FEMA Q3 Data

The term “100-year flood” is misleading. It is not a flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1 percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time. But how can you tell if your land is in a flood zone? In the old days we’d use a FIRM (Flood Insurance Rate Map), and a few tools that might lnclude a scale, a dividers and a rubber-band – and then hope we were close.

But today, you can digitally download and/or purchase FIRM’s from FEMA as well as other sites. But a great place to start is with the download of Q3 Flood Data. Digital Q3 Flood Data is created by scanning the effective Flood Insurance Rate Map (FIRM) paper maps and digitizing selected features and lines. Usually, FIRMs for the cities and towns within a county are digitized and combined with other towns to produce a countywide map.

For $25 you can sign up for 150Mb of download at the GIS Data Depot. Q3 data is bundled by county, and downloadable in a zip file (usually about 5Mb). Therefore, typical Q3 data for an entire county will run you about 83 cents! And while you’re there, check out the plethora of available data. Please note that Q3 data is not yet available for the entire continental US.