Your flood map designation includes ratings from at least the following data: Base Flood Elevation; LiMWA or Limit of Moderate Wave Action; and SWEL, or Stillwater Flood Elevation. All of these go into determining your flood zone designation.

One of the confusing pieces of the new flood maps is that your flood zone designation may remain the same, BUT your Base Flood Elevation increased – and that could mean higher insurance rates.

Your house didn’t move. All of these changes will likely affect whether you will be required to purchase flood insurance and the premium rates you will pay.

The changes in what is called “datum” are occurring to ensure more accurate elevation measurements. The current system is NGVD 29, which stands for National Geodetic Vertical Datum of 1929. FEMA provided this helpful graphic at the Floodmap Open Houses that helps to explain the change.

It is a system that has been used by surveyors and engineers for most of the 20th Century. It has been the basis for relating ground and flood elevations.

But it has been replaced by the more-accurate North American Vertical Datum of 1988 (NAVD 88). Your home’s flood plain designation is now based on NAVD88.

The average, Base Flood Elevation increase around the county due to the change in datum is about 1.5 feet.

Existing home elevation certificates are still valid, but may use NGVD 29 measurements. To compare an existing elevation certificate to the new flood maps requires a conversion of the old elevation data to NAVD 88.

You can learn more about these data changes: https://www.fema.gov/media-library-data/20130726-1755-25045-0634/ngvd_navd.pdf

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