With all the talk about Flood Insurance I decided to re-visit this old blog post.
I have an old survey of my property and it shows the finished floor (F.F. EL) of my house as say 8.1′ but this is based upon the NGVD 1929. How do I convert that to the NAVD 1988 which the new flood maps use?
Why would I need this? I like to know the answer to a question before I ask it. If my Lender or Insurer tells me I need an elevation certificate or I think I want one then I want to know if I should: A) fight them to apply the correct logic and classify the building and not my land, or 2) Have an elevation certificate done and ask my insurer to apply the post firm rates because I have enough of an elevation difference between the BFE and my finished floor to really get the rates down IF they insist on incorrectly classifying my building. In order to make this decision I want to know the 1988 FF EL before I push one way or the other. Also, it can be helpful to dtermine if a forcast storm surge will affect my property.
The Flood Insurance Rate Maps (FIRM) Maps were developed based upon the National Geodetic Vertical Datum of 1929 or NGVD 29. However there is a more current version of this which is the North American Vertical Datum of 1988 (NAVD 88) and the new (as of October 2017) flood zone maps use this new 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.
So, when we see on the new flood zone maps ZONE AE (EL 6) this means that the Base Flood Elevation (BFE) has been determines and it is 6′ above the NAVD 1988.
To Convert these elevations to the NAVD 88 Datum we use this page…
https://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl
So, let’s look at 300 Xanadu Place. Looking this up on the County GIS site or where ever we get the Lat/Lon: 26.9247024, 080.0694433 and we see the difference is -1.59′. (Note that I recently looked at a property on the St Lucie River in Palm City and the conversion there was -1.45′) These adjustments vary by location but this is very close to the published Palm Beach County wide ‘ball park’ number of -1.56′. So in this example location if say the elevation shown on an old survey was 8.1 according to the 1929 datum then the 1988 datum elevation is 8.1′-1.59′ = 6.51′.
And, the flood zones are defined as:
Special Flood Hazard Area (SFHA). An area having special flood, mudflow or flood-related erosion hazards and shown on a Flood Hazard Boundary Map (FHBM) or a Flood Insurance Rate Map (FIRM) Zone A, AO, A1-A30, AE, A99, AH, AR, AR/A, AR/AE, AR/AH, AR/AO, AR/A1-A30, V1-V30, VE or V. For the purpose of determining Community Rating System (CRS) premium discounts, all AR and A99 zones are treated as non-SFHAs.Shaded Zone X is a moderate risk areas within the 0.2-percent-annual-chance floodplain, areas of 1-percent-annual-chance flooding where average depths are less than 1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 1-percent-annual-chance flood by a levee. No BFEs or base flood depths are shown within these zones. (Zone X (shaded) is used on new and revised maps in place of what was Zone B.)
