Online tools, GIS, data

An overview of vertical datums and the
District’s conversion process

The St. Johns River Water Management District is in the process of changing the reference point that represents “zero feet” in defining elevations. This new measurement could impact anyone who uses District data.

About vertical datums

When people casually speak about sea level, such as when talking about the elevation of their property, what do they really mean? Sea level is measured by a defined reference point, known as vertical datum.

To develop the vertical datum, or sea level reference elevation, scientists measured water levels at 26 tide stations in the United States and Canada. This established the most widely known vertical datum in North America, becoming known as the Sea Level Datum of 1929, and later, known as the National Geodetic Vertical Datum of 1929 (NGVD 29). By decree, mean sea level was held fixed as a 0.00-foot elevation at each of the tide stations, and from them, 106,724 kilometers of survey leveling was run, forming the backbone for most of the elevation surveys performed.

Inherent inaccuracies were uncovered as technology grew and the demand for tighter tolerances and accuracies were required. Scientists learned that localized densities in the earth’s crust affected elevations, and improvements in global positioning satellite (GPS) systems made surveying with great accuracy over large areas economically feasible. New knowledge and technology all highlighted problems with NGVD 29.

In 1991, the North American Vertical Datum of 1988 (NAVD 88) was established by Canada, the United States and Mexico. It holds fixed a single point as the reference point from which all other elevations are measured. The District is in the process of converting to the use of NAVD 88 as its vertical datum.

How vertical datums are used

Among its duties, the District is tasked with managing the flow of water through both the Upper St. Johns River Basin and the Upper Ocklawaha River Basin to ensure the safety of area inhabitants and the environment, and to sustain sufficient water resources for a myriad of uses.

To accomplish this, the District relies on accurate surveys of water management features (dams, pump stations, weirs, etc.) and water measurement devices (water level gauges, well readings, flow meters, etc.) to make important decisions that meet the demands of its constituents and the environment. Vertical datums establish the starting point for all of these elevation measurements.

Why the District is converting

As mentioned above, fundamental flaws in the older vertical datum (NGVD 29) have emerged as improved technologies develop. With the implementation of NAVD 88, staff can more precisely measure elevations and relative heights.

Federal government agencies (National Geodetic Survey [NGS], U.S. Geological Survey, Federal Emergency Management Agency [FEMA], etc.), other state agencies (Florida Department of Transportation, Florida Department of Environmental Protection etc.), and counties and municipalities within the District are moving to the new datum and will no longer support the old datum.

What the District is doing about converting its vertical datum

The District has developed a plan to implement the Vertical Datum Conversion Project. This systematic approach has defined the steps, roles, responsibilities, time lines and cost estimates to complete the project.

The project team has worked closely with District organizations to accurately and cost-efficiently convert information, systems and processes to be able to employ the NAVD 88 values. The conversion work is expected to be complete in late 2010, followed by a year of testing, with the District operating in NAVD 88 before the 2012 storm season.

The benefits of converting

Benefits include the following:

• Elevation data will be more accurate.
• District stakeholders and constituents will realize reduced costs of surveys.
• Duplicate databases will be eliminated.
• Communications will be improved with other agencies and private entities.

The impacts of converting

Because the NAVD 88 datum is a more accurate reflection of the height of the earth’s surface, the values or numbers that refer to the height of water will change. The difference between NGVD 29 and NAVD 88 heights at any particular point can be determined and expressed as an offset value, or a difference between the two heights. This difference within the District’s boundaries ranges from 0.7 feet in the northwest to 1.5 feet in the southwest. NAVD 88 values will always be lower than the NGVD 29 value for a specific location.

People and organizations will need to utilize these new values to effectively communicate with the District and other users of elevation data. FEMA flood maps have been or soon will be converted to NAVD 88 elevations. Survey benchmarks will be in the new values, so local agencies that depend on published heights will need to adjust their reference points. Water management decisions will be made using NAVD 88 elevations. Construction, storm water and consumptive use permits will be issued based on NAVD 88 elevations.

The biggest impact to people across the District is that the numerical value for elevations will change. Nothing else will change. Your home will still be at the same place above flood stage. Rivers, creeks and groundwater will still be at the same level referenced to the surrounding land surface. Only the numerical value associated with that elevation will change.

NGS has created a calculator to help people identify the difference in elevation between NAVD 88 and NGVD 29 values using a specific latitude and longitude. Select the “height conversion” option when accessing the vertical conversion tool, or Vertcon. Other tools are also available on the Internet, such as through Google Earth.

For additional information

To learn more about the vertical datum conversion efforts at the District, please contact one of the following: