This is a follow up post to THIS ONE on converting elevations on old building documents and elevation certificates where the elevation given is by the older 1929 NGVD to the now used 1988 NAVD datum.
The datum is defined as: Datum: A reference surface used to ensure that all elevation records are properly related. The National Flood Insurance Program previously used the National Geodetic Vertical Datum (NGVD) of 1929, but all recent FIRMs have used the North American Vertical Datum (NAVD) of 1988. Both datum planes express elevations in relation to sea level.
The “adjustment” or conversion from the 1929 to 1988 datums is NOT a static number. It varies from one location to another. However, for most home owners and folks reading this blog the ballpark adjustment for Martin County Florida to convert something that says say 8′ by the NGVD datum to the now used NAVD datum is to SUBTRACT 1.45′ from this number. So +8′ by the NGVD datum is the same as +6.55′ by the NAVD datum in Martin County. BTW, this adjustment in Palm Beach County is typically -1.56′.
