DFIRM Vertical Datums and Elevations

Understanding Vertical Datums and Elevations on DFIRMs

All Flood Insurance Studies (FIS) and FIRMs are referenced to a specific vertical datum.  The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared.  Until recently, the standard vertical datum in use for newly created or revised FISs and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD29).  With the finalization of the North American Vertical Datum of 1988 (NAVD88), many FIS reports and FIRMs are being prepared using NAVD 88 as the referenced vertical datum. 

All flood elevations shown in the "L" series Flood Insurance Study (FIS) report and on the FIRM panels are referenced to NAVD88.  Structure and ground elevations in the community must, therefore, be referenced to NAVD88. At any specific location, the NAVD88 elevation can be 1.72 to 2.39 feet higher than the old NGVD29 elevation.  This does not mean that the ground surface or water surface elevation has physically changed, it just means that the baseline (mean sea level) has changed. 

For more information on NAVD 88, see "Converting the National Flood Insurance Program to the North American Vertical Datum of 1988", FEMA Publication FIA-20/June 1992, contact the Vertical Network Branch, National Geodetic Survey, Coast and Geodetic Survey, National Oceanic and Atmospheric Administration, Rockville, Maryland 20910, or talk with a surveyor.