Flood Hazard Identification, Mapping, and Flood Risk Data
The map set
A Flood Insurance Study (FIS) is the text: hydrology, hydraulics, and flood profiles. The Flood Insurance Rate Map (FIRM) is the graphic; the Digital FIRM (DFIRM) is its digital form, and the National Flood Hazard Layer (NFHL) is FEMA's national GIS dataset. The base flood is the flood having a 1% chance of occurring in any year - a frequency statement, not a guarantee of one flood per century.
Zones and elevations
A and A99 zones generally carry approximate analysis with no published base flood elevation (BFE). AE, AH, and AO zones come with BFEs or depths; A99 is a protected area between levees or within a restoration zone. VE is the coastal high-hazard zone where the 3-foot wave criterion is exceeded; the Limit of Moderate Wave Action (LiMWA) marks roughly the 1.5-foot wave limit and separates Coastal A from inland areas. Shaded Zone X is the 0.2% annual-chance event; unshaded X is minimal risk but still subject to local drainage and stormwater flooding.
Making and changing maps
FEMA issues a preliminary map, runs a 90-day appeal period, then publishes an effective date that starts a 6-month window for the community to adopt compliant ordinances. Letters of Map Amendment (LOMA) correct property identification; Letters of Map Revision Based on Fill (LOMR-F) address placement of fill; Conditional LOMRs precede construction. Risk MAP is the strategy that couples mapping products with risk communication and mitigation planning.
Traps: the BFE is a water surface elevation, not a depth; vertical datum matters (NGVD 29 versus NAVD 88 can shift a number by feet); an approximate A zone has no BFE, so use best available data plus freeboard; Zone X never means no risk.
Rule of thumb: before reading anything else on a FIRM, confirm four things - panel and effective date, zone, BFE, and datum.