Elevation Data for Floodplain Mapping (2007) / Chapter Skim
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5 Assessment of Floodplain Mapping Technologies
5 Assessment of Floodplain Mapping Technologies
Pages 89-114
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From page 89... ...
This chapter first summarizes the committee's observations about the FEMA Map Modernization process and provides recommendations about the collection of orthoimagery base map information for FEMA floodplain mapping. The chapter concludes with a set of recommendations about the manner in which adequate digital elevation data might be collected to support FEMA's Digital Flood Insurance Rate Map (DFIRM)
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From page 90... ...
to specify the best practices and processes. 5.1.4 FEMA's Use of Elevation Data The principal factor impacting the reliability of the floodplain boundary delineation is the quality of the input digital elevation information.
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From page 91... ...
. 5.2 CoLLECTIoN oF oRTHoIMAGERY BASE MAPS "A Flood Insurance Rate Map (FIRM)
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From page 92... ...
• Soft copy photogrammetric techniques can be used to efficiently and accurately pro duce base maps that meet FEMA minimum floodplain mapping specifications. • Digital orthophoto base maps are a valuable backdrop for many GIS applications in addition to FEMA floodmaps.
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From page 93... ...
Therefore, the committee recommends that radar orthoimagery for FEMA base mapping applications be collected only when and where it is not possible to obtain traditional passive optical orthoimagery.
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From page 94... ...
The proposed timetable is shown in Figure 5.1. orthophotos produced to the current NDoP specifications also meet FEMA's DFIRM base map minimum specifications.
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From page 95... ...
. 5.2.3.3 .S. Department of agricUltUre national agricUltUral imagery program U The NAIP acquires imagery during the agricultural growing seasons in the continental United States.
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From page 96... ...
. This will increase the demand for accurate elevation models to support the NAIP program.
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From page 97... ...
The committee especially likes the flexibility inherent in the program that allows the various interest groups to buy up and obtain even higher-resolution orthoimagery if desired. If funded, the Imagery for the Nation program will ensure that highspatial-resolution digital orthoimagery is obtained for the United States on a timely basis that can be used for FEMA FIRM base mapping.
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From page 98... ...
resolution to pay the differential • 100% cost for better elevation • 100% cost for better elevation data costs above the standard data products products base product for each • 100% cost for removal of • 100% of cost for removal of buy-up requested building lean (true orthophoto) building lean (true orthophoto)
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From page 99... ...
The committee's study of available elevation data has shown that the average age of USGS topographic map sheets is 35 years and their equivalent contour accuracy does not meet the FEMA flood mapping standards. Approximately three-quarters of the streams completed under FEMA's Map Modernization program define the spatial extent of flood inundation but lack the BFE data by which it is possible to judge whether a structure lying within the floodplain has a first floor elevation above the BFE.
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From page 100... ...
• Aerial photography can be used to generate multiple products such as orthophotos, planimetric maps, and digital elevation models. • Widely varying requirements for scale, detail, and accuracy can be addressed ef fectively by selecting an appropriate camera focal length and flying height.
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From page 101... ...
Direct georeferencing and digital workflows have greatly reduced the time required to produce photogrammetric products; however extraction of bare-earth elevation models is still a very labor intensive, time consuming, and therefore, relatively expensive process. Photogrammetry alone is not cost or time effective enough to support the current demand for accurate, up-to-date elevation data to support the FEMA floodplain mapping mission.
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From page 102... ...
• Existing guidelines and standards describing the accuracy of lidar-derived elevation models do not adequately address all potential sources of error. 5.3.3.3 liDar SUmmary Lidar is the most robust and cost-effective technology to address FEMA's needs for elevation data to support floodplain mapping.
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From page 103... ...
, states and local entities, and academia to define guidelines for professional practice and mapping specifications for application domains, such as floodplain mapping. For FEMA floodplain mapping, these specifications should address the need for 4-foot equivalent contour accuracy in complex, hilly terrain and 2-foot equivalent contour accuracy for the remainder of the nation as required by FEMA, in addition to the 1-foot equivalent contour accuracy in vulnerable coastal or very flat inland floodplains as recommended by this committee.
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From page 104... ...
. Why North Carolina Established a Statewide Floodplain Mapping Program • Hurricane Floyd revealed flood hazard data and map limitations: approximately 55 percent of North Carolina flood maps were at least 10 years old, and 75 percent were at least 5 years old; • Federal flood mapping budgets were finite; on average, the state received an updated flood study for only one county per year; and • Most counties indicated that they do not have the resources to take on this responsibility.
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From page 105... ...
and leak (gas) management • Snow removal Forestry • Use of multiple-return lidar data for tree cover and canopy analysis Municipal Engineering • Improve generally ALL municipal engineering projects • Improved sewer design base information • GPS location planning • Quality Control Checks for existing field surveys of sewer manholes • Greenway creation • Landfill sight screening • Assistance in mapping stormwater outfalls -- Environmental Protection Agency Clean Water Act Department of Transportation • Planning-level design savings • Hydrologic analysis and design savings SOURCE: http://www.ncfloodmaps.com.
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From page 106... ...
Therefore, it is difficult to produce high-accuracy elevation models or imagery base maps in dense urban areas. • In vegetated areas, IFSAR rarely measures the ground surface unambiguously.
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From page 107... ...
5.3.5 Sources of Elevation Data for FEMA Floodplain Mapping The USGS began creating topographic maps in the 1920s and had largely completed that task by the 1990s. The USGS also interpolated contours from the 1:24,000-scale topographic map sheets onto an elevation grid at 1-arc-second (30-meter)
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From page 108... ...
. While this accuracy is better than the quoted RMSE of 7 meters for USGS DEMs, it does not approach the FEMA requirement for 4-foot and 2-foot equivalent contour accuracy, or the committee's recommended 1-foot equivalent contour accuracy for coastal areas (equivalent to 37-, 18.5-, and 9.25-centimeter RMSEs, respectively)
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From page 109... ...
In the absence of a coordinated national effort, a number of states have initiated elevation mapping programs to meet a variety of needs. States such as West Virginia and Indiana have acquired elevation data to support digital orthophoto production, but these datasets, like the overall NED RMSE, fall short of FEMA accuracy requirements for floodplain mapping.
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From page 110... ...
In addition, the committee recommends consideration of a 1-foot equivalent contour accuracy requirement in very flat terrain, both for inland floodplains and for coastal areas vulnerable to hurricane storm surges. The distinctions between very flat, relatively flat, and rolling to hilly are not well defined; therefore, assessing the impact of these recommendations on a seamless national program is difficult.
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From page 111... ...
budgeting purposes, the committee recommends the development of 2-foot equivalent contour accuracy data as the nominal standard, 1-foot equivalent contour accuracy data in areas classified with near-zero slope, and 4-foot equivalent contour accuracy data in areas classified with slope of 10 degrees or greater. The reference to slope is the committee's attempt to relate to FEMA's requirements, which state that higher-accuracy elevation data are required in floodplains that are essentially flat (FEMA, 2003)
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From page 112... ...
The modernization guidelines suggest that participants use best-practice remote sensing technology to obtain much of the required base map imagery and elevation information. The committee believes that the information contained in FEMA DFIRMs is of significant value and is based on sound logic; the organization of the floodplain maps by "stream or coastal miles" is superior to organizing the FEMA DFIRM inventory according to typical "map sheets or panels." The principal factor impacting the reliability of the predicted BFE and floodplain boundary delineation is the quality of the input digital elevation information.
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From page 113... ...
However, extraction of bare-earth elevation models is still a very labor intensive, time-consuming, and therefore, relatively expensive process. Thus, photogrammetry alone is not cost or time effective enough to support the current demand for accurate, up-to-date elevation to support the FEMA floodplain mapping mission.
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From page 114... ...
Professional organizations with elevation mapping and surveying experience should be called on to assist public agencies such as FEMA, the USGS, NoAA, states or local entities, and academia to define guidelines for professional practice and mapping specifications for application domains, such as floodplain mapping. Specifications should address FEMA's requirements for 4-foot equivalent contour accuracy in complex, hilly terrain and 2-foot equivalent contour accuracy elsewhere, as well as the 1-foot equivalent contour accuracy in vulnerable coastal or very flat inland floodplains as recommended for the nation by this committee.
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