Datum

An arbitrary surface that is used as a common reference for the elevations of points above or below it, to ensure that all elevation records are properly related. Elevation datum are identified in terms of feet, meters, or other units of measure as negative or positive, depending on whether they are above or below the mean sea level datum. The National Geodetic Vertical Datum (NGVD) of 1929 and North American Vertical Datum (NAVD) of 1988 are commonly used elevation datums. The National Flood Insurance Program previously used the NGVD, but all recent Flood Insurance Rate Maps have used the NAVD of 1988. Many communities have their own datum, developed before there was a national standard. The Flood Insurance Rate Map indicates which datum applies to a given community.