Flood Zones

The National Flood Hazard Layer (NFHL) data incorporates all Flood Insurance Rate Map (FIRM) 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. It is updated on a monthly basis. The FIRM Database is the digital, geospatial version of the flood hazard information shown on the published paper 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 NFHL is available as State or US Territory data sets. Each State or Territory data set consists of all FIRM Databases and corresponding LOMRs available on the publication date of the data set. 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.国家洪水灾害图层（National Flood Hazard Layer, NFHL）数据集整合了美国联邦紧急事务管理局（Federal Emergency Management Agency, FEMA）发布的全部洪水保险费率图（Flood Insurance Rate Map, FIRM）数据库，以及自这些数据库发布之日起针对其出具的所有地图修订函（Letters of Map Revision, LOMRs）。该数据集每月更新一次。 洪水保险费率图数据库是正式发布的纸质洪水保险费率图的数字化地理空间版本，用于呈现用于生成洪水风险数据的洪水风险信息及配套支撑数据。其采用的核心风险分级包括年度发生概率1%的洪水事件、年度发生概率0.2%的洪水事件，以及最低洪水风险区域。 该数据库源自洪水保险研究（Flood Insurance Studies, FISs）、此前发布的洪水保险费率图、为支撑洪水保险研究与洪水保险费率图所开展的洪水灾害分析，以及可获取的最新测绘数据。洪水保险研究与洪水保险费率图均由美国联邦紧急事务管理局发布。 国家洪水灾害图层可按州或美国领地数据集的形式获取。每个州或领地数据集包含该数据集发布当日可获取的全部洪水保险费率图数据库及其对应的地图修订函。 洪水保险费率图数据库的水平控制规范与1:12000比例尺测绘的相关要求保持一致。该文件采用地理坐标系（Geographic Coordinate System, GCS）与1983北美大地基准面（North American Datum of 1983）完成地球表面的地理配准。