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3 Elevation and Height Data A flood map is the final outcome of a multitude 3. Water surface elevation. The depth of water in of measurement, engineering, and data Âanalysis rivers, lakes, and streams and the point at which water tasks. The purpose of a flood study is to pre- overtops their banks and spreads across the landscape dict the height of water and the extent to which it will are the subjects of riverine flood studies. The depth of inundate the landscape in a modeled flood event. The water in the ocean and the impact of extreme events elevations of the land, water, and hydraulic structures such as hurricane-induced storm surge or earthquake- (e.g., bridges) are key elements in a flood study, and the induced tsunamis are the subjects of coastal flood accuracy to which these elements are determined is a studies. The height of water surfaces is measured with critical factor in the accuracy of the final flood map. The stream and tide gages. The location and elevation of Federal Emergency Management Agencyâs (FEMAâs) the gages themselves must be determined accurately in accuracy standards for land surface elevations are sum- order to correctly relate water surface measurements to marized in Box 3.1. This chapter explains how eleva- other elevations. tion is measured and examines the impact of elevation 4. Structure elevation. The vulnerability of build- uncertainties in flood studies. ings and infrastructure to flood damage is directly The data components of a flood study that involve related to their location with respect to the floodplain a measurement of height or elevation can be grouped and the elevation and orientation of critical structural into four general categories: components with respect to the height of potential floodwaters. In addition, structures within the floodway 1. Elevation reference surface. Before elevation can (such as bridges, dams, levees, and culverts) influence be measured or the data used in engineering analysis, a the conveyance of water in a stream channel during a measurement system must be established. The location flood event, affecting flood heights. of âzeroâ and a physical reference for elevation zero (in other words, a vertical datum) must be established on These categories are described in more detail the Earth, where it can be used for all types of height below. measurements. 2. Base surface elevation. Two types of base sur- ESTABLISHING A REFERENCE SURFACE faces are important to flood studies: land surface elevation (topography) and its underwater equivalent To measure something with a ruler, we place the ( Â bathymetry). Topography is expressed as the height zero mark at the end of the object and measure length of a location above the geodetic datum and is in most or distance relative to that mark. The term datum cases a positive value. Bathymetry is expressed as the refers to a reference surface against which position depth of the land surface below rivers, lakes, and oceans; measurements are made; it defines the location of zero positive depth is equivalent to negative elevation. on the measurement scale. Three fundamentally differ- 25
26 MAPPING THE ZONE BOX 3.1â FEMA Land Surface Elevation Accuracy Standards FEMA has established two land surface elevation accuracy stan- dards, depending on whether the terrain is flat or rolling to hilly (FEMA, 2003, Appendix A): 1. Two-foot contour interval equivalent for flat terrain ( Â 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. 2. 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 impor- FIGURE 3.1â Relationship of the Earthâs surface, the geoid, tance of variations in the way elevation is measured and defined and a geocentric ellipsoid. The height difference between the in the flood mapping process. geoid and the ellipsoid is the geoid separation. SOURCE: Kevin Figure 3-1.eps McMaster, URS Corporation. Used with permission. bitmap image ent types of vertical datumsâellipsoidal, orthometric, Orthometric Height Datums and tidalâare relevant to flood studies. In the United States, establishing and maintaining vertical datums is Modeling the flow of water across the Earthâs surface the responsibility of the National Oceanic and Atmo- requires a reference surface defined by constant gravita- spheric Administrationâs (NOAAâs) National Geodetic tional potential; this surface is referred to as the geoid. Survey (NGS). Heights measured with respect to an Â equipotential gravity surface are called orthometric heights, and the difference between the ellipsoid and the geoid at any Ellipsoidal Datums particular location on the Earth is called the geoid The Global Positioning System (GPS) provides height, or geoid separation (Figure 3.1). Geoid Âmodels the most accurate and efficient means for establishing developed and maintained by the NGS are used to fundamental reference marks (also called monuments) convert ellipsoid heights to orthometric heights. on the Earthâs surface, and it forms the basis for most The orthometric height datum for surveying and land and aerial surveys performed today. Land surveys mapping the North American continent is the North are performed using handheld and tripod-mounted American Vertical Datum of 1988 (NAVD 88). NAVD GPS equipment; airborne photogrammetric or remote 88 supersedes the National Geodetic Vertical Datum sensing surveys employ GPS and inertial measure- of 1929 (NGVD 29), which was used in many early ment systems to track the position of the sensor and flood maps and provided the basis for many engineering project the data into accurate ground coordinates. flood studies still in use today. The height differences GPS satellite systems measure distances to the Earthâs between NGVD 29 and NAVD 88 can be large (Fig- surface relative to a mathematically idealized (smooth) ure 3.2), ranging from â49 cm (â1.6 feet) in Florida to ellipsoid that closely approximates the shape of the +158 cm (+5.2 feet) in Colorado. Elevation differences Earth (Figure 3.1). Heights computed with respect to between NGVD 29 and NAVD 88 are immaterial to this surface are referred to as ellipsoid heights. How- flood mapping as long as elevations are referenced to ever, neither the Earthâs surface nor its gravity field, as the same datum. A potential problem arises when old delineated by the undulating geoid surface, matches See <http://geodesy.noaa.gov/faq.shtml> and Maune (2007) for this idealized ellipsoid. a description of the differences between the two datums.
ELEVATION AND HEIGHT DATA 27 FIGURE 3.2â 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 ing measurements from the continental United States. new studies, based otherwise on NAVD 88. Although Therefore, uniform national standards for FEMA flood conversion programs are available, the old surveys and maps cannot be met until an improved orthometric methods used to establish NGVD 29 elevations are height datum and geoid model exist. The NGS is not a robust substitute for new measurements made engaged in this task through geodetic leveling in U.S. with modern surveying technology and tied to well- territorial islands and implementation of the Gravity founded, well-maintained NAVD 88 control monu- for the Redefinition of the American Vertical Datum ments. FurtherÂmore, the NGVD 29 elevations for (GRAV-D) project, which is estimated to be completed benchmarks in areas of active subsidence frequently in 2017 (NOAA, 2007). If local island vertical datums were not adjusted to account for movement of the are established, efforts should be made to ensure that terrain. the observations conform to national geodetic stan- dards and that the data are archived and easily available Finding. FEMA is justified in requiring that all for later adjustments. s Â urvey data be referenced to the NAVD 88 datum. The NGS Height Modernization Program includes the development of a high-accuracy geoid model and Establishing an orthometric height datum that tools to assist with datum transformations. Height can provide centimeter-level height accuracy requires moderniÂzation has been implemented in only a few the use of either geodetic survey leveling observations states (Figure 3.3). Yet it is essential for ongoing or GPS measurements and a high-accuracy geoid maintenance and expansion of NAVD 88 to support model. The current version of NAVD 88 does not FEMAâs standards and requirements for flood studies apply to islands, which cannot be reached with level- and floodplain mapping. The control monumenta-
28 MAPPING THE ZONE FIGURE 3.3â Location of NGS height modernization stations as of March 2007. SOURCE: Courtesy of D. Zilkoski, NOAA. Figure 3-3.eps bitmap image tion established by the program can be used as a basis local water levels; therefore, tidal datums are location for remote sensing surveys of topographic surfaces specific and cannot be extended to areas with different and hydrographic Â surveys of bathymetric surfaces. oceanographic characteristics without substantiating Establishing additional high-accuracy control points measurements. Importantly for floodplain mapping, throughout the nation would make it possible to tie mean sea level at two different locations will not be local structure surveys, including those performed for on the same equipotential gravity surface. Thus, when Elevation Certificates, to the common vertical refer- performing engineering studies or making maps over ence system, ensuring a precise comparison to com- large coastal areas, water surface elevations referenced puted base flood elevations and accurate evaluation of to any tidal datum must be converted to the ortho- flood risk. metric height datum used to reference the topographic surface. The relationship between tidal and orthometric Tidal Datums height datums is shown in Figure 3.4. The choice of an appropriate vertical datum There are numerous tidal datums (e.g., mean sea depends on a number of factors, including whether level), each defined by a certain phase of the tide and the primary interest is the height of land or the depth targeted to a particular application. The principal tidal of water. Regardless, it is essential to have access to datums in the United States are measured at tide gage well-maintained control monuments whose elevation stations over 19-year periods. Tide gages measure with respect to the desired datum(s) is known with very high accuracy so they can be used as reference points Further for further elevation measurements. information is available at <http://tidesandcurrents. noaa.gov/datum_options.html>.
ELEVATION AND HEIGHT DATA 29 B A H(29) (Physical location of tidal datums) H(88) MHHW H(MLLW) MHW NGVD 29 MTL NAVD 88 MLW MLLW FIGURE 3.4â Where is zero on this scale? Height differences between tidal datums such as mean lower low water (MLLW) and Âgeodetic 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. 3-4 revised ESTABLISHING BASE SURFACES photographic sources used to create the topographic maps. However, these methods are being superseded by Topographic Surfaces new remote sensing technologies, particularly lidar (light detection and ranging) and IFSAR (interferometric The goal of topographic mapping is to develop a s Â ynthetic aperture radar), which can quickly produce detailed and accurate three-dimensional model of the highly accurate surface models over large areas. bare Earth, without vegetation or man-made structures, Although land surface elevation is stable in many to be used as a base map surface. Topography can be areas, natural processes and human activities can cause mapped directly using traditional surveying instruments elevation changes on the order of inches per year. Con- such as theodolites and levels or remotely using photo- tinual monitoring of subsidence and updating of eleva- grammetry (aerial surveying). Photogrammetry was used tion databases every few years may be required in these to produce the majority of elevation contours shown areas (e.g., coastal Louisiana, Texas, and Â Mississippi; on U.S. Geological Survey (USGS) 1:24,000-scale central valley of California). In geologically stable topographic maps (Figure 3.5). Digital elevation models areas, topographic changes caused by construction (DEMs) were historically derived from these contours and development can be tracked locally and fed into a or from photogrammetric data compiled from the aerial national database.
30 MAPPING THE ZONE FIGURE 3.5â Portion of a USGS topographic map in Centre County, Pennsylvania, depicting elevation contours derived photogram- metrically from stereo aerial photography. Figure 3-5.eps bitmap image The National Elevation Dataset (NED), which is Bathymetric Surfaces maintained by the USGS, is composed largely of USGS digital elevation models at 30-meter and 10-meter post The bottom surface of rivers, lakes, and oceans spacing, but also includes some high-resolution, more is keenly important to hydraulic and storm surge accurate datasets acquired by the USGS and state and modeling. However, no technology exists for obtain- local governments. A shaded relief map created from ing accurate and detailed measurements of the entire the NED is shown in Figure 3.6. Independent tests bottom surface for all types of rivers, lakes, and coastal have shown that the overall vertical accuracy of eleva- areas of interest in a flood study. Hydrographic surveys tion data in the NED is 14.9 feet at the 95 percent can be performed from boats, using sounding devices confidence level (NRC, 2007). Although local NED to produce profiles and samples of the bottom surface. accuracy may meet FEMA accuracy requirements in Bathymetric lidar can be used to the extent that the limited areas of the country, the overall value falls far blue-green laser light can penetrate the water. It is quite short of these requirements, which are 1.2 feet in flat useful in clear water (e.g., around Hawaiian coral reefs), terrain and 2.4 feet in hilly terrain at the 95 percent somewhat useful in shallow areas (e.g., along barrier confidence level (Box 3.1). islands of the southeastern United States), but ineffec- tive in turbid rivers, lakes, streams, and oceans. Finding. The National Elevation Dataset and the River bathymetry is defined using field-surveyed tagged vector contour data from 1:24,000 topographic cross sections (e.g., Figure 3.7) immediately upstream maps used to create it have an elevation uncertainty and downstream of bridges and culverts. Traditional that is about 10 times larger than that defined by survey instruments (e.g., levels, total stations) or GPS FEMA as acceptable for floodplain mapping. are typically used to determine water surface elevations
ELEVATION AND HEIGHT DATA 31 FIGURE 3.6â 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. ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Figure 3-6.eps SOURCE: USGS, <http://erg.usgs. gov/isb/pubs/factsheets/fs10602.html>. bitmap image FIGURE 3.7â Example of a riverine cross-section survey. Elevations are measured at all significant breaks in gradient and at inter- mediate points depending on the width and depth of the river. SOURCE: FEMA (2003). Figure 3-7 redraft.eps bitmap image
32 MAPPING THE ZONE FIGURE 3.8â Areas where VDatum is currently available to transform coastal measurements to a common vertical datum. SOURCE: ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Bang Le, NOAA. Figure 3-8.eps bitmap image along the water edge. FEMA guidelines require cross- work, but funding shortfalls have slowed its completion section surveys to include an elevation at the deepest until 2013. Areas where sufficient input data (hydro- part of the channel (FEMA, 2003). Cross-section dynamic models and sea surface topographic grids) surveys derive elevation from nearby geodetic control exist to use the tool are shown in Figure 3.8. monuments, applying observed height differences between these known points and the newly surveyed MEASURING AND MONITORING WATER points to establish their elevation with respect to the SURFACE ELEVATIONS vertical datum. NOAAâs National Ocean Service (NOS) is respon- Water surfaces are dynamic by nature, changing sible for mapping bathymetry in coastal areas, and over a wide range of time scales as a result of varia- the U.S. Army Corps of Engineers is responsible for tions in the amount of rainfall, the influence of diurnal mapping the bathymetry of navigable inland water- tides, the dynamics of ocean circulation, and changes in ways. Because bathymetric charts are used for marine global sea level. Measurements of water surface eleva- navigation, they display depth below a tidal datum. To tions must be monitored continuously over long periods produce coastal flood hazard maps, bathymetric data of time to identify trends and cycles. must be converted to NAVD 88. A NOAA software tool (VDatum) enables coastal water surface eleva- Riverine Water Surfaces tion measurements, which are made relative to a tidal datum, to be related to the orthometric height datum Stream gages are the most common way to monitor used as the reference surface for FEMA maps and riverine water surfaces. Stream gages measure stream studies. This makes it possible to merge topographic stage, or height of the water relative to the gage. Dis- and bathymetric surfaces to create the seamless eleva- charge, which is the volume of water passing the gage tion surface needed to support storm surge modeling, location in a given interval of time, can be calculated coastal flood studies, and coastal floodplain mapping. from stream stage height using a rating curve based on Recent hurricanes along the Gulf Coast and the sub- historical measurements of flow and stage at the gage sequent imperative to update storm surge models and coastal flood hazard maps demand continuation of this See <http://vdatum.noaa.gov>.
ELEVATION AND HEIGHT DATA 33 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. A typi- cal USGS stream gage is shown in Figure 3.9. Stream gages usually survive flood events and provide much needed information about riverine water surface eleva- tions 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 differ- ent 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 sys- tematic efforts are made to archive them in a national repository of flood data. Figure 3-9.eps FIGURE 3.9â Typical USGS stream gage. The box on top of the bitmap image Coastal Water Surfaces 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 Tide gages measure water heights relative to the the gage-height reading. The data are transmitted to computers gage. To determine water level with respect to any tidal via satellite radio. SOURCE: USGS. 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 measure- ments are used to develop tidal datums, it is prudent to Finding. There are significant long-term linear maintain the height of the tide gage with respect to a trends in sea levels around the U.S. coastline; in most more solidly fixed orthometric height datum. cases, sea levels are rising with respect to the land The NOS maintains tide gages as part of the National surface. The rate of change of sea level is significant Water Level Observation Network (NWLON). The when compared to flood map accuracy standards. network includes approximately 200 long-term, con- tinuously operating water level stations throughout the Measuring the extreme water elevations caused United Statesâincluding islands, territories, and the by storm surge has been a challenge. Gages are often Great Lakesâvertically referenced to nearby geodetic destroyed by the surge and waves, so water surface control monuments. NWLON stations provide the elevations are usually estimated by surveying high water reference for tide prediction products, serve as controls marks left on buildings and other elevated objects that for determining tidal datums for short-term water survive the storm. Such surveys require deployment level stations, and are a key component of NOAAâs of numerous technicians during the height of rescue tsunami and storm surge warning systems. The data and recovery activities because data must be collected continuity, vertical stability, and careful referencing of before they are altered or destroyed by cleanup efforts. NWLON stations also enable the data to be used to A pre-storm deployed network of temporary gages estimate relative sea level trends, such as those shown designed to survive extreme events was established by in Figure 3.11. the USGS after the 2005 hurricane season to begin Stream building a record of the timing, extent, and magnitude gage data are available through the National Water Information System, <http://waterdata.usgs.gov/nwis/rt>. of storm surge.
34 MAPPING THE ZONE FIGURE 3.10â Color-coded image map of floodwater surface elevation above the ellipsoid using lidar in Iowa City, Iowa. Areas in Figure 3-10.eps the darkest blue (160 meters) have the lowest ellipsoid heights. The lighter blue areas indicate higher water surface elevations (163 bitmap image meters). Water is flowing from right to left so the flooded regions on the left side of the picture are âdownslopeâ from the flooded areas on the right side of the picture. The lidar data were collected by the National Science Foundationâs 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 some communities, can also provide good estimates of hydraulic structure dimensions. Hydraulic Structures Buildings For detailed studies, FEMA guidelines specify that the dimensions and elevations of all hydraulic structures Elevation Certificates provide elevation informa- and underwater sections adjacent to the structures must tion necessary to document compliance with commu- be obtained from available sources or by field survey nity floodplain management ordinances, to determine where necessary (FEMA, 2003). Aerial surveys are the proper insurance premium rate, and to support not permitted. Data required for detailed studies of requests for map amendment or revision. Surveys for h Â ydraulic structures are summarized in Table 3.1. Elevation Certificates have traditionally been made For limited detailed studies, bridges and hydraulic using differential levels and total stations, with differ- structures are typically modeled using field measure- ential elevations relative to the nearest available (not ments or as-built records, rather than precise survey necessarily the most accurate) benchmark to minimize measurements. For approximate studies, bridge, survey costs. In recent years these methods have been culvert, dam, and weir data may be estimated from supplemented with GPS surveys and GPS-derived photographs, orthophotos, or existing topographic elevations relative to the most accurate control monu- mapping without performing field surveys (FEMA, ment in the community. GPS-derived structural eleva- 2003). Oblique aerial digital imagery, now available in tion data on Elevation Certificates are estimated to be accurate to Â±0.5 foot at the 95 percent confidence level (FEMA, 2005b). Presentation to the committee by Paul Rooney, FEMA, on Data from Elevation Certificates are rarely avail- August 20, 2007. able in digital format for all buildings in a community.
ELEVATION AND HEIGHT DATA 35 FIGURE 3.11â Sea level trends throughout the twentieth century determined from continuously operating water level stations. Sea level is increasing at Charleston, South Carolina, and Galveston, 3-11.epsdecreasing at Juneau, Alaska, indicating that the land Figure Texas. It is level is rising faster through postglacial rebound than thebitmap image NOAA, <http://tidesandcurrents.noaa.gov/sltrends/ sea level. ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ SOURCE: sltrends.shtml>.
36 MAPPING THE ZONE TABLE 3.1â Data Requirements for Detailed Studies of Hydraulic Structures Bridges Culverts Dams and Weirs â¢ Size and shape of openings â¢ Size and shape of openings â¢ Top-of-dam elevation â¢ Upstream and downstream channel invert â¢ Upstream and downstream channel invert â¢ Normal pool elevation elevations elevations â¢ Principal spillway type, inlet and outlet â¢ Entrance conditions (e.g., wingwalls, vertical â¢ Entrance conditions (i.e., headwall, wingwalls, elevations, and dimensions abutments) mitered to slope, projecting) â¢ Emergency spillway type (if applicable), â¢ Bridge deck thickness, low-steel elevation, and â¢ Height of road surface above culvert invert and elevation, and dimensions bridge parapet type (i.e., solid railing, open railing) vertical dimensions of guardrails â¢ Roadway embankment side-slope rate â¢ Roadway embankment side-slope rate â¢ Type and width of roadway pavement â¢ Type and width of roadway pavement â¢ Top-of-road section of sufficient length for â¢ Top-of-road section of sufficient length for weir-flow calculations weir-flow calculations A FEMA (2005b) report examined whether it is tech- horizontal extent of flooding across the landscape. The nically feasible to mass-produce Elevation Certificates point at which the water surface intersects the terrain inexpensively using aerial remote sensing. If so, an becomes the floodplain boundary. Elevation errors in elevation registry could be populated with elevation the terrain surface can therefore affect the horizontal data for all structures in a community for electronic location of the floodplain boundary. rating of flood insurance policies and for geographic information system (GIS) analysis of flood risks. USGS Digital Elevation Models and Although the study found that lowest adjacent grade Floodplain Mapping elevations of reasonable accuracy could be produced from aerial surveys, other elevation data (e.g., eleva- The accuracy of the terrain surface is a function tion of basement floors) cannot be determined without of the accuracy of the survey methods used to produce on-site land surveys. Therefore, there are no current it. Land or airborne surveys determine elevations at a plans to establish an elevation registry of all structures limited number of points on the ground, and a continu- in or near floodplains. ous terrain surface is created by interpolating between the points. The density and spacing of the measure- IMPACT OF ELEVATION UNCERTAINTIES ments depend on the survey technology used and have IN A FLOOD STUDY a significant effect on cost. Therefore, it is important to establish the optimum point spacing and density to The base flood elevation (BFE) is the critical piece represent the terrain surface: too few points, and key of water surface elevation data portrayed on a flood features may be left out or smoothed over; too many map. The accuracy of the BFE depends on the accuracy points, and cost and data management may become of other elevation components described above. burdensome. Throughout the history of the FEMA floodplain Vertical and Horizontal Uncertainties mapping program, a mixture of data has been used to define topography. In detailed studies of high-flood-risk The BFE is expressed as a height above NAVD 88. areas, data of accuracy equivalent to 4-foot contours or There are three sources of uncertainty implicit in this better have generally been used, at least for the main elevation: (1) geodetic uncertainty in defining the true rivers and streams. In approximate studies of lower- elevation of the datum itself, (2) terrain uncertainty in flood-risk areas, USGS digital elevation data are more measuring the height of the ground surface above the commonly used, either as tagged vector contour data datum, and (3) hydraulic uncertainty in calculating or as digital elevation models derived from such data. the floodwater depth above the stream channel and However, the USGS DEM has three shortÂcomings for floodplain surface. Once the BFE has been determined, floodplain mapping (NRC, 2007): it is mapped on the terrain surface to determine the
ELEVATION AND HEIGHT DATA 37 1. 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. 2. 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. 3. 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 Figure 3-12 top.eps (i.e., one or more points for every square meter on the bitmap image ground), and absolute errors in elevations are measur- able 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, Figure 3-12 middle.eps bitmap image 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 aver- age 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 differ- ence between lidar and NED at this location. FIGURE 3.12âFigure 3-12 bottom.eps the USGS NED Elevation differences between and the North Carolina Floodplain Mapping Program lidar bitmap image along rivers in three counties in North Carolina. Areas in red The elevation differences have important implica- and pink are lower than appear on FEMA flood maps and tions for predicting the extent of expected flooding. suggest that the floodplain extends further than expected. Top: Eastern coastal plain (Ahoskie Creek, elevation ranging from Figure 3.13 depicts the difference in predicted flood 1 foot to 74 feet). Middle: Central piedmont (Long Creek, eleva- inundation in Pamlico Sound using a USGS digital tion ranging from 566 to 767 feet). Bottom: Western mountains elevation model and the NCFMP lidar data. Uncer- (Swannanoa River, elevation ranging from 1,966 to 2,202 feet). tainties in the amount of land inundated are much SOURCE: Courtesy of T. Langan, North Carolina Floodplain Mapping Program. Used with permission.
38 MAPPING THE ZONE TABLE 3.2â Elevation Difference Statistics, NED Minus Lidar Stream Mean (ft) Standard Deviation (ft) Minimum (ft) Maximum (ft) Ahoskie Creek â 0.5 â 3.9 34.8 â â25.3 Long Creek 14.7 15.6 81.5 â â46.0 Swannanoa River â2.0 17.5 89.7 â139.3 FIGURE 3.13â Inundation maps of Beaufort County, North Carolina, where the Tar-Pamlico River empties into Pamlico Sound. The Figure 3-13.eps 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. bitmap image 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 â¢ Elevation for the Nation. The North Carolina potential error in determination of the flood boundary case study demonstrates the sensitivity of flood studies and, thus, the flood risk. and floodplain boundary determinations to the resolu- tion and accuracy of topographic data. Clearly, the stan- CONCLUSIONS dard practice of using the best available elevation data does not meet the needs of FEMAâs floodplain map- It is neither trivial nor inexpensive to accurately ping program. As concluded by the National Research measure and monitor the elevation of land, water, and Council (NRC, 2007), a seamless, high-resolution, structures across a vast geographic area. However, the high-accuracy topographic dataset is needed nation- committeeâs analysis shows that the accuracy of eleva- wide to support floodplain mapping. The governance tion data has an enormous impact on the accuracy of and implementation of Elevation for the Nation is flood maps. Ensuring that future flood studies are currently being considered (along with similar initia- based on the most accurate and consistent foundation tives for nationwide imagery, transportation, and parcel possible requires (1) continuation of a suite of agency data) by the National Geospatial Advisory Committee. elevation programs and (2) acquisition of accurate, Elevation for the Nation would rely on nationwide high-resolution elevation data. Key elements of this availability of high-accuracy control monumentation foundation include the National Height Moderniza- provided by national height modernization. tion program, VDatum, and improved measurement of terrain and of streamflow and storm surge during flood Recommendation. FEMA should increase collabora- events. Major efforts include the following: tion with the USGS and state and local government
ELEVATION AND HEIGHT DATA 39 agencies to acquire high-resolution, high-accuracy elevations, measured and monitored through NOAAâs topographic data throughout the nation. NWLON program, provide essential information for FEMAâs coastal flood maps. The information provided â¢ National Water Information System. Stream gage by NWLON tide gages is also critical to the develop- data, available through the USGS National Water ment of VDatum, which in turn is needed to develop Information System, provide the necessary riverine seamless topographic-bathymetric surfaces for coastal discharge information required for flood studies. Flood flood studies. maps can be produced with much greater accuracy when a long and consistent history of stream gage Elevation and height data are analogous to the information, and therefore discharges during flooding, foundation of a skyscraper; even if the engineering is available. design and construction are flawless, the entire build- â¢ USGS Storm Surge Network. The USGS cur- ing is at risk of failure if the foundation is inadequate. rently deploys short-duration storm surge gages prior It would be wise to lay a strong foundation before to expected landfall of hurricanes. These gages are a investing additional time, effort, and money in fur- considerable improvement over post-storm watermark ther construction of a building. Yet we have not taken surveys, which are subject to significant errors and such an approach to elevation data as they pertain to uncertainties in the peak storm surge and wave condi- floodplain mapping. The technology and knowledge tions. Accurate storm surge measurements are critical to build and maintain a comprehensive and accurate for verifying coastal storm surge models using selected elevation measurement system have been available for historical storms (see Chapter 5). 15 to 20 years. The main hurdle to implementing such â¢ National Water Level Observation Network. a system nationwide has been cost. The relative costs Flood risk is increasing rapidly in coastal areas due and benefits of investing substantially in elevation data to a combination of land subsidence, sea level rise, to produce more accurate flood maps are discussed in population growth, and development. Coastal water Chapter 6.