The short version
The base flood elevation (BFE) is the height that floodwater has a 1% chance of reaching or exceeding in any given year. People often call it the “100-year flood”, which sounds rarer than it is: over a 30-year mortgage, a 1% yearly chance adds up to roughly a 26% chance of at least one such flood.
Where you'll find it
FEMA publishes BFEs on Flood Insurance Rate Maps for many high-risk zones, usually zone AE and some V zones. Zone A areas often have no published BFE, which makes local knowledge and a survey more important.
Datums: comparing like with like
An elevation is only meaningful against a reference surface called a datum. Modern maps use NAVD88. Some older maps use NGVD29, and the gap between the two can be more than a foot. Aeonis Flood converts NGVD29 values with NOAA VDatum before comparing them with ground elevation from USGS.
Ground vs. building
We compare the ground at your pin with the BFE. Insurance pricing and building rules care about the lowest floor of the building, which can be higher or lower than the ground. Only an Elevation Certificate from a licensed surveyor or engineer measures that.
What to do with the number
- Ground well above the BFE is good news, but not a guarantee: heavy rain floods places outside mapped zones too.
- Ground near or below the BFE is a reason to ask for an Elevation Certificate and talk with an insurance agent early.
- Check nearby map amendments (LOMR and LOMA): they may change the picture for a specific lot.