Description: FEMA’s National Flood Hazard’s Layer is a very complex dataset for a web-based GIS to process over a cloud-based infrastructure. This is due in part because the lines that define polygons are very detailed. The distance between many vertices is in places less than 30 cm. This amount of data is just too much for the system to process, resulting in significant lag times and crashing of widgets. Further, this level of detail is unnecessary for the scale at with a user is going to use the GIS.Thus, the objective is to remove the maximum number of vertices defining a flood polygon without excessively modifying the polygon shape. That is accomplished through a three-step process: 1) removing the class “minimal flood hazard”, 2) eliminating very small polygons, and 3) simplifying line structure by reducing vertex density.MethodsRemove all polygons of “Minimal Flood Hazard” Select by attribute all polygons of “minimal flood hazard” and then switch the selection so all polygons with not minimal food hazard are now selected. Export the selected features to create a copy of the data (the new copy will exclude those features of minimal flood hazard). Temporarily remove all polygons classified as “Floodway”Select by attribute polygons of “floodway” and export the selected features.Switch the selection and again export the selected (this will create a flood hazards layer without the Floodway polygons).Eliminate all polygons with an area less than 10 square meters. Working in the flood hazards layer without the Areas of Minimal Flood Hazard of Floodway, use the ELIMINATE tool (with “by border” as criteria) to remove polygons less than the indicated 10 square meter minimum mapping unit. This process will merge small polygons with the neighboring polygon that exhibits the largest neighboring border length.Re-add the “floodway” layer. Use UPDATE tool to insert the individual “floodway” layer back into flood hazards layer (now simplified to remove small polygons).Eliminate all polygons under 5 square meters, again by border. Simplify all polygons to maximize the reduction of vertices with minimal changes in shape. Use the SIMPLIFY POLYGON tool using the Weighted Area method with 1 square meter toleranceThis will reduce the file size by 38.1%The resulting layer, when incorporated into the web GIS, significantly increases speed and performance, allowing for effective and efficient use of the system.*****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.
Copyright Text: These data are provided courtesy of the FEMA National Flood Hazards Layer.