[an error occurred while processing this directive] [an error occurred while processing this directive]
ArcUser Online
 

July - September 2007
Search ArcUser
 
ArcUser Main Current Issue Previous Issues Subscribe Advertise Submit An Article
 

E-mail to a Friend

Floodplain Mapping Updates

Digital format now firmly established as data source for GIS

As part of its mission to reduce loss of life and property from all types of hazards, the Federal Emergency Management Agency (FEMA) administers the National Flood Insurance Program (NFIP). The agency's Mitigation Division maintains and updates the maps produced for this program. Updated versions of these maps and data are being released in GIS formats.

In 1968, Congress passed the National Flood Insurance Act and created NFIP. This act required that flood zones be established to define locations subject to higher probability of flooding. Maps were created that showed the location of the 100-year floodplain, known as Special Hazard Flood Areas (SHFAs).

Zones were assigned to these areas, which triggered specific building standards and flood insurance rates. SHFAs were further divided into specific risk zones designated by a letter or letters that denote the type and risk of flooding. For example, AE Zone identifies a floodplain with a 100-year flood elevation and VE Zone indicates flooding along a coastline. Shaded X Zone areas indicate low to moderate risk of flooding located in a 500-year floodplain and X Zone areas are outside a 500-year floodplain.

FEMA's floodplain maps, also called Flood Insurance Rate Maps (FIRM), are the nationally accepted source of data for determining if a building is located in a flood zone. These maps are used to determine the type of construction allowed and assign flood insurance rates. The first paper FIRM maps, the result of detailed topographic and hydraulic studies, rolled off the presses in 1973 and were state of the art for that time.

In the 1980s, digital data, mostly in the form of CADD files, began to be used to process and produce the paper FIRMs. GIS came of age in the early 1990s as Windows-based applications spread to desktops across the nation. FEMA responded by releasing digital versions of the FIRM data, called q3, in popular GIS formats.

This data represented the agency's best efforts to provide the GIS community with accurate floodplain data. However, q3 data had some problems. Base flood elevations, river cross sections, study data, river depths, and other features shown on paper maps were missing from q3 data. The data provided was compressed into a single layer of zones and panels. Producing effectively symbolized maps from q3 data was challenging. Quality control was not as stringent, and the data occasionally contained anomalies. These shortcomings were evident to FIRM users, and use of paper maps continued despite the debut of digital versions. Although GIS users made maps using q3 data, these maps could not be used for final flood zone determinations.

It took an act of Congress to usurp the power of paper and restore trust in digital FIRM data. In 2003, Congress and President George W. Bush began a multiyear, billion dollar program called Map Modernization. In addition to updating paper maps, this effort will also provide reliable digital FIRM data (dFIRMs) to the GIS community.

Map Modernization is well under way today. It has just undergone a midcourse adjustment. Almost half the new maps and data have been released to the public. Digital FIRM data comes in countywide coverages and is usually available from a local GIS or tax assessor's office or online at FEMA's Map Services Center (msc.fema.gov).

[an error occurred while processing this directive]