAVD 88.

The Virginia Department of Transportation (VDOT) requires that all control surveys be referenced to the North American Vertical Datum of 1988 (NAVD 88).

This brief overview of a potentially confusing situation should underscore the necessity of determining the required datum prior to conducting field surveys and preliminary design. Conversions of topographic surveys, calculations, and the resulting floodplain studies and RPA plans based on incorrect datums are expensive and time consuming.

For more information, contact Richard Hudson, LS or Yeoanny Venetsanos, LS.

Excellent On-Line Resources:

National Geodetic Survey

United States Geological Survey

Federal Emergency Management Agency

City of Alexandria Geographic Information Systems (GIS) Department

Arlington County GIS Department

Fairfax County GIS Department

Fairfax County Public Facilities Manual

City of Fairfax GIS Department

Fauquier County GIS Department

Loudoun County GIS Department

Prince William County GIS Department