SRWMD Using a Newer Vertical Datum

Suwannee River Water Management District (SRWMD) Using a Newer Vertical Datum for River Level Measurements

Providing more accurate river level information is important to the citizens in the Suwannee River Water Management District (District), especially those who own property along the rivers of the District that are affected by periodic flooding. One part of providing more accurate river levels is upgrading to the newest vertical datum (North American Vertical Datum of 1988 - NAVD 88) in August 2015.

The new vertical datum will affect river level values on the following rivers:

Flooding at Santa Fe River Estates

  • Alapaha
  • Aucilla
  • Econfina
  • Fenhollaway
  • Ichetucknee
  • New
  • Santa Fe
  • Steinhatchee
  • Suwannee
  • Withlacoochee
  • Waccasassa
  • Wacissa Rivers

What is a Datum?

Flood TotemA datum is a standard or reference by which things are mapped from either horizontally or vertically. Using the same datum, a surveyor may relate and compare positions or elevations of various stations. Horizontal datums are used to establish specific locations on the earth. Latitudes and longitudes are one coordinate system used to provide these locations. By using the same vertical datum on surveys, a surveyor is able to provide an elevation for a location and this elevation can be compared with other elevations at different locations.

For example, the District uses horizontal and vertical datums to establish the locations and the elevations of each river level station. When a surveyor provides a vertical survey for constructing a home near the river, the new homeowner can compare his elevation to the elevation of the river at the nearest river level measurement station. Using this relationship, the homeowner can estimate when floodwaters may impact his home, property or even the prevent access to his home.

National Geodetic Vertical Datum of 1929

All river level stations in the District have been using the vertical datum referred to as the National Geodetic Vertical Datum of 1929 (NGVD 29).

NAVD 88 was developed to improve the accuracy of a reference point for vertical measurements. It is a result of years of new leveling, newer technology and addressing other factors that affect the accuracy of the NGVD 29.

Illustration of Elevation Differences

Using the vertical datum NAVD 88 will result in a reduction of 0.64 to 0.90 feet in the values used to represent the river levels, depending on the location of the river level station. At the Suwannee River at Branford station, using NAVD 88 results in a reduction in the numerical value used to represent the elevation by ¾ of foot (0.75 feet). If the river level for Branford reads, 24.81 feet using the NGVD 29 datum, it would read 0.75 foot less or 24.06 feet using the vertical datum NAVD 88. Both values, 24.81 and 2406, represent the same actual river level elevation. There is no change in the actual river level, only a change in the value used to represent the level.

The use of the new datum, NAVD 88, will only affect river levels at this time. Vertical datums associated with lake levels and groundwater levels will be done at a later date.

More Information on Vertical Datums

Some more information about vertical datums may be found at the following links:

  • The National Geodetic Survey (NGS) provides information on datums at their Vertical Datums page and  Frequently Asked Questions page.
  • The Federal Emergency Management Agency (FEMA) has information about the conversion in their "NGVD to NAVD?" Brochure (PDF).
  • If you want to determine the change at your location and you know the latitude and longitude, you may go to the VERTCON website to determine what the change in your elevation would be. It is important to know what datum was used to determine your elevation. The data represents the shift from NGVD 29 to NAVD 88 or vice versa depending on your selections. It is a negative number and lists the datum change in meters. If you don’t know the latitude and longitude for your location, GPSCoordinates.org provides a tool to determine it.
  • View a map displaying differences between vertical datums (JPG).

This is a cooperative effort between the Florida Division of Emergency Management, National Weather Service, United States Geological Survey and the District.