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3 Elevation and Height Data
Pages 25-40

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From page 25...
... 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.
From page 26...
... Used with permission. bitmap image ent types of vertical datums -- ellipsoidal, orthometric, Orthometric Height Datums and tidal -- are relevant to flood studies.
From page 27...
... s ­ 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.
From page 28...
... 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 orthoflood risk. metric height datum used to reference the topographic surface.
From page 29...
... , 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.
From page 30...
... are typically used to determine water surface elevations
From page 31...
... bitmap image 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.
From page 32...
... 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.
From page 33...
... 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.
From page 34...
... have the lowest ellipsoid heights. The lighter blue areas indicate higher water surface elevations (163 bitmap image meters)
From page 35...
... It is level is rising faster through postglacial rebound than thebitmap image NOAA, .
From page 36...
... 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)
From page 37...
... FIGURE 3.12 Figure 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.
From page 38...
... However, the high-accuracy topographic dataset is needed nationcommittee's 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.
From page 39...
... 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.


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