An overview of vertical datums and the District’s conversion process
The St. Johns River Water Management District has completed its preparations for 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 has completed its preparations to use NAVD 88 as its vertical datum, and is waiting for other agencies to complete their preparations before fully moving all operations to the new 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 did to convert its vertical datum
In 2008, the District approved a plan to implement the agency’s move to the use of NAVD 88. This action resulted in the establishment of the Vertical Datum Conversion Project. The project team’s systematic approach defined the steps, roles, responsibilities, time lines and cost estimates to complete the project. The conversion was completed, tested and evaluated by September 2011. The District is now able to operate in NAVD 88 elevations, and much of the information within the District is provided in NAVD 88 values. However, until more external agencies complete their conversions, the District will continue to operate in both vertical datums.
District data is labeled with the appropriate vertical datum.
The benefits of converting
Benefits include the following:
- Elevation data is more accurate.
- District stakeholders and constituents have realized reduced costs of surveys.
- Duplicate databases have been eliminated.
- Communications have 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 within the District boundary.
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 are now 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 has changed. Nothing else changed. Your home is still at the same place above flood stage. Rivers, creeks and groundwater are still at the same level referenced to the surrounding land surface. Only the numerical value associated with that elevation has changed.
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.
Updated on 11-21-2011