The District is upgrading the datum, or starting point, used to record water level elevation at its data collection sites. Water elevation is measured in feet above mean sea level.
Within the boundaries of the District, the NAVD 88 elevation number is a shift of approximately 0.7 to 1.1 feet lower than the elevation in the NGVD 29 standard. This variation is due to geographical differences. For example, a water level for Lake Panasoffkee of 39.24 feet in NGVD 29 will be 38.36 in the NAVD 88 standard. The difference of -0.88 feet, is referred to as the “shift” between datums. Only the numerical value for the elevation changes; the depth of the lake remains the same.Data.Maps@WaterMatters.org. In addition, the National Geodetic Survey (NGS) has created a tool to help people estimate the “shift” between the two datums for a specific latitude and longitude.
Once a data collection site is converted to NAVD 88, all historic and future water level data will be published in both datum standards (NAVD 88 and NGVD 29) and made available through the District’s Water Management Information System (WMIS). The datum change, which involves a surveyed recalibration of active District water-level data collection sites to the NAVD 88 standard, is expected to be complete by the end of 2016.
All federal agencies, including the Federal Emergency Management Agency, are required to use NAVD 88. Both the South Florida and St. Johns River water management districts are also migrating to NAVD 88. Many cities and counties such as Hillsborough County are upgrading to the NAVD 88 standard as well.