This is a bit of a rehash on a post I did some time ago on converting elevations from the older NGVD 1929 standard to the now in use NAVD 1988 standard. I noted the other day in looking at the previous version of a flood map for the Palm City area that it gave the Base Flood Elevation in terms of the NAD 83 (GRS 80) datum. I suspect that some folks have elevation certificates or construction documents which give elevations of their homes in this datum so I wanted to cover it. And sort of compare the 3. First, the NGVD 1929 and NAVD 1988 datums are what are know as Orthometric where the the NAD 83 is an Ellipsoidal 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 recently adopted FIRMs have used the North American Vertical Datum (NAVD) of 1988. Historic Elevation Certificates may use any one of these but newer ones use either the 1929 or 1988 datums. Both of these datum planes express elevations in relation to “sea level”. So, when we see on the new flood zone map something like ZONE AE (EL 6) this means that the Base Flood Elevation (BFE) has been determined and it is 6 feet above the datum stated on the map which again, for newer FIRMS, is NAVD 1988. This is what is referred to as Datum Shift. They quite literally moved zero.
Here’s the bottom line. After looking literally everywhere for this answer I found it in an Army Corp paper and what I was able to determine is that in terms of the vertical elevations the NAVD 1988 elevations are the same as the NAD 83 (GRS80) elevations shown on some older FIRM’s.
