NAVD 88 is the abbreviation for the North American Vertical Datum of 1988. NAVD 88 is one of five current National Geodetic Datums, which are coordinate systems that act as standard reference lines to measure points on the earth’s surface in the region that they apply.
Elevation values from maps using different vertical datums (e.g., NGVD 29, NAVD 88, MLLW, and MHHW) are not directly comparable because they are based on different frameworks and starting (or zero) points. For a depiction of various vertical datums and their elevation differences at a particular location, see the figure below.
NAVD 88 was established in 1991 by the minimum-constraint adjustment of geodetic leveling observations in Canada, the United States, and Mexico. It held fixed the height of the primary tide gauge benchmark (surveying) , referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) height value, at Rimouski , Quebec , Canada .
Within the boundaries of the District, the NAVD 88 elevation number is a shift of approximately 0.7 to 1.1 feet lower that 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.
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Elevations in NAVD 88 for floodplain management and flood insurance purposes (e.g., elevation certificates) in Broward County should be used as of August 18, 2014, the effective date of the current flood maps.
Jul 26, 2013 · Accurate elevation surveying starts from a dependable elevation reference mark. A community and the surveyors in the community may normally use NAVD 88 for most purposes, but if the community’s FIRM uses NGVD 29, then NGVD 29 must be used for all flood, ground, and building elevations on Elevation Certificates and other NFIP applications.
The upgrade to NAVD 88 means that the values we associate with the height of water – what many people think of as mean sea level – will change. Within the SFWMD boundaries, measurements of water levels in NAVD 88 will be approximately -0.6 feet to -1.8 feet lower than they are in NGVD 29, as elevation difference varies by geographic location.
Converting Existing Elevations When the Flood Map Vertical Datum Changes from NGVD 29 to NAVD 88 Elevations referenced to NGVD 29 can be converted to NAVD 88 by using a conversion factor. This conversion factor is just added to the NGVD 29 elevation to compute the equivalent NAVD 88 elevation. The simplest method is to use the offset used by FEMA.
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