FEMA Standards for LiDAR and Other High Quality Digital Topography
To implement FEMA’s Risk MAP vision and provide the high quality topographic data necessary to meet Risk MAP’s goals, FEMA has provided upgraded guidance concerning the accuracy and processing of high quality topographic data including Light Detection and Ranging (LIDAR) data.
FEMA is aligning Appendix A of the Guidelines and Specifications for Flood Hazard Mapping Partners (Guidelines) to theto modernize the FEMA specifications to current industry practice, leverage the expertise of the USGS Geography discipline, maintain Federal standards across agencies, and support the use of elevation products acquired as part of Risk MAP by other agencies for other purposes thus maximizing the Government’s investment.
Existing elevation data, not acquired by FEMA, but planned for use in a new flood hazard analysis for National Flood Insurance Program (NFIP) regulatory products must comply with the accuracy, density and the final product metadata requirements detailed in , but is not required to comply with the other specifications included and referenced below.
What is LiDAR?
LiDAR is an acronym for “light detection and ranging”. It is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. In the mapping industry, LiDAR is used to describe an airborne laser profiling system that produces location and elevation data to define the surface of the earth and heights of above-ground features. The manner in which lidar data is collected is via a pulse of laser light being transmitted towards the earth’s surface from a flown aircraft. Similar to radar technology, the range to an object is determined by measuring the time-laps between the transmission of a pulse and the detection of the reflected signal. After capturing the raw data each reflected signal can be classified into different layers e.g. Ground, Buildings and vegetation. The final output from the point cloud is either a Digital Terrain Model (DTM) or a Digital Surface Model (DSM).
Collection of elevation data using LiDAR has several advantages over most other techniques. Chief among them are higher resolutions, centimeter accuracies, and penetration in forested terrain. The resulting product is a densely spaced network of highly accurate georeferenced elevation points—often called a point cloud—that can be used to generate three-dimensional representations of the Earth’s surface and its features.
This topographic information can be processed to produce a "bare earth" Digital Terrain Model (DTM) suitable for floodplain delineation.
FEMA Standards (as of 3/29/2011)
When procuring topographic data to be used for floodplain mapping under the Risk MAP Program, the topographic data collected must meet the specifications detailed in and it's attachments. The full document as well as additional resources are listed below.
The attachments align FEMA’s high quality topographic specifications, found in Appendix A of the Guidelines, with the United States Geological Survey (USGS) Lidar Guidelines and Base Specifications v13 so that data procured and used by the Federal government is consistent across agencies and is updated to industry standards. Further, adherence to these specifications will support the Risk MAP Program by closing gaps in existing flood hazard data; supporting risk assessments; and better communicating risks to community officials and the public.
Because FEMA’s needs for elevation data are specific to NFIP floodplain mapping, FEMA has some unique requirements that differ from the USGS specifications. To supplement the existing USGS specifications, FEMA-specific items such as cross section surveys, bridges, and other features in Appendix A of the Guidelines remain valid except where superseded by more current information provided in this attachment. Table 1 summarizes the sections in Appendix A that are fully superseded, partially superseded or not superseded by this Procedure Memorandum.
|A.1||Introduction||Is not superseded and remains valid.|
|A.2||Industry Geospatial Standards||Remains valid but is appended by additional standards which use newer standards from the National Digital Elevation Program (NDEP) and American Society for Photogrammetry and Remote Sensing (ASPRS) to test elevation data for Fundamental Vertical Accuracy (FVA), Supplemental Vertical Accuracy (SVA), and Consolidated Vertical Accuracy (CVA).|
|A.3||Accuracy Guidelines||Partly superseded, especially Table 2, below, that specifies variablevertical accuracy standards and nominal pulse spacing (NPS), depending on the risk level and terrain slope within the floodplain being mapped.|
|A.4||Data Requirements||Major portions are superseded. Subsection A.4.2.3 pertaining to breaklines, subsection A.4.3 pertaining to elevation data vertical accuracy, and subsection A.4.5 pertaining to mapping area, are superseded. Subsection A.4.11 pertaining to other digital topographic data requirements, including Table A-3, Digital Topographic Data Requirements Checklist, is now superseded by other FEMA procurement guidelines. Subsection A.4.9 on data formats is partially superseded by the addition of lidar LAS formatted datasets. Subsections pertaining to cross sections (A.4.6)and hydraulic structures (A.4.7) remain valid.|
|A.5||Ground Control||Is not superseded and remains valid.|
|A.6||Ground Surveys||Is not superseded and remains valid.|
- FEMA Guidelines and Specificatiions
- FEMA Procedural Memorandum 61
- USGS-NGP Base LiDAR Specifications v1.3