It has taken me a while to comprehend the FEMA elevation certificate, but I think I have finally grasped the subtleties of this little document.  Over the past 30+ years, FEMA has conducted an enormous number of flood studies across the nation, and these range in degree of accuracy and reliabilty, but they do provide a planning guide and in some cases a reasonable guess as to the likelyhood of severe flooding in many drainages.

The earlier rate maps are ink and are of varying scale (frequently 400' to the inch) and they comprise numerous communities.  Maps are available here: FEMA LINK. The newer Flood rate maps are superimposed on aerial photographs so that it is possible to distinguish individual buildings, bridges and other structures.  In areas where FEMA has performed a detailed study it is possible to determine the location of a proposed or existing structure in relation to the "Base Flood Elevation," determined by FEMA.  This is the area with a 1% chance of flooding during any given season.  Elevations may be derived conventionally (by locating existing benchmarks and levelling to structures) or with GPS.  The relationship between the structure and the FEMA determined Base flood elevation is entered in section C of the certificate and will affect mandatory flood insurance requirements and rates.

Areas where Fema has concluded that there is a flood hazard, but has not conducted detailed studies are classified as flood zones A, AO or V.  Elevation certificates in these zones do not need the elevation information but still require the structure type and the relationship of the structure to the surrounding ground.  If there is a local determination of the 100-year flood, or an appropriate hydraulogical study, these results may be used.  Section E of the certificate describes the structure, whether it is likely to fill with water and what the hydrostatic pressures might be if the area floods.