| Addition FEMA Resources can be found on the FEMA Website (www.fema.gov) | ||||||||||||||||||||||||
| FLOOD INSURANCE RATE MAP data | ||||||||||||||||||||||||
In no event shall the Pasco County Property Appraiser's Office have any responsibility or liability for any consequence of use or misuse, or of the reliability of the information herein. No representation is made regarding the accuracy, adequacy, or applicability of the contents. FEMA FIRM data is provided for informational purposes only. Flood Hazard Zones are only indicated on those parcels lying within a single zone. Flood Hazard Zones for individual parcels are determined by using computer software to intersect FEMA FIRM maps with Pasco County, Property Appraiser ownership maps. |
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FEMA zones indicated on this site do not reflect FEMA approved revisions based on fill determinations. |
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| Note: BFEs - Base Flood Elevations
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Prepared under
FEMA Contract No. EMW-92-C-3888
by
Harvard Design and Mapping Company, Inc.,
ARC/INFO(r) is a registered trademark of Environmental Systems Research Institute, Inc. MapInfo(r) is a registered trademark of Mapping Information Systems Corporation. IBM(r) is a registered trademark of International Business Machines. Microsoft(r) is a registered trademark of Microsoft Corporation. Windows(tm) is a trademark of Microsoft Corporation. FIRM-DLG Index System(c) Copyright 1994, Harvard Design and Mapping Company, Inc.
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FIRM-DLG DEFINED:
The FIRM-DLG is a product developed by digitizing and/or scanning the existing hardcopy FIRM to create a thematic overlay of flood risks. The format of this product is the U.S. Geological Survey Digital Line Graph Level 3 Optional format, as described in the FEMA specifications for digital FIRMs. These products differ from the DFIRM as they are not tied to a base map, not used to produce a new version of the hardcopy FIRM, not subjected to community review, and are not accompanied by a Metadata file. FIRM-DLGs are intended to faithfully duplicate the existing hardcopy FIRM and to provide users with automated flood risk data that is comparable to that they would derive from the hardcopy FIRM. To this end, edge-matching errors, overlaps and underlaps in coverage, and similar problems are not corrected during digitizing or scanning as they are during the DFIRM-DLG production.
* See the "Standards for Digital Flood Insurance Rate Maps "Glossary for definitions of the DFIRM, and DFIRM-DLG.
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BACKGROUND
Flood Insurance Rate Map - Digital Line Graphs
Developments in the fields of automated cartography and Geographic Information Systems (GIS) technology during the 1980's led to the Federal Emergency Management Agency's (FEMA) study of the feasibility for automating National Flood Insurance Program (NFIP) mapping and engineering. After a series of technology assessments and pilot projects, FEMA concluded that existing technology made conversion of NFIP mapping and engineering to an automated basis feasible. A plan was implemented to begin automation, starting with the conversion of flood insurance rate maps (FIRMs) to a digital format.
A key consideration in the design of the digitizing process was to identify and adopt a standard format for digital FIRM data. After evaluating digital map data formats, FEMA determined that the U.S. Geological Survey (USGS) Digital Line Graph Level 3 (DLG-3), Optional Format in the public domain, best met the needs of the NFIP.
In the development and use of digital maps, users must consider the inherent accuracy of source maps. National map accuracy standards for horizontal and vertical position have been developed. When applied to a 1:24,000 scale USGS 7.5-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 1/50th of an inch (0.05 centimeters) on the map" (from USGS pamphlet "Map Accuracy", obtainable from: DOI-USGS, Earth Science Information Center, 507 National Center, Reston, Virginia, 22092). At the 1:24,000 map scale, 1/50th of an inch is equivalent of 40 feet.
Map accuracy standards should be a consideration of hard copy and FIRM-DLG users. Hardcopy, FIRMs are published at scales ranging from 1 inch = 400 feet to 1 inch = 2000 feet. However, FIRM's published at scales larger than 1 inch = 2000 feet are normally only an enlargement of smaller scale maps. 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 hardcopy 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 hardcopy 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 FIRM-DLG product is intended to convey all thematic information required implementing the mandate of NFIP. Base map data, such as roads, are not included with the FIRM-DLG 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 1:24000. As such, the FIRM-DLGs are optimized for overlay of the USGS 7.5-minute topographic quadrangle maps.
Regardless of the base map source and scale of digital data used in conjunction with FIRM-DLGs, users should bear in mind that the FIRM-DLG is foremost the result of hydrologic and hydraulic engineering processes, not a cartographic process. Inherent in these engineering processes are many uncertainties of both data and methodology. The resulting regulatory Base Flood Elevations (BFEs) and the SFHA delineations are also impacted by tolerances and specifications used in the development of hardcopy FIRM maps.
FIRM-DLG users must bear in mind that the simple conversion of FIRMs to a digital format does not inherently improve the engineering quality of the product. Many of the same difficulties with interpretation of flood risk data and the requirement for users to apply sound judgment in methods selected for decision making and map interpretation remain unchanged.
The FIRM-DLG data are developed in a countrywide format. All incorporated and unincorporated areas are sewn together as continuous data. This however creates the possibility of mismatches in flood hazard data, particularly at corporate boundaries, since analyses for individual communities were often performed at different times. In some cases, these mismatches have been resolved. In other cases, a multi-source polygon has been added to indicate that the data in the area comes from one FIRM. For these areas, the user should go back to the original engineering source materials.
Since existing FIRMs are not controlled horizontally, a fitting process is used to establish the needed control. The fitting process is defined as locating the floodplain delineation's from an uncontrolled FIRM on a horizontally controlled USGS quadrangle. This is accomplished by scaling, moving and/or rotating the panel data overlay and align common features on the FIRM and quad with the floodplain. Features include bridges, Roads, railroads, stream drainage, and contours.
Certain standards described below were developed for FIRM-DLGs. The FIRM-DLG thematic data are digital versions of the source hardcopy FIRM. The digital FIRM data are collected in logical categories that comprise four unique themes: (1) Political Areas - political subdivisions, (e.g. city, county), (2) Map Panel Areas - portions of communities covered by a specific map sheet; (3) FEMA Hydrography - both water control structures and surface water; and (4) Flood Hazard Zones - inundated areas for given flood events. All thematic data related to flood risks portrayed on the hardcopy FIRMs, as well as features such as FIRM panel boundaries and political units, are described by this topology. Base map information, such as road networks, address information, and similar data are not provided.
The projection used for the FIRM-DLGs is the Universal Transverse Mercator or UTM. The horizontal datum is the North America Datum of 1927 and the vertical datum is the National Geodetic Vertical Datum of 1929. For further documentation and a complete definition of features, see the FEMA document, Standards for Digital Flood Insurance Rate Maps, October 1993.
There are two ways to implement the line graph concept in DLG files: The area case and the network case. These cases are differentiated by the nature of the map category information. In the area case, the DLG is used to represent aerial features such as political entities of flood hazard areas in digital form. Area line graphs correspond directly to the general line case in that each closed area on the map is represented by a distinct area element of polygon. In the network case, the DLG is used to represent linear features such as streams or shorelines in digital form. The network case differs from the area case in that, irrespective of the number of closed areas (or polygons) forming the map, only two area elements are encoded: (1) the area outside the map termed the outside area; and (2) the area within the map, termed the background area. All lines except the map boundary are considered to be contained within the background area.
FEMA has determined that the majority of flood hazard data are better represented as a DLG area case as opposed to a network case. Therefore, the Political Areas, Map Panel Areas, and Flood Hazard Zones DLGs are area case DLGs. Under this concept, the map is subdivided into areas or zones. Each area contains an area centroid located somewhere within its boundaries. Each area centroid carries attribute codes indicating the type of area. The DLG was further simplified by creating four separate files specific to the aforementioned four unique themes. The FEMA Hydrography DLG is a network case DLG. In it, the map contains only two areas, the areas inside and outside the map boundary. All of the linear features within the map are defined by their attribute codes.
It should be noted that Flood Insurance Rate Maps continually undergo revisions and updates. Letter (Letter of Map Revision, Letter of Map Amendment) affects some of these revisions. DLG data files may not reflect the most current information. To obtain the latest information, contact:
Division Director
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