dem users manual - asprs

Chapter 2: Vertical Datums
Chapter 2 explains why there are so many vertical datums used in the United States and why the “official”
North American Vertical Datum of 1988 (NAVD 88) is normally the only one that ought to be used for topographic
applications. It also explains the different tidal datums and how they are used. It is essential that DEM users
understand what the geoid is, the difference between ellipsoid heights and orthometric heights, and reasons why
DEM users should not combine data sets that are referenced to different datums. This is the only chapter with
relatively minor changes between the 1 st and 2 nd editions.
Chapter 3: Accuracy Standards and Guidelines
Chapter 3 explains the National Standard for Spatial Data Accuracy (NSSDA) and compares it with the
traditional National Map Accuracy Standard (NMAS) published in 1947, as well as the ASPRS Standard for Large
Scale Maps published in 1990 (ASPRS 90), both of which were replaced by the NSSDA. Chapter 3 also explains
the relevant standards for hydrographic surveys and the new National Ocean Service “Hydrographic Surveys
Specifications and Deliverables.” This 2 nd edition also includes the new Appendix A, Aerial Mapping and
Surveying, to FEMA’s “Guidelines and Specifications for Flood Hazard Mapping Partners;” the National Digital
Elevation Program (NDEP) “Guidelines for Digital Elevation Data;” and the “ASPRS Guidelines, Vertical Accuracy
Reporting for Lidar Data,” the covers of which are shown in Figures 2a, 2b and 2c. If you don’t know the meanings
of Fundamental Vertical Accuracy (FVA), Consolidated Vertical Accuracy (CVA) and Supplemental Vertical
Accuracy (SVA), Chapter 3 explains what you need to know; and Chapter 12 (DEM Quality Assessment) provides
actual examples of FVA, CVA and SVA statistics and reporting.
Figure 2a. New FEMA Guidelines Figure 2b. New NDEP Guidelines Figure 2c. New ASPRS Guidelines
Chapter 4: National Elevation Dataset
Chapter 4 explains the National Elevation Dataset (NED) which includes the best available standard DEMs
produced and/or distributed nationwide by USGS, comprising the elevation layer of The National Map. NED data
are available to the public at minimal cost, and all users are encouraged to share their data sets with the NED which
places DEM data in the public domain. This 2 nd edition does a better job explaining the three NED resolution layers
available, and how such data can be obtained by the public. Samples of NED data are provided on the DVD
included with this 2 nd edition. Figure 3 helps explain procedures for obtaining NED data.
Chapter 5: Photogrammetry
Chapter 5 has been rewritten to incorporate the latest information from the 5 th Edition of the Manual of
Photogrammetry published by ASPRS. In addition to film cameras, it addresses the latest airborne digital systems
and satellite imagery. For production of digital elevation data, it addresses planning considerations, georeferencing
and aerotriangulation, photogrammetric data collection methods, post processing, quality control, data deliverables,
supporting technologies, calibration procedures, capabilities and limitations compared with competing and/or
complementary technologies, user applications, cost considerations, and technological advancements for the future.
MAPPS/ASPRS 2006 Fall Conference
November 6 – 10, 2006 * San Antonio, Texas