Base Map Inputs for Floodplain Mapping Committee on Floodplain Mapping Technologies Board on Earth Sciences and Resources Division on Earth and Life Studies August 2006-January 2007

Background I  Flood Insurance Rate Maps (FIRMs) produced by the Federal Emergency Management Agency (FEMA) are integral to the National Flood Insurance Program (NFIP).

Background II  The NFIP and the flood maps associated with it provide an alternative to federal disaster assistance to reduce the costs of repairing damage caused by floods and are used by communities for land planning to avoid flood damages.  FEMA is conducting an ambitious 5-year program ( ) to update old, paper FIRMs to digital FIRMs (DFIRMs) in a ‘Flood Map Modernization Program’.  FEMA’s Flood Map Modernization program is nationally supported with a budget of $200 million per year.

Background III  Concerns have been expressed to Congress that the underlying base map information used as input to the flood mapping process is not of adequate quality in much of the nation to properly support the new digital flood map creation.  Various technologies can be used to generate the base map information used in floodplain maps.  This study was commissioned by the National Academies for the purpose of informing Congress about these technologies and their ability to support DFIRM creation.  The National Academies established the Committee on Floodplain Mapping Technologies to address the following Statement of Task:

Statement of Task 1. Identify the current mapping technologies being used by FEMA to develop flood hazard maps; 2. Identify mapping technologies that are currently available; and 3. Determine if newer technologies are appropriate and would be of additional benefit to floodplain mapping.

Committee Membership DAVID R. MAIDMENT, Chair, University of Texas, Austin SCOTT EDELMAN, Watershed Concepts, Greensboro, North Carolina ELVIN R. HEIBERG III, Heiberg Associates, Inc., Arlington, Virginia JOHN R. JENSEN, University of South Carolina, Columbia DAVID F. MAUNE, Dewberry and Davis, Fairfax, Virginia KAREN SCHUCKMAN, URS Corporation, Gaithersburg, Maryland RAMESH SHRESTHA, University of Florida, Gainesville National Research Council Staff ELIZABETH A. EIDE, Study Director JARED P. ENO, Senior Project Assistant

Study Timeline  Three meetings (September, October, November 2006)  September 2006 – Committee only; discussion & planning  October 2006 – Key information gathering meeting - two days, four panel sessions - external panelists representing federal, research, practitioner and private sector perspectives - cover all areas of Statement of Task  November 2006 – Writing meeting  January 31, 2007 – Report delivery to Congress & FEMA

Construction projects (impervious surfaces and fill) continuously raise flood levels

Old, paper FIRM New, digital (D)FIRM - The conversion from FIRMs to DFIRMs was intended to create national, digital floodplain map coverage - The ideal DFIRM: more accurate than paper FIRM, more flexible to use and update, more versatile for community use

FEMA Floodplain Map

This study addressed the technologies producing Imagery and Elevation data components of the DFIRM ‘Base map information’ *New, 2-year NRC study sponsored by FEMA will look at Flood Map Accuracy and include analysis of Flood Data component of the DFIRM *

FEMA Study Types Redelineation Approximate Studies ($500 per stream mile) Limited Detailed Study ($1,500 per stream mile) Detailed Study ($20,000 per stream mile)

Redelineation Used in parts of the country where flood risks have not significantly changed since the date of the effective map panel of an existing FIRM Effective detailed study data is used with updated digital elevation data. Base Flood Elevations (BFEs) are transferred to the current land surface, resulting in a floodplain fit to existing terrain conditions Flood boundaries are revised at minimal cost

Redelineation with New Elevation Data New elevation data typically change the areas defined as flood prone. Some areas previously under predicted are now added to the special flood hazard areas, whereas other areas previously over predicted are now removed from the special flood hazard areas.

Approximate Study Used in parts of the country where flood risks are lower Semi-automated hydrologic, hydraulic, and mapping tools, coupled with digital elevation data, are used to predict floodplain limits. Mapping tools are used without benefit of any field survey data, e.g., no field-surveyed cross sections, dam, culvert or bridge openings. Base Flood Elevations (BFEs) are not computed.

Limited Detailed Study Tools are used with some data collected in the field – for example, sketches of bridges to determine the clear openings. Sometimes Base Flood Elevations (BFEs) are published on the maps, depending on accuracy of the elevation data and the bridges, dams and culverts that may impede flow on the flooding source.

Detailed Study The same tools and the same base map are used, but with the addition of field-surveyed cross sections and field surveys of bridges, culverts and dams. Base Flood Elevations (BFEs) are published on the maps, as well as flood profiles and usually a floodway where land development is not allowed.

DFIRM and Base Imagery Digital orthophotos show two-dimensional features like roads, buildings, vegetation, and rivers, which are important for identification and location purposes Photogrammetry, the technology used to produce orthophotos, is mature, established— using either airborne cameras or satellite sensors Digital orthophotos are available nationwide, are routinely updated, and easily meet FEMA standards for base maps in ‘leaf-off’ conditions Orthophotos are the ideal base imagery for FEMA Map Modernization purposes

National Base Imagery Digital orthophotos have many applications outside floodplain maps (e.g. agriculture, tax assessment, regional & resource planning, emergency management) USGS and USDA each host digital orthophoto programs, National Aerial Photography Program (NAPP) & National Agricultural Imagery Program (NAIP), and are part of larger federal consortium, National Digital Orthophoto Program (NDOP) NDOP develops and maintains national specifications for orthoimagery that meet FEMA’s requirements Imagery for the Nation, a collaborative initiative at federal level, currently unfunded, could make nationwide orthophoto coverage available publicly with set standards for collection, distribution and archiving

Elevation Mapping Technologies Photogrammetry Mature technology Direct georeferencing has increased productivity and reduced cost Best practices yield consistent results Error models and product accuracies are well-understood Bare earth DEM generation is still labor- intensive and time-consuming

Elevation Mapping Technologies Lidar Sampling density and accuracies exceed photogrammetry at comparable flying heights Better at mapping bare-earth under canopy than photogrammetry Completely dependent on direct georeferencing. Error propagation is not rigorously modeled by product accuracy assessment techniques

Elevation Mapping Technologies Lidar Breaklines and linear features are not captured directly, but lidargrammetry offers an effective solution Best practices are not well documented Current methods of testing and reporting do not fully characterize data quality.

Elevation Mapping Technologies IFSAR Does not map bare earth as well as either photogrammetry or lidar Contains other artifacts (foreshortening and layover) that affect product consistency and make end use interpretation more difficult Limited commercial competition Is very useful for detecting small changes over time

Elevation Mapping Technologies IFSAR Technology is not mature Further development is highly dependent on government Best-practices are not documented Guidelines and standards do not exist

Floodplain Mapping Applications: Photogrammetry Aerial photography can be used to generate multiple products: imagery, planimetric maps and elevation data Oblique aerial photos can provide detailed information on structures. Large number of providers ensures competitiveness in the marketplace, in terms of quality, cost and delivery schedules.

Floodplain Mapping Applications: Photogrammetry Acquisition limited by season and sun- angle Photogrammetry alone is not cost or time effective enough to support the demand for accurate, up-to-date elevation data for floodplain mapping.

Floodplain Mapping Applications: Lidar Lidar sees through vegetation better than photogrammetry or IFSAR Lidar produces the highest-resolution 3D data in a wide variety of land cover types. Acquisition windows (seasonal and diurnal) provide greater productivity

Floodplain Mapping Applications: Lidar Lidargrammetry offers the potential for superior planimetric data capture. Terrestrial lidar can be used to capture floodway structures Large number of providers ensures competitiveness in the marketplace, in terms of quality, cost and delivery schedules.

Floodplain Mapping Applications: Lidar Error modeling is less robust than for photogrammetry. Product characterization and accuracy assessment techniques could be improved. Best-practices and guidelines need further attention.

Floodplain Mapping Applications: IFSAR Collection can occur day/night and in cloudy conditions. IFSAR provides the most rapid, wide- area coverage. IFSAR does not adequately map bare earth in all land cover types at accuracies required to support floodplain mapping.

Floodplain Mapping Applications: IFSAR IFSAR is the best technology to use in remote areas with little vegetation where lower accuracies are acceptable for floodplain mapping IFSAR is the most viable technology to use in large areas with perpetual cloud cover (Alaska). Small number of vendors limits competition.

Conclusions 1. Base Imagery Data: New DFIRMs typically use a digital orthophoto as the base map image. FEMA requirements for orthophoto standards for use in flood maps are satisfactory. The nation has adequate image mapping to support Flood Map Modernization through the National Digital Orthophoto Program ( and National Agricultural Imagery Program ( SUMMARY: Digital orthophotos are appropriate for flood map image base; current standards are adequate; existing programs should be supported

Conclusions 2. Base Elevation Data: Rational flood management for the nation requires a three-dimensional view, quantifying both the variable Base Flood Elevations (BFEs) throughout the floodplain (vertical) and the areal extent (horizontal) of the 1% annual chance (100-year) flood where the BFEs intersect the terrain surface as depicted by digital elevation models. To support the NFIP, analyzing these features in three-dimensions requires high-accuracy base map elevation data. FEMA requirements for elevation data in DFIRMs are 5-10 times more demanding than most existing elevation data in the National Elevation Database (NED), used by many of FEMA’s state and local partners. SUMMARY: A new digital mapping program for this land surface elevation is needed, which the committee has termed Elevation for the Nation

Recommendations 1. Elevation for the Nation should employ Light Detection and Ranging (lidar) as the primary technology for digital elevation data acquisition. Lidar is the technology most capable of producing the (bare-earth) elevation accuracy that meets FEMA’s requirements for national floodplain mapping in all terrain types. 2. A seamless nationwide elevation model produced with lidar has application beyond the FEMA Map Modernization program. As part of Elevation for the Nation, federal, state, and local mapping partners should have the option to request data that exceed minimum specifications if they pay the additional cost of data collection and processing required to achieve higher accuracies. 3.The new data collected in Elevation for the Nation should be disseminated to the public as part of an updated National Elevation Dataset.

Recommendations 4.The Elevation for the Nation database should contain the original lidar mass points and edited bare-earth surface, as well as any breaklines required to define essential linear features. 5. In addition to elements for the national database, secondary products including triangulated irregular networks, hydrologically corrected digital elevation models, and hydrologically corrected stream networks and shorelines should be created to support FEMA floodplain mapping with new standards and interchange formats. Comprehensive standards for lidar data collection and processing are also needed. Professional societies and federal agency consortia are appropriate entities to lead development of these standards; funding to support these efforts should be considered as part of a nationwide effort.

The committee concludes that the nation’s base map information for land surface elevation is inadequate to support FEMA’s Flood Map Modernization needs and that a new program called Elevation for the Nation is needed. We recommend lidar data with 1-ft equivalent contour accuracy in very flat coastal or inland floodplains; 4-ft equivalent contour accuracy in mountainous terrain; and 2-ft equivalent contour accuracy in most other areas. The Elevation for the Nation database should contain the original lidar mass points, edited bare-earth surface, and breaklines required to define essential linear features. A seamless nationwide elevation model, with applications beyond FEMA Map Modernization, should be disseminated to the public as part of an updated National Elevation Dataset. Comprehensive standards for lidar data collection and processing are also needed. Funding to support these efforts should be considered as part of a nationwide effort. Summary

1. Existing elevation data are old, and the gap between their accuracy and the accuracy required for floodplain mapping is great. 2. The required elevation mapping technology exists and has been commercially deployed such that implementing Elevation for the Nation is technically feasible with lidar. Regardless of whether “best-available” elevation data are used or new elevation data are being acquired for a flood study, informed judgments must be made about the appropriateness of these datasets and their influence on the flood data computations. Why is Elevation for the Nation Needed?

Age of USGS topographic maps in the National Elevation Dataset (NED) Elevation for the Nation Terrain data in USGS topographic maps are on average 35 years old and flood mapping requires data that are either collected or considered for updating within the last 7 years. The root mean square error (RMSE) of the terrain data in the NED is 2.34 m (7.7 ft) and FEMA’s requirements for flood plain mapping are for data with RMSE of 18.5 cm (0.61 ft) or 2 ft equivalent contour accuracy in flat areas and 37 cm (1.22 ft) or 4 ft equivalent contour accuracy in rolling or hilly areas. Existing data are thus ~ 1/10 accuracy and about 5 times older than needed for the flood mapping task.

Elevation for the Nation Recommend elevation data at: 2-foot equivalent contour accuracy for national ‘average’; 1-foot equivalent contour accuracy for zero slope (red) areas; 4-foot equivalent contour accuracy for mountainous or sparsely populated areas. Lidar data can meet all of these accuracy standards Slope map of the U.S. gives approximate scope of a national elevation program 11% of the continental US and Alaska has zero slope in the current National Elevation Dataset, much of which is in high-risk, coastal flooding areas.