FLOOD02115

• County: Statewide
• Basin: Statewide
• Title: National Flood Insurance Program Standards for Digital Flood Insurance Rate Maps
• Date: 10/1/1993
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

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fl'ct with existing USGS schema. (This file coding concept allows flexibility
con h~ use of specific data and minimizes computer processing of superfluous
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APplications of Digital FIRMs

the development and use of digital maps, users must consider the inherent
~:curacy of source maps. National map accuracy standards for horizontal and
vertical position have been developed. When applied to a 1:24,000 scale USGS
7.s-minute quadrangle topographic map, the standard map used for creating FIRM
base maps, the horizontal accuracy standard requires that, "90\ of all points
tested must be accurate to within l/SOth of an inch (O.OS centimeters) on the
map" (from USGS pamphlet "Map Accuracy., obtainable from: DOI-USGS, Earth Science
Information Center, S07 National Center, Reston, VA, 22092). At the 1: 24,000 map
scale l/SOth of an inch is the equivalent of 40 feet.

Map accuracy standards should be a consideration of hardcopy and digital FIRM
(DFIRM) users. Paper fIRMs are published at ,scales ranging from 1 inch = SOO
feet, to 1 inch = 2000 feet. However, FI~s published at scales larger than
1:2000 are normally only an enlargement of smaller scale~aps. Therefore, map
users cannot use the scale of the published FIRM as the basis for estimating the
horizontal accuracy of FIRM data. The cartographic procedures used to produce
FIRMs were not designed with the objective of meeting national map accuracy
standards. This is reflected by the use of an approximate scale and the lack of
horizontal control on FIRMs.

In general, cartographic procedures used in the production of FIRMs stress
preserving the correct relative relationship between Special Flood Hazard Area
(SFHA) boundaries and zones with hydrographic features and principle roads in and
near the SFHA.

The DFIRMs product is intended to convey all thematic information required to
implement the mandate of the NFIP. Base map data, such as roads, are not
included with the DLG-3 FIRM product. Specifications for the digitizing of FIRMs
to create the DLG-3 thematic flood risk overlay are generally consistent with
those required for mapping at the scale of 1:24,000. As such, the DLG-3 FIRMs
are optimized for overlay of the USGS 7.S-minute topographic, quadrangle maps.

At present, all USGS 7. S-minute topographic quadrangle maps have not been
digitized. Available public domain digital map data that may provide adequate
base map informatioN for some applications of digital FIRMs (DFIRMs) include the
USGS 1:100,000 scale quadrangle maps and the U.S. Bureau of the Census TIGER
files. However, users should familiarize themselves with the limitations of
these products. For example, according to national map accuracy standards, the
horizontal accuracy of 1:100,000 scale maps can be stated as 90% of all points
deviating from their true position on the earth by less than approximately 167
feet. The Census TIGER data was compiled from multiple sources, including the
USGS 1:100,000 scale data, and the quality of horizontal control is variable.
Other issues Common to both the USGS and TIGER data include completeness of the
road work and hydrography, "Y" features at road intersections that are artifacts
of the scan digitizing and vectorization processes used to create much of these
data, and lack of shape of points. Finally, users must constantly keep in mind

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