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Mapping the Zone: Improving Flood Map Accuracy
3
Elevation and Height Data
A flood map is the final outcome of a multitude of measurement, engineering, and data analysis tasks. The purpose of a flood study is to predict the height of water and the extent to which it will inundate the landscape in a modeled flood event. The elevations of the land, water, and hydraulic structures (e.g., bridges) are key elements in a flood study, and the accuracy to which these elements are determined is a critical factor in the accuracy of the final flood map. The Federal Emergency Management Agency’s (FEMA’s) accuracy standards for land surface elevations are summarized in Box 3.1. This chapter explains how elevation is measured and examines the impact of elevation uncertainties in flood studies.
The data components of a flood study that involve a measurement of height or elevation can be grouped into four general categories:
Elevation reference surface. Before elevation can be measured or the data used in engineering analysis, a measurement system must be established. The location of “zero” and a physical reference for elevation zero (in other words, a vertical datum) must be established on the Earth, where it can be used for all types of height measurements.
Base surface elevation. Two types of base surfaces are important to flood studies: land surface elevation (topography) and its underwater equivalent (bathymetry). Topography is expressed as the height of a location above the geodetic datum and is in most cases a positive value. Bathymetry is expressed as the depth of the land surface below rivers, lakes, and oceans; positive depth is equivalent to negative elevation.
Water surface elevation. The depth of water in rivers, lakes, and streams and the point at which water overtops their banks and spreads across the landscape are the subjects of riverine flood studies. The depth of water in the ocean and the impact of extreme events such as hurricane-induced storm surge or earthquake-induced tsunamis are the subjects of coastal flood studies. The height of water surfaces is measured with stream and tide gages. The location and elevation of the gages themselves must be determined accurately in order to correctly relate water surface measurements to other elevations.
Structure elevation. The vulnerability of buildings and infrastructure to flood damage is directly related to their location with respect to the floodplain and the elevation and orientation of critical structural components with respect to the height of potential floodwaters. In addition, structures within the floodway (such as bridges, dams, levees, and culverts) influence the conveyance of water in a stream channel during a flood event, affecting flood heights.
These categories are described in more detail below.
ESTABLISHING A REFERENCE SURFACE
To measure something with a ruler, we place the zero mark at the end of the object and measure length or distance relative to that mark. The term datum refers to a reference surface against which position measurements are made; it defines the location of zero on the measurement scale. Three fundamentally differ-
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Mapping the Zone: Improving Flood Map Accuracy
BOX 3.1
FEMA Land Surface Elevation Accuracy Standards
FEMA has established two land surface elevation accuracy standards, depending on whether the terrain is flat or rolling to hilly (FEMA, 2003, Appendix A):
Two-foot contour interval equivalent for flat terrain (vertical accuracy = 1.2 feet at the 95 percent confidence level). This means that 95 percent of the elevations in the dataset will have an error with respect to true ground elevation that is equal to or smaller than 1.2 feet.
Four-foot contour interval equivalent for rolling to hilly terrain (vertical accuracy = 2.4 feet at the 95 percent confidence level.)
These standards provide a benchmark for determining the importance of variations in the way elevation is measured and defined in the flood mapping process.
ent types of vertical datums—ellipsoidal, orthometric, and tidal—are relevant to flood studies. In the United States, establishing and maintaining vertical datums is the responsibility of the National Oceanic and Atmospheric Administration’s (NOAA’s) National Geodetic Survey (NGS).
Ellipsoidal Datums
The Global Positioning System (GPS) provides the most accurate and efficient means for establishing fundamental reference marks (also called monuments) on the Earth’s surface, and it forms the basis for most land and aerial surveys performed today. Land surveys are performed using handheld and tripod-mounted GPS equipment; airborne photogrammetric or remote sensing surveys employ GPS and inertial measurement systems to track the position of the sensor and project the data into accurate ground coordinates. GPS satellite systems measure distances to the Earth’s surface relative to a mathematically idealized (smooth) ellipsoid that closely approximates the shape of the Earth (Figure 3.1). Heights computed with respect to this surface are referred to as ellipsoid heights. However, neither the Earth’s surface nor its gravity field, as delineated by the undulating geoid surface, matches this idealized ellipsoid.
FIGURE 3.1 Relationship of the Earth’s surface, the geoid, and a geocentric ellipsoid. The height difference between the geoid and the ellipsoid is the geoid separation. SOURCE: Kevin McMaster, URS Corporation. Used with permission.
Orthometric Height Datums
Modeling the flow of water across the Earth’s surface requires a reference surface defined by constant gravitational potential; this surface is referred to as the geoid. Heights measured with respect to anequipotential gravity surface are called orthometric heights, and the difference between the ellipsoid and the geoid at any particular location on the Earth is called the geoid height, or geoid separation (Figure 3.1). Geoid models developed and maintained by the NGS are used to convert ellipsoid heights to orthometric heights.
The orthometric height datum for surveying and mapping the North American continent is the North American Vertical Datum of 1988 (NAVD 88). NAVD 88 supersedes the National Geodetic Vertical Datum of 1929 (NGVD 29), which was used in many early flood maps and provided the basis for many engineering flood studies still in use today.1 The height differences between NGVD 29 and NAVD 88 can be large (Figure 3.2), ranging from −49 cm (−1.6 feet) in Florida to +158 cm (+5.2 feet) in Colorado. Elevation differences between NGVD 29 and NAVD 88 are immaterial to flood mapping as long as elevations are referenced to the same datum. A potential problem arises when old
1
See <http://geodesy.noaa.gov/faq.shtml> and Maune (2007) for a description of the differences between the two datums.
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FIGURE 3.2 Differences in heights (NAVD 88 minus NGVD 29) in units of centimeters. In the eastern United States, NGVD 29 is generally higher than NAVD 88, with differences of 30 cm along the Carolina coasts and nearly half a meter in some parts of Florida. In the western United States, NAVD 88 is higher than NGVD 29 and height differences are greater than in the east, more than a meter in many locations. SOURCE: Maune (2007). Reprinted with the permission of the American Society for Photogrammetry and Remote Sensing.
engineering analyses, based on NGVD 29, are used for new studies, based otherwise on NAVD 88. Although conversion programs are available, the old surveys and methods used to establish NGVD 29 elevations are not a robust substitute for new measurements made with modern surveying technology and tied to well-founded, well-maintained NAVD 88 control monuments. Furthermore, the NGVD 29 elevations for benchmarks in areas of active subsidence frequently were not adjusted to account for movement of the terrain.
Finding. FEMA is justified in requiring that all survey data be referenced to the NAVD 88 datum.
Establishing an orthometric height datum that can provide centimeter-level height accuracy requires the use of either geodetic survey leveling observations or GPS measurements and a high-accuracy geoid model. The current version of NAVD 88 does not apply to islands, which cannot be reached with leveling measurements from the continental United States. Therefore, uniform national standards for FEMA flood maps cannot be met until an improved orthometric height datum and geoid model exist. The NGS is engaged in this task through geodetic leveling in U.S. territorial islands and implementation of the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project, which is estimated to be completed in 2017 (NOAA, 2007). If local island vertical datums are established, efforts should be made to ensure that the observations conform to national geodetic standards and that the data are archived and easily available for later adjustments.
The NGS Height Modernization Program includes the development of a high-accuracy geoid model and tools to assist with datum transformations. Height modernization has been implemented in only a few states (Figure 3.3). Yet it is essential for ongoing maintenance and expansion of NAVD 88 to support FEMA’s standards and requirements for flood studies and floodplain mapping. The control monumenta-
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FIGURE 3.3 Location of NGS height modernization stations as of March 2007. SOURCE: Courtesy of D. Zilkoski, NOAA.
tion established by the program can be used as a basis for remote sensing surveys of topographic surfaces and hydrographic surveys of bathymetric surfaces. Establishing additional high-accuracy control points throughout the nation would make it possible to tie local structure surveys, including those performed for Elevation Certificates, to the common vertical reference system, ensuring a precise comparison to computed base flood elevations and accurate evaluation of flood risk.
Tidal Datums
There are numerous tidal datums (e.g., mean sea level), each defined by a certain phase of the tide and targeted to a particular application. The principal tidal datums in the United States are measured at tide gage stations over 19-year periods.2 Tide gages measure local water levels; therefore, tidal datums are location specific and cannot be extended to areas with different oceanographic characteristics without substantiating measurements. Importantly for floodplain mapping, mean sea level at two different locations will not be on the same equipotential gravity surface. Thus, when performing engineering studies or making maps over large coastal areas, water surface elevations referenced to any tidal datum must be converted to the orthometric height datum used to reference the topographic surface. The relationship between tidal and orthometric height datums is shown in Figure 3.4.
The choice of an appropriate vertical datum depends on a number of factors, including whether the primary interest is the height of land or the depth of water. Regardless, it is essential to have access to well-maintained control monuments whose elevation with respect to the desired datum(s) is known with very high accuracy so they can be used as reference points for further elevation measurements.
2
Further information is available at <http://tidesandcurrents.noaa.gov/datum_options.html>.
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FIGURE 3.4 Where is zero on this scale? Height differences between tidal datums such as mean lower low water (MLLW) and geodetic datums are derived by leveling from a tidal benchmark (A), to which tidal datums are referenced, to a geodetic benchmark (B), and comparing heights. NOTE: MHHW = mean higher high water, MHW = mean high water, MLW = mean low water, MTL = mean tide level. SOURCE: Courtesy of D. Zilkoski, NOAA.
ESTABLISHING BASE SURFACES
Topographic Surfaces
The goal of topographic mapping is to develop a detailed and accurate three-dimensional model of the bare Earth, without vegetation or man-made structures, to be used as a base map surface. Topography can be mapped directly using traditional surveying instruments such as theodolites and levels or remotely using photo-grammetry (aerial surveying). Photogrammetry was used to produce the majority of elevation contours shown on U.S. Geological Survey (USGS) 1:24,000-scale topographic maps (Figure 3.5). Digital elevation models (DEMs) were historically derived from these contours or from photogrammetric data compiled from the aerial photographic sources used to create the topographic maps. However, these methods are being superseded by new remote sensing technologies, particularly lidar (light detection and ranging) and IFSAR (interferometric synthetic aperture radar), which can quickly produce highly accurate surface models over large areas.
Although land surface elevation is stable in many areas, natural processes and human activities can cause elevation changes on the order of inches per year. Continual monitoring of subsidence and updating of elevation databases every few years may be required in these areas (e.g., coastal Louisiana, Texas, and Mississippi; central valley of California). In geologically stable areas, topographic changes caused by construction and development can be tracked locally and fed into a national database.
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FIGURE 3.5 Portion of a USGS topographic map in Centre County, Pennsylvania, depicting elevation contours derived photogrammetrically from stereo aerial photography.
The National Elevation Dataset (NED), which is maintained by the USGS, is composed largely of USGS digital elevation models at 30-meter and 10-meter post spacing, but also includes some high-resolution, more accurate datasets acquired by the USGS and state and local governments. A shaded relief map created from the NED is shown in Figure 3.6. Independent tests have shown that the overall vertical accuracy of elevation data in the NED is 14.9 feet at the 95 percent confidence level (NRC, 2007). Although local NED accuracy may meet FEMA accuracy requirements in limited areas of the country, the overall value falls far short of these requirements, which are 1.2 feet in flat terrain and 2.4 feet in hilly terrain at the 95 percent confidence level (Box 3.1).
Finding. The National Elevation Dataset and the tagged vector contour data from 1:24,000 topographic maps used to create it have an elevation uncertainty that is about 10 times larger than that defined by FEMA as acceptable for floodplain mapping.
Bathymetric Surfaces
The bottom surface of rivers, lakes, and oceans is keenly important to hydraulic and storm surge modeling. However, no technology exists for obtaining accurate and detailed measurements of the entire bottom surface for all types of rivers, lakes, and coastal areas of interest in a flood study. Hydrographic surveys can be performed from boats, using sounding devices to produce profiles and samples of the bottom surface. Bathymetric lidar can be used to the extent that the blue-green laser light can penetrate the water. It is quite useful in clear water (e.g., around Hawaiian coral reefs), somewhat useful in shallow areas (e.g., along barrier islands of the southeastern United States), but ineffective in turbid rivers, lakes, streams, and oceans.
River bathymetry is defined using field-surveyed cross sections (e.g., Figure 3.7) immediately upstream and downstream of bridges and culverts. Traditional survey instruments (e.g., levels, total stations) or GPS are typically used to determine water surface elevations
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FIGURE 3.6 A shaded relief representation of the conterminous Unites States created from the National Elevation Dataset. Elevation is shown as a range of colors, from dark green for low elevations to white for high elevations. SOURCE: USGS, <http://erg.usgs.gov/isb/pubs/factsheets/fs10602.html>.
FIGURE 3.7 Example of a riverine cross-section survey. Elevations are measured at all significant breaks in gradient and at intermediate points depending on the width and depth of the river. SOURCE: FEMA (2003).
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FIGURE 3.8 Areas where VDatum is currently available to transform coastal measurements to a common vertical datum. SOURCE: Bang Le, NOAA.
along the water edge. FEMA guidelines require cross-section surveys to include an elevation at the deepest part of the channel (FEMA, 2003). Cross-section surveys derive elevation from nearby geodetic control monuments, applying observed height differences between these known points and the newly surveyed points to establish their elevation with respect to the vertical datum.
NOAA’s National Ocean Service (NOS) is responsible for mapping bathymetry in coastal areas, and the U.S. Army Corps of Engineers is responsible for mapping the bathymetry of navigable inland waterways. Because bathymetric charts are used for marine navigation, they display depth below a tidal datum. To produce coastal flood hazard maps, bathymetric data must be converted to NAVD 88. A NOAA software tool (VDatum) enables coastal water surface elevation measurements, which are made relative to a tidal datum, to be related to the orthometric height datum used as the reference surface for FEMA maps and studies. This makes it possible to merge topographic and bathymetric surfaces to create the seamless elevation surface needed to support storm surge modeling, coastal flood studies, and coastal floodplain mapping. Recent hurricanes along the Gulf Coast and the subsequent imperative to update storm surge models and coastal flood hazard maps demand continuation of this work, but funding shortfalls have slowed its completion until 2013.3 Areas where sufficient input data (hydrodynamic models and sea surface topographic grids) exist to use the tool are shown in Figure 3.8.
MEASURING AND MONITORING WATER SURFACE ELEVATIONS
Water surfaces are dynamic by nature, changing over a wide range of time scales as a result of variations in the amount of rainfall, the influence of diurnal tides, the dynamics of ocean circulation, and changes in global sea level. Measurements of water surface elevations must be monitored continuously over long periods of time to identify trends and cycles.
Riverine Water Surfaces
Stream gages are the most common way to monitor riverine water surfaces. Stream gages measure stream stage, or height of the water relative to the gage. Discharge, which is the volume of water passing the gage location in a given interval of time, can be calculated from stream stage height using a rating curve based on historical measurements of flow and stage at the gage
3
See <http://vdatum.noaa.gov>.
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Mapping the Zone: Improving Flood Map Accuracy
location. The USGS operates a network of more than 7,000 stream gages nationwide and provides real-time data, recorded at 15- to 60-minute intervals.4 A typical USGS stream gage is shown in Figure 3.9. Stream gages usually survive flood events and provide much needed information about riverine water surface elevations used to calibrate flood models and determine flood frequencies.
Lidar offers another way to monitor water surface elevations. Figure 3.10 shows an inundation map of part of the Iowa River made using lidar data during flooding in the summer of 2008. Such real-time, high-accuracy measurements of water surface elevation could also be used to evaluate the relative accuracies of different types of flood studies (e.g., detailed, approximate). Currently, high-water marks of historical floods are used for this purpose, but they are sparse and no systematic efforts are made to archive them in a national repository of flood data.
Coastal Water Surfaces
Tide gages measure water heights relative to the gage. To determine water level with respect to any tidal or orthometric height datum, the height of the gage must be known with respect to that datum. Since tidal datums change over time and since tide gage measurements are used to develop tidal datums, it is prudent to maintain the height of the tide gage with respect to a more solidly fixed orthometric height datum.
The NOS maintains tide gages as part of the National Water Level Observation Network (NWLON). The network includes approximately 200 long-term, continuously operating water level stations throughout the United States—including islands, territories, and the Great Lakes—vertically referenced to nearby geodetic control monuments. NWLON stations provide the reference for tide prediction products, serve as controls for determining tidal datums for short-term water level stations, and are a key component of NOAA’s tsunami and storm surge warning systems. The data continuity, vertical stability, and careful referencing of NWLON stations also enable the data to be used to estimate relative sea level trends, such as those shown in Figure 3.11.
FIGURE 3.9 Typical USGS stream gage. The box on top of the metal pipe contains a data logger that has a pulley with a metal wire holding a float at one end. As the water in the stream moves up and down, the float moves, turning the pulley and changing the gage-height reading. The data are transmitted to computers via satellite radio. SOURCE: USGS.
Finding. There are significant long-term linear trends in sea levels around the U.S. coastline; in most cases, sea levels are rising with respect to the land surface. The rate of change of sea level is significant when compared to flood map accuracy standards.
Measuring the extreme water elevations caused by storm surge has been a challenge. Gages are often destroyed by the surge and waves, so water surface elevations are usually estimated by surveying high water marks left on buildings and other elevated objects that survive the storm. Such surveys require deployment of numerous technicians during the height of rescue and recovery activities because data must be collected before they are altered or destroyed by cleanup efforts. A pre-storm deployed network of temporary gages designed to survive extreme events was established by the USGS after the 2005 hurricane season to begin building a record of the timing, extent, and magnitude of storm surge.
4
Stream gage data are available through the National Water Information System, <http://waterdata.usgs.gov/nwis/rt>.
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FIGURE 3.10 Color-coded image map of floodwater surface elevation above the ellipsoid using lidar in Iowa City, Iowa. Areas in the darkest blue (160 meters) have the lowest ellipsoid heights. The lighter blue areas indicate higher water surface elevations (163 meters). Water is flowing from right to left so the flooded regions on the left side of the picture are “downslope” from the flooded areas on the right side of the picture. The lidar data were collected by the National Science Foundation’s National Center for Airborne Laser Mapping in June 2008 for IIHR-Hydroscience and Engineering at the University of Iowa. SOURCE: Courtesy of Ramesh Shrestha, University of Florida, and Witold Krajewski, IIHR-Hydroscience and Engineering. Used with permission.
SURVEYING STRUCTURE ELEVATIONS
Hydraulic Structures
For detailed studies, FEMA guidelines specify that the dimensions and elevations of all hydraulic structures and underwater sections adjacent to the structures must be obtained from available sources or by field survey where necessary (FEMA, 2003). Aerial surveys are not permitted. Data required for detailed studies of hydraulic structures are summarized in Table 3.1.
For limited detailed studies, bridges and hydraulic structures are typically modeled using field measurements or as-built records, rather than precise survey measurements.5 For approximate studies, bridge, culvert, dam, and weir data may be estimated from photographs, orthophotos, or existing topographic mapping without performing field surveys (FEMA, 2003). Oblique aerial digital imagery, now available in some communities, can also provide good estimates of hydraulic structure dimensions.
Buildings
Elevation Certificates provide elevation information necessary to document compliance with community floodplain management ordinances, to determine the proper insurance premium rate, and to support requests for map amendment or revision. Surveys for Elevation Certificates have traditionally been made using differential levels and total stations, with differential elevations relative to the nearest available (not necessarily the most accurate) benchmark to minimize survey costs. In recent years these methods have been supplemented with GPS surveys and GPS-derived elevations relative to the most accurate control monument in the community. GPS-derived structural elevation data on Elevation Certificates are estimated to be accurate to ±0.5 foot at the 95 percent confidence level (FEMA, 2005b).
Data from Elevation Certificates are rarely available in digital format for all buildings in a community.
5
Presentation to the committee by Paul Rooney, FEMA, on August 20, 2007.
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FIGURE 3.11 Sea level trends throughout the twentieth century determined from continuously operating water level stations. Sea level is increasing at Charleston, South Carolina, and Galveston, Texas. It is decreasing at Juneau, Alaska, indicating that the land level is rising faster through postglacial rebound than the sea level. SOURCE: NOAA, <http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml>.
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TABLE 3.1 Data Requirements for Detailed Studies of Hydraulic Structures
Bridges
Culverts
Dams and Weirs
Size and shape of openings
Upstream and downstream channel invert elevations
Entrance conditions (e.g., wingwalls, vertical abutments)
Bridge deck thickness, low-steel elevation, and bridge parapet type (i.e., solid railing, open railing)
Roadway embankment side-slope rate
Type and width of roadway pavement
Top-of-road section of sufficient length for weir-flow calculations
Size and shape of openings
Upstream and downstream channel invert elevations
Entrance conditions (i.e., headwall, wingwalls, mitered to slope, projecting)
Height of road surface above culvert invert and vertical dimensions of guardrails
Roadway embankment side-slope rate
Type and width of roadway pavement
Top-of-road section of sufficient length for weir-flow calculations
Top-of-dam elevation
Normal pool elevation
Principal spillway type, inlet and outlet elevations, and dimensions
Emergency spillway type (if applicable), elevation, and dimensions
A FEMA (2005b) report examined whether it is technically feasible to mass-produce Elevation Certificates inexpensively using aerial remote sensing. If so, an elevation registry could be populated with elevation data for all structures in a community for electronic rating of flood insurance policies and for geographic information system (GIS) analysis of flood risks. Although the study found that lowest adjacent grade elevations of reasonable accuracy could be produced from aerial surveys, other elevation data (e.g., elevation of basement floors) cannot be determined without on-site land surveys. Therefore, there are no current plans to establish an elevation registry of all structures in or near floodplains.
IMPACT OF ELEVATION UNCERTAINTIES IN A FLOOD STUDY
The base flood elevation (BFE) is the critical piece of water surface elevation data portrayed on a flood map. The accuracy of the BFE depends on the accuracy of other elevation components described above.
Vertical and Horizontal Uncertainties
The BFE is expressed as a height above NAVD 88. There are three sources of uncertainty implicit in this elevation: (1) geodetic uncertainty in defining the true elevation of the datum itself, (2) terrain uncertainty in measuring the height of the ground surface above the datum, and (3) hydraulic uncertainty in calculating the floodwater depth above the stream channel and floodplain surface. Once the BFE has been determined, it is mapped on the terrain surface to determine the horizontal extent of flooding across the landscape. The point at which the water surface intersects the terrain becomes the floodplain boundary. Elevation errors in the terrain surface can therefore affect the horizontal location of the floodplain boundary.
USGS Digital Elevation Models and Floodplain Mapping
The accuracy of the terrain surface is a function of the accuracy of the survey methods used to produce it. Land or airborne surveys determine elevations at a limited number of points on the ground, and a continuous terrain surface is created by interpolating between the points. The density and spacing of the measurements depend on the survey technology used and have a significant effect on cost. Therefore, it is important to establish the optimum point spacing and density to represent the terrain surface: too few points, and key features may be left out or smoothed over; too many points, and cost and data management may become burdensome.
Throughout the history of the FEMA floodplain mapping program, a mixture of data has been used to define topography. In detailed studies of high-flood-risk areas, data of accuracy equivalent to 4-foot contours or better have generally been used, at least for the main rivers and streams. In approximate studies of lower-flood-risk areas, USGS digital elevation data are more commonly used, either as tagged vector contour data or as digital elevation models derived from such data. However, the USGS DEM has three shortcomings for floodplain mapping (NRC, 2007):
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On average, USGS DEM data contained in the NED are more than 35 years old, while FEMA flood mapping standards call for data measured within the last 7 years.
The standard gridded digital elevation model in the NED has 30-meter point spacing, but many land features (e.g., levees, berms, small streams, drains) are less than 30 meters wide and may be missing from the terrain surface generated from the DEMs.
The original surveys were performed from high-altitude photography, and the absolute elevation error is on the order of meters.
Lidar is capable of taking dense measurements (i.e., one or more points for every square meter on the ground), and absolute errors in elevations are measurable in centimeters, rather than meters, which is in accordance with current FEMA requirements (FEMA, 2003). To quantify the differences between NED and lidar data, the committee requested the North Carolina Floodplain Mapping Program (NCFMP) to produce flood maps made using each type of data in the North Carolina case study areas. Figure 3.12 and Table 3.2 show the elevation differences around streams in flat Hereford County, hilly Mecklenburg County, and mountainous Buncombe County.
Ground truthing proves that the lidar data meet FEMA requirements for floodplain mapping (NCFMP, 2008) and supports the NRC (2007) recommendation for nationwide collection of high-resolution, high-accuracy topographic data.
Finding. At Ahoskie Creek and the Swannanoa River, the stream and topographic data are well aligned for both lidar data and the NED, so while there are random differences between then, the average difference is small. At Long Creek, the stream and topographic data are aligned for the lidar data but not for the NED, so there is a large systematic difference between lidar and NED at this location.
The elevation differences have important implications for predicting the extent of expected flooding. Figure 3.13 depicts the difference in predicted flood inundation in Pamlico Sound using a USGS digital elevation model and the NCFMP lidar data. Uncertainties in the amount of land inundated are much
FIGURE 3.12 Elevation differences between the USGS NED and the North Carolina Floodplain Mapping Program lidar along rivers in three counties in North Carolina. Areas in red and pink are lower than appear on FEMA flood maps and suggest that the floodplain extends further than expected. Top: Eastern coastal plain (Ahoskie Creek, elevation ranging from 1 foot to 74 feet). Middle: Central piedmont (Long Creek, elevation ranging from 566 to 767 feet). Bottom: Western mountains (Swannanoa River, elevation ranging from 1,966 to 2,202 feet). SOURCE: Courtesy of T. Langan, North Carolina Floodplain Mapping Program. Used with permission.
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TABLE 3.2 Elevation Difference Statistics, NED Minus Lidar
Stream
Mean (ft)
Standard Deviation (ft)
Minimum (ft)
Maximum (ft)
Ahoskie Creek
0.5
3.9
34.8
−25.3
Long Creek
14.7
15.6
81.5
−46.0
Swannanoa River
−2.0
17.5
89.7
−139.3
FIGURE 3.13 Inundation maps of Beaufort County, North Carolina, where the Tar-Pamlico River empties into Pamlico Sound. The figure on the left is based on a 30-meter DEM created from the USGS NED. The figure on the right is based on a 3-meter DEM created from NCFMP lidar data. The dark blue tint represents land that would become inundated with 1 foot of storm surge or sea level rise. The light blue area represents uncertainty in the extent of inundation at the 95 percent confidence level. SOURCE: Gesch (2009).
greater with the DEM. The large differences represent potential error in determination of the flood boundary and, thus, the flood risk.
CONCLUSIONS
It is neither trivial nor inexpensive to accurately measure and monitor the elevation of land, water, and structures across a vast geographic area. However, the committee’s analysis shows that the accuracy of elevation data has an enormous impact on the accuracy of flood maps. Ensuring that future flood studies are based on the most accurate and consistent foundation possible requires (1) continuation of a suite of agency elevation programs and (2) acquisition of accurate, high-resolution elevation data. Key elements of this foundation include the National Height Modernization program, VDatum, and improved measurement of terrain and of streamflow and storm surge during flood events. Major efforts include the following:
Elevation for the Nation. The North Carolina case study demonstrates the sensitivity of flood studies and floodplain boundary determinations to the resolution and accuracy of topographic data. Clearly, the standard practice of using the best available elevation data does not meet the needs of FEMA’s floodplain mapping program. As concluded by the National Research Council (NRC, 2007), a seamless, high-resolution, high-accuracy topographic dataset is needed nationwide to support floodplain mapping. The governance and implementation of Elevation for the Nation is currently being considered (along with similar initiatives for nationwide imagery, transportation, and parcel data) by the National Geospatial Advisory Committee. Elevation for the Nation would rely on nationwide availability of high-accuracy control monumentation provided by national height modernization.
Recommendation. FEMA should increase collaboration with the USGS and state and local government
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agencies to acquire high-resolution, high-accuracy topographic data throughout the nation.
National Water Information System. Stream gage data, available through the USGS National Water Information System, provide the necessary riverine discharge information required for flood studies. Flood maps can be produced with much greater accuracy when a long and consistent history of stream gage information, and therefore discharges during flooding, is available.
USGS Storm Surge Network. The USGS currently deploys short-duration storm surge gages prior to expected landfall of hurricanes. These gages are a considerable improvement over post-storm watermark surveys, which are subject to significant errors and uncertainties in the peak storm surge and wave conditions. Accurate storm surge measurements are critical for verifying coastal storm surge models using selected historical storms (see Chapter 5).
National Water Level Observation Network. Flood risk is increasing rapidly in coastal areas due to a combination of land subsidence, sea level rise, population growth, and development. Coastal water elevations, measured and monitored through NOAA’s NWLON program, provide essential information for FEMA’s coastal flood maps. The information provided by NWLON tide gages is also critical to the development of VDatum, which in turn is needed to develop seamless topographic-bathymetric surfaces for coastal flood studies.
Elevation and height data are analogous to the foundation of a skyscraper; even if the engineering design and construction are flawless, the entire building is at risk of failure if the foundation is inadequate. It would be wise to lay a strong foundation before investing additional time, effort, and money in further construction of a building. Yet we have not taken such an approach to elevation data as they pertain to floodplain mapping. The technology and knowledge to build and maintain a comprehensive and accurate elevation measurement system have been available for 15 to 20 years. The main hurdle to implementing such a system nationwide has been cost. The relative costs and benefits of investing substantially in elevation data to produce more accurate flood maps are discussed in Chapter 6.
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Mapping the Zone: Improving Flood Map Accuracy
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