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MassGIS Data: FEMA National Flood Hazard Layer

July 2023

The National Flood Hazard Layer (NFHL) dataset represents the current effective flood risk data for those parts of the country where maps have been modernized by the Federal Emergency Management Agency (FEMA).

Table of Contents

Downloads

Overview

This layer is a compilation of effective Flood Insurance Rate Map (FIRM) databases and any Letters of Map Revision (LOMR) that have been issued against those databases since their publication date. The NFHL is updated as new data reaches its designated effective date and becomes valid for regulatory use under the National Flood Insurance Program (NFIP).

All data included in this layer are considered "final" by FEMA. Any preliminary or draft data that appear on maps displayed at community meetings, etc., are not included here.

The FIRM 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 FIRM Database depicts flood risk information and supporting data used to develop the risk data. 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 FIRM Database is derived from Flood Insurance Studies (FISs), previously published 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 specification for the horizontal control of FIRM Databases is consistent with those required for mapping at a scale of 1:12,000. This file is georeferenced to the Earth's surface using the Geographic Coordinate System (GCS) and North American Datum of 1983 (NSRS-2007).

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 located or to be located in identified areas having special flood hazards, " 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 FIRM Database presents the flood risk information depicted on the FIRM in a digital format suitable for use in electronic mapping applications. The FIRM Database serves to archive the information collected during the Flood Risk Project.

The layer is named FEMA_NFHL_POLY and includes data published by FEMA as of July 2, 2023 (Latest Study Effective Date = 6/21/2023; Latest LOMR Effective Date = 11/28/2022).

NFHL data for portions of Massachusetts are currently available. See the Status Map (PDF) (based on the layer FEMA_NFHL_POLY_STATUS) .

Use Limitations

The hardcopy FIRM and FIRM Database and the accompanying FIS are the official designation of SFHAs and Base Flood Elevations (BFEs) for the NFIP. For the purposes of the NFIP, changes to the flood risk information published by FEMA may only be performed by FEMA and through the mechanisms established in the NFIP regulations (44 CFR Parts 59-78). These digital data are produced in conjunction with the hardcopy FIRMs and generally match the hardcopy map exactly. Acknowledgement of FEMA would be appreciated in products derived from these data.

Production

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.

MassGIS downloaded all data for Massachusetts in one file geodatabase from the FEMA Flood Map Service Center and did the following: projected the S_FLD_HAZ_AR feature class from NAD1983 Geographic to NAD1983 Mass. State Plane Mainland Meters; removed polygons where (FLD_ZONE = 'X' AND ZONE_SUBTY = 'AREA OF MINIMAL FLOOD HAZARD') OR (FLD_ZONE = 'OPEN WATER'); and added and calculated the LABEL field. MassGIS also created the FEMA_NFHL_POLY_STATUS feature class, starting with the Municipalities layer with generalized coast. 

Conceptual Consistency

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.

Completeness Omission

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.

Attributes

The polygon attribute table for FEMA_NFHL_POLY contains the following fields:

DFIRM_IDStudy 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_IDVersion Identifier. Identifies the product version and relates the feature to standards according to how it was created.
FLD_AR_IDFlood Area ID. Primary key for table lookup. Assigned by table creator.
STUDY_TYPStudy 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:

AAEAHAOVE: 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)
AREA NOT INCLUDED: Areas excluded from flood mapping within a city or town where NFHL flood data has been published.

See Definitions of FEMA Flood Zone Designations.

ZONE_SUBTYFlood 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_TFSpecial 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_BFEStatic 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_DATUMVertical 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.
DEPTHDepth, 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_UNITLength 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.
VELOCITYThe 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_UNITVelocity Unit. This is the unit of measurement for the velocity. This field is populated when the VELOCITY field is populated.
AR_REVERTFlood 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_SUBTRVFlood 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_REVERTFlood 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_REVERTFlood 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_ZONEFlood 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_CITSource 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.
LABELText description (added by MassGIS) of flood zones for map legends based on the FLD_ZONE and ZONE_SUBTY fields (to replicate the symbology used in FEMA's NFHL Viewer). Current values include:
1% Annual Chance Flood Hazard - where FLD_ZONE IN ('A', 'AE', 'AH', 'AO', 'VE') AND ZONE_SUBTY IS NULL
Regulatory Floodway - where FLD_ZONE = 'AE' AND ZONE_SUBTY IN ('FLOODWAY', 'FLOODWAY CONTAINED IN CHANNEL')
Area of Undetermined Flood Hazard - where FLD_ZONE = 'D'
0.2% Annual Chance Flood Hazard - where FLD_ZONE = 'X' AND ZONE_SUBTY IN ('0.2 PCT ANNUAL CHANCE FLOOD HAZARD', '1 PCT DEPTH LESS THAN 1 FOOT', '1 PCT DRAINAGE AREA LESS THAN 1 SQUARE MILE')
Area with Reduced Risk Due to Levee - where FLD_ZONE = 'X' AND ZONE_SUBTY = 'AREA WITH REDUCED FLOOD RISK DUE TO LEVEE'
Area Not Included - where FLD_ZONE = 'AREA NOT INCLUDED'
 
GFIDCounty Code from FEMA

The polygon layer FEMA_NFHL_POLY_STATUS contains the fields TOWN_IDTOWN, and NFHL_STAT fields. Values for NFHL_STAT (status of NFHL data) are:

  • FEMA NFHL Data Available
  • Area Not Included (Areas excluded from flood mapping within a city or town where NFHL flood data has been published.)
  • Area with no DFIRM - Paper FIRMs in Effect (digital data is available in the Q3 Flood layer)
  • No Flood Mapping Available (Franklin County only).

See the Status Map (PDF).

Additional documentation

For more information on this dataset and its creation process, please see the metadata attached to the downloadable data and the following from FEMA:

Maintenance

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 Flood Map Service Center.

To view the NFHL data online with other features developed by FEMA, see FEMA's National Flood Hazard Layer (NFHL) Viewer.

Older Q3 FEMA Flood data exists for all areas of the state except Franklin County.

Date published: July 27, 2023

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