Elevation Data for Floodplain Mapping (2007) / Chapter Skim
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4 Remote Sensing Technologies for Floodplain Mapping
4 Remote Sensing Technologies for Floodplain Mapping
Pages 57-88
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From page 57... ...
one must be familiar with the categories of available mapping technology. This chapter is intended to provide an introduction to remote sensing technologies sufficient to understand the availability of adequate elevation data to address the floodplain management challenges faced by our nation.
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From page 58... ...
, and some basic concepts of geodesy are explained before delving into the applications of remote sensing technology to floodplain mapping. The mathematical model that is most often used to approximate the earth's shape is an oblate ellipsoid, a spheroid that has been slightly flattened at the north and south poles.
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From page 59... ...
Conversion from NGVD29 to NAVD88 can be accomplished using the NGS program, VERTCoN, except in regions of significant subsidence as discussed in Chapter 3. Digital elevation models (DEMs)
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From page 60... ...
All spatial coordinates are computed measurements; therefore accuracy itself can only be estimated, never known absolutely. The quantification of error and the language of accuracy assessment rely heavily on principles of statistics and probability.
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has 30-meter post spacing; the term is synonymous with grid spacing. It is not exactly the same as GSD in reference to elevation models.
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. • Technology X is capable of producing elevation data that meet 36.3-centimeter vertical accuracy at the 95 percent confidence level.
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From page 63... ...
This broad definition could be applied to all of the technologies discussed in this chapter; however it is used here to refer specifically to mapping performed using film or digital aerial photography. Products created from photogrammetry include the following: • Two-dimensional planimetric maps and three-dimensional feature datasets, • Elevation models, and • Digital orthophoto base maps.
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From page 64... ...
E L E VAT I O N D ATA F O R F L O O D P L A I N M A P P I N G FIGURE 4.2 Camera orientation and scale effects for vertical and oblique aerial photographs. SOURCE: Wolf and Dewitt, 2000.
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From page 65... ...
This application of technology, measuring the location of the camera focal point and the angular orientation of the focal plane at the time of exposure, is known as direct georeferencing. These measurements are included in the aerotriangulation, replacing ground control while increasing redundancy and adding statistical significance to the adjustment results.
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From page 66... ...
Traditionally, orthophotos are created using bare-earth elevation models; tops of buildings are not corrected to their true positions and, because of camera perspective, appear to lean away from the center of the photo. Rectification with a DSM, on the other hand, which includes building heights, produces a "true orthophoto" in which the rooftops are aligned correctly with building footprints (Figure 4.5)
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From page 67... ...
Digital orthophotos, whether true or conventional, make very useful base maps for geographic information systems (GIS) and have become very popular with local, state, and federal government agencies for a wide variety of purposes, from tax assessment, to urban and regional planning, resource management, and emergency response.
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From page 68... ...
In today's photogrammetric production environment, virtually all aerotriangulation, elevation, and feature extraction are performed in an all-digital, or soft copy, work flow. There is no development being done on aerial film cameras, and commercial manufacturers have discontinued their production as digital cameras mature and become more affordable.
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From page 69... ...
maps of roads, buildings, drainage features, utilities, and so forth; • Digital orthophoto base maps; and • Elevation data in a variety of forms: profile, cross section, contour, DEM, DTM, DSM, or triangulated irregular network (TIN)
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From page 70... ...
Distortions are caused by motion of the aircraft and are removed using direct georeferencing information collected with GPS-IMU sensors integrated with the camera system on the aircraft. SOURCE: EarthData International.
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From page 71... ...
Black and white base map imagery is often preferable if a lot of vector data need to be plotted as an overlay; depending on the nature of the land itself, it may be difficult to find colors for plotting the vector data that consistently stand out when overlaid on a color image. 4.2.4 Section Summary Photogrammetry is a mature technology that has benefited from decades of development and practical experience.
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From page 72... ...
Accurate georeferencing of lidar data requires careful mounting and calibration of the sensor in the aircraft; best practices dictate that calibration checks be conducted as part of every lidar project. The elevation model produced from each flight mission should also be checked against a distribution of ground control points with published ellipsoidal and orthometric heights in the appropriate mapping datum.
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From page 73... ...
More recently, advanced systems are implementing single-photon ranging techniques, whereby the distribution of target heights can be efficiently built up from very low power, very high repetition-rate laser pulses. Commercial mapping lidar systems are most often of the discrete-return type, recording up to five reflections per transmitted FIGURE 4.9 Multiple lidar echoes from a single transmitted pulse are returned from tops of trees and branches.
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From page 74... ...
Lidar has proved to be far less ambiguous; however, the problem of filtering nonground points out of the bare-earth elevation model remains with lidar data and must be addressed with further data processing, editing, and quality control. Images can also be created from lidar returns by recording the amount of energy, or intensity, reflected back from the object.
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From page 75... ...
A recent breakthrough in lidar mapping is the technology of lidargrammetry, a process for creating pseudostereo pairs of images from lidar intensity data. These images can be used in conventional soft copy photogrammetry systems to digitize linear features such as roads, buildings, edges of water bodies, and DTM breaklines very accurately in three dimensions.
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From page 76... ...
Lidar systems can also be mounted on tripods; vehicles or ships are used to map structures in urban or industrial environments or to monitor environmental parameters. Although these other applications are not discussed in this report, bathymetric lidar can make important contributions to coastal storm surge modeling and flood hazard mapping, and ground-based lidar systems can be used to survey bridges, culverts, and other structures of importance in hydraulic modeling.
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From page 77... ...
4.3.4 Section Summary Lidar is a powerful and cost-effective means for high-speed acquisition of three-dimensional point data to suit a wide variety of user requirements and is the most robust remote sensing technology for the creation of seamless statewide and nationwide elevation models. Like any remote sensing technology, human interaction is still required for the production of clean, bare-earth datasets and linear feature mapping.
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From page 78... ...
The attempts by NDEP, ASPRS, and FEMA to establish guidelines and specifications are a step in the right direction, but they do not go far enough. For example, the relationship between lidar point spacing and elevation model accuracy is complex and not easily quantified, particularly with rapidly changing technology that allows dense point spacings to be easily achieved and processed to bare-earth elevation models.
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Synthetic aperture radar (SAR) refers to a technique used to synthesize a very long antenna from the motion of the aircraft along the flight track.
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The range difference is determined from the phase difference between two coherent radar signal echoes using a technique called interferometry. Such SAR systems are referred to as interferometric synthetic aperture radar (IFSAR or InSAR)
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Rough surfaces appear bright; flat surfaces appear dark. Surfaces inclined toward the radar reflect more energy than surfaces inclined away from the radar and appear bright; surfaces inclined away from the radar reflect less energy and appear dark.
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From page 82... ...
These sloping features appear closer together in planimetric view, compressed or bunched, compared to their actual position; they will also appear bright due to strong backscatter. Slopes facing away will conversely be dark and expanded or stretched compared to their actual positions.
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Remote Sensing Technologies for Floodplain Mapping FIGURE 4.14 Foreshortening, layover, and shadow. The three-dimensional world is collapsed to two dimensions in conventional SAR imaging.
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From page 84... ...
The system has also been augmented with a profiling lidar that collects elevation data at nadir with 15- to 20-centimeter RMSE accuracy. The lidar data are used to calibrate the GeoSAR data and to support processing of bare-earth terrain models with observations of the ground surface beneath canopy.
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From page 85... ...
. Contractors performed the final data editing and DEM product generation to NGA specifications.
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From page 86... ...
A local statistical height error map can be generated from the phase correlation measurements and provides the user with a point-by-point estimate of vertical DEM accuracy.
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From page 87... ...
Geosynchronous systems pointing continuously at a site of interest could measure very small changes in the surface conditions on an hour-by-hour basis, applications of which are diverse and important but of little benefit to the development of seamless bare-earth elevation models for the nation. 4.5 CHAPTER SUMMARY The purpose of this chapter is to provide the fundamental concepts of remote sensing technologies for creation of elevation datasets and other base map products of interest to a national floodplain mapping program.
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From page 88... ...
Improvements in accuracy assessment and reporting are needed to fully characterize lidar-derived elevation datasets and to leverage them most effectively for a broad range of engineering and planning applications. IFSAR is a unique and important technology for the creation of elevation models on a global scale and in localized regions of the earth perpetually covered by clouds.
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