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The National Flood Hazard Layer (NFHL) data incorporates all Digital Flood Insurance Rate Map (DFIRM) databases published by the Federal Emergency Management Agency (FEMA), and any Letters Of Map Revision (LOMRs) that have been issued against those databases since their publication date.
All data included in this layer are considered "final" by FEMA. Any preliminary data that appear on maps displayed at community meetings, etc., are not included here.
The DFIRM Database is the digital, geospatial version of the flood hazard information (location and attributes for boundaries of flood insurance risk zones) shown on the published paper Flood Insurance Rate Maps (FIRMs). The primary risk classifications used are the 1-percent-annual-chance flood event, the 0.2-percent-annual-chance flood event, and areas of minimal flood risk. The NFHL data are derived from Flood Insurance Studies (FISs), previously published Flood Insurance Rate Maps (FIRMs), flood hazard analyses performed in support of the FISs and FIRMs, and new mapping data where available. The FISs and FIRMs are published by FEMA. The specifications for the horizontal control of DFIRM data are consistent with those required for mapping at a scale of 1:12,000.
The FIRM is the basis for floodplain management, mitigation, and insurance activities for the National Flood Insurance Program (NFIP). Insurance applications include enforcement of the mandatory purchase requirement of the Flood Disaster Protection Act, which "... requires the purchase of flood insurance by property owners who are being assisted by Federal programs or by Federally supervised, regulated or insured agencies or institutions in the acquisition or improvement of land facilities" (Section 2 (b) (4) of the Flood Disaster Protection Act of 1973). In addition to the identification of Special Flood Hazard Areas (SFHAs), the risk zones shown on the FIRMs are the basis for the establishment of premium rates for flood coverage offered through the NFIP. The NFHL data present the flood risk information depicted on the FIRM in a digital format suitable for use in electronic mapping applications. The NFHL database is a subset of the information created for the FIS and serves as a means to archive a portion of the information collected during the FIS.
Currently this dataset is available for portions of Massachusetts. See the Status Map (PDF).
In ArcSDE the layer is named FEMA_NFHL_POLY.
The individual components of the National Flood Hazard Layer data are compiled in conjunction with the hardcopy FIRM and the final FIS report. The specifics of the hydrologic and hydraulic analyses performed are detailed in the FIS reports available for each community. The results of these studies are submitted in digital format to FEMA. These data and unrevised data from effective FIRMs are compiled onto the base map used for DFIRM publication and checked for accuracy and compliance with FEMA standards. As new DFIRM data sets are received the individual DFIRM layers are sewn into the nationwide layers of the NFHL. LOMRs for the DFIRMs in the NFHL are cut directly into the NFHL data layers as they are being produced and finalized.
In the October 2013 update, MassGIS added data for Norfolk, Plymouth, Barnstable, Essex and Hampden counties. Other counties for which data had been available were replaced with the latest data from FEMA. All county data were merged and projected to the Mass State Plane Meters coordinate system and the FLD_ZONE field was edited to include "Area with no DFIRM - Paper FIRMs in Effect". The LABEL field was also added and populated by MassGIS.
When FEMA revises an FIS, adjacent studies are checked to ensure agreement between flood elevations at the boundaries. Likewise flood elevations at the confluence of streams studied independently are checked to ensure agreement at the confluence. The FIRM and the FIS are developed together and care is taken to ensure that the elevations and other features shown on the flood profiles in the FIS agree with the information shown on the FIRM. However, the elevations as shown on the FIRM are rounded whole-foot elevations. They must be shown so that a profile recreated from the elevations on the FIRM will match the FIS profiles within one half of one foot.
Information contained in the National Flood Hazard Layer data reflects the content of the source materials. Features may have been eliminated or generalized on the source graphic, due to scale and legibility constraints. With new mapping, FEMA plans to maintain full detail in the spatial data it produces. However, older information is often transferred from existing maps where some generalization has taken place. Flood risk data are developed for communities participating in the NFIP for use in insurance rating and for floodplain management. Flood hazard areas are determined using statistical analyses of records of river flow, storm tides, and rainfall; information obtained through consultation with the communities; floodplain topographic surveys; and hydrological and hydraulic analysis. Both detailed and approximate analyses are employed. Generally, detailed analyses are used to generate flood risk data only for developed or developing areas of communities. For areas where little or no development is expected to occur, FEMA uses approximate analyses to generate flood risk data. Typically, only drainage areas that are greater than one square mile are studied. The National Flood Hazard Layer data reflects the most current information available when the distribution data set was created.
Quantitative Attribute Accuracy
The National Flood Hazard Layer (NFHL) data incorporates all Digital Flood Insurance Rate Map(DFIRM) databases published by FEMA, and any Letters Of Map Revision (LOMRs) that have been issued against those databases since their publication date. The DFIRM Database consists of community based vector files and associated attributes produced in conjunction with the hard copy FEMA FIRM. The published effective FIRM and DFIRM maps are issued as the official designation of the Special Flood Hazard Areas (SFHAs). As such they are adopted by local communities and form the basis for administration of the National Flood Insurance Program (NFIP). For these purposes they are authoritative. Provisions exist in the regulations for public review, appeals and corrections of the flood risk information shown to better match real world conditions. As with any engineering analysis of this type, variation from the estimated flood heights and floodplain boundaries is possible. Details of FEMA's requirements for the FISs and flood mapping process that produces these data are available in the Guidelines and Specifications for Flood Hazard Mapping Partners. Attribute accuracy was tested by manual comparison of source graphics with hardcopy plots and a symbolized display on an interactive computer graphic system. Independent quality control testing of FEMA's DFIRM databases was also performed. To obtain more detailed information in areas where Base Flood Elevations (BFEs) and/or floodways have been determined, users are encouraged to consult the Flood Profiles and Floodway Data and/or Summary of Stillwater Elevations tables contained within the FIS report that accompanies the individual components of the DFIRM data. Users should be aware that BFEs shown in the S_BFE table represent rounded whole-foot elevations. These BFEs are intended for flood insurance rating purposes only and should not be used as the sole source of flood elevation information. Accordingly, flood elevation data presented in the FIS report must be used in conjunction with the FIRM for purposes of construction and/or floodplain management. The 1-percent-annual-chance water-surface elevations shown in the S_XS table match the regulatory elevations shown in the FIS report.
Absolute External Horizontal Positional Accuracy
The National Flood Hazard Layer data consists of community based vector files and associated attributes produced in conjunction with the hardcopy FEMA FIRM. The published effective FIRM and DFIRM maps are issued as the official designation of the SFHAs. As such they are adopted by local communities and form the basis for administration of the NFIP. For these purposes they are authoritative. Provisions exist in the regulations for public review, appeals and corrections of the flood risk information shown to better match real world conditions. As with any engineering analysis of this type, variation from the estimated flood heights and floodplain boundaries is possible. Details of FEMA's requirements for the FISs and flood mapping process that produces these data are available in the Guidelines and Specifications for Flood Hazard Mapping Partners. Vertical accuracy was tested by manual comparison of source graphics with hardcopy plots and a symbolized display on an interactive computer graphic system. Independent quality control testing of the individual DFIRM data sets was also performed.
No warranty expressed or implied is made by FEMA regarding the utility of the data on any other system nor shall the act of distribution constitute any such warranty. FEMA will warrant the delivery of this product in a computer-readable format, and will offer appropriate adjustment of credit when the product is determined unreadable by correctly adjusted computer input peripherals, or when the physical medium is delivered in damaged condition. Requests for adjustment of credit must be made within 90 days from the date of this shipment from the ordering site.
The polygon attribute table contains the following fields:
DFIRM_ID Study Identifier. For a single jurisdiction flood risk project, the value is composed of the two-digit State FIPS code and the four-digit FEMA CID code (e.g., 480001). For a countywide flood risk project, the value is composed of the two-digit State FIPS code, the three-digit county FIPS code, and the letter “C” (e.g., 48107C). Within each FIRM Database, the DFIRM_ID value will be identical. VERSION_ID Version Identifier. Identifies the product version and relates the feature to standards according to how it was created. FLD_AR_ID Flood Area ID. Primary key for table lookup. Assigned by table creator. STUDY_TYP Study Type. This describes the type of flood risk project performed for flood hazard identification. Acceptable values for this field are listed in the D_Study_Typ table. "NP" = Not Populated. FLD_ZONE
Flood Zone. This is a flood zone designation. These zones are used by FEMA to designate the SFHAs and for insurance rating purposes. NOTE: The symbol ‘%’ is a reserved symbol in most software packages, so the word ‘percent’ was abbreviated to ‘PCT.’ Values for data in Massachusetts include:
A, AE, AH, AO, VE: 1% Annual Chance Flood Hazard
AE: Regulatory Floodway (FLOODWAY Zone Subtype)
D: Area of Undetermined Flood Hazard
X: 0.2% Annual Chance Flood Hazard or Area with Reduced Risk Due to Levee (based on the value in the ZONE_SUBTY field)
ZONE_SUBTY Flood Zone Subtype. This field captures additional information about the flood zones not related to insurance rating purposes. For example, insurance rate zone Shaded X could have “PROTECTED BY LEVEE” or “0.2 PCT ANNUAL CHANCE FLOOD HAZARD CONTAINED IN STRUCTURE.” Types of floodways are also stored in this field. Floodways are designated by FEMA and adopted by communities to provide an area that will remain free of development to moderate increases in flood heights due to encroachment on the floodplain. Normal floodways are specified as ‘FLOODWAY.’ NOTE: The symbol ‘%’ is a reserved symbol in most software packages, so the word ‘percent’ was abbreviated to ‘PCT.’ Acceptable values for this field are listed in the D_Zone_Subtype table. SFHA_TF Special Flood Hazard Area. If the area is within a SFHA this field would be true. This field will be true for any area coded as an A or V flood zone area. It should be false for any X or D flood areas. STATIC_BFE Static Base Flood Elevation. This field will be populated for areas that have been determined to have a constant Base Flood Elevation (BFE) over a flood zone. The BFE value will be shown beneath the zone label. In this situation the same BFE applies to the entire polygon. This normally occurs in lakes or coastal zones. V_DATUM Vertical Datum. The vertical datum indicates the reference surface from which the flood elevations are measured. Normally this would be North American Vertical Datum of 1988 for new studies. This field is only populated if the STATIC_BFE field is populated. Acceptable values for this field are listed in the D_V_Datum table. DEPTH Depth, for Zone AO areas. This value is shown beneath the zone label on the FIRM. This field is only populated if a depth is shown on the FIRM. LEN_UNIT Length Units. This unit indicates the measurement system used for the BFEs and/or depths. Normally this would be feet. This field is only populated if the STATIC_BFE or DEPTH field is populated. VELOCITY The velocity measurement of the flood flow in the area. Normally this is applicable to alluvial fan areas (certain Zone AO areas). This value is shown beneath the zone label on the FIRM. This field is only populated when a velocity is associated with the flood zone area. VEL_UNIT Velocity Unit. This is the unit of measurement for the velocity. This field is populated when the VELOCITY field is populated. AR_REVERT Flood Control Restoration Zones – Zone AR Classification. If this area is Zone AR in FLD_Zone field, this field would hold the zone that area would revert to if the AR zone were removed. This field is only populated if the corresponding area is Zone AR. Acceptable values for this field are listed in the D_Zone table, but should only include one of AE, AO, AH, A, and X domain values. Does not apply to data in Massachusetts at present. AR_SUBTRV Flood Control Restoration Zones – Zone AR Classification Zone Subtype. If this area is Zone AR in FLD_Zone field, this field would hold the zone subtype that area would revert to if the AR zone were removed. This field is only populated if the corresponding area is Zone AR. NOTE: The symbol ‘%’ is a reserved symbol in most software packages, so the word ‘percent’ was abbreviated to ‘PCT.’ Acceptable values for this field are listed in the D_Zone_Subtype_ table and must be one of the allowable subtypes for Zones AE, AO, AH, A, or X. Does not apply to data in Massachusetts at present. BFE_REVERT Flood Control Restoration Zones – BFE Revert. If zone is Zone AR in FLD_Zone field, this field would hold the static base flood elevation for the reverted zone. This field is populated when Zone equals AR and the reverted zone has a static BFE. Does not apply to data in Massachusetts at present. DEP_REVERT Flood Control Restoration Zones – Depth Revert. If zone is Zone AR in FLD_Zone field, this field would hold the flood depth for the reverted zone. This field is populated when Zone equals AR and the reverted zone has a depth assigned. Does not apply to data in Massachusetts at present. DUAL_ZONE Flood Control Restoration Zones – Dual Zone Classification. If the flood hazard areas shown on the effective FIRM shall be designated as “dual” flood insurance rate zones (i.e., Zone AR/AE, Zone AR/AH, Zone AR/AO, Zone AR/A), this field will be coded as true. It should be false for any for AR Zones that revert to Shaded X. Acceptable values for this field are listed in the D_TrueFalse table. Does not apply to data in Massachusetts at present. SOURCE_CIT Source Citation. Abbreviation used in the metadata file when describing the source information for the feature. The abbreviation must match a value in L_Source_Cit. LABEL Text description of flood zones for map legends based on the FLD_ZONE and ZONE_SUBTY fields. Current values include:
A: 1% Annual Chance of Flooding, no BFE
AE: 1% Annual Chance of Flooding, with BFE
AE: Regulatory Floodway
AH: 1% Annual Chance of 1-3ft Ponding, with BFE
AO: 1% Annual Chance of 1-3ft Sheet Flow Flooding, with Depth
D: Possible But Undetermined Hazard
VE: High Risk Coastal Area
X: 0.2% Annual Chance of Flooding
X: Reduced Flood Risk due to Levee
Area Not Included
Area with no DFIRM - Paper FIRMs in Effect
For more information on this dataset and its creation process, please see the metadata attached to the downloadable data and the following from FEMA:
- NFHL Resources
- National Flood Hazard Layer (NFHL): New Products and Services for FEMA's Flood Hazard Map Data
- NFHL GIS Data: Perform Spatial Analyses and Make Custom Maps and Reports
- Guidelines and Standards for Flood Risk Analysis and Mapping
- Technical References
- Guidance Documents
- Guidelines and Specifications for Flood Hazard Mapping Partners (specifically Appendix L: Guidance for Preparing Draft Digital Data and DFIRM Databases)
- Map Modernization
As FEMA makes more final NFHL data available MassGIS will incorporate updates and modifications into the dataset available for download from this page. Questions about this dataset should be directed to FEMA at 1-877-FEMA MAP (336-2627).
Users may also visit FEMA's Map Service Center.
To view the NFHL data online with other features developed by FEMA, see FEMA's National Flood Hazard Layer map.
Older Q3 FEMA Flood data exists for all areas of the state except Franklin County.
Last Updated 1/22/2014