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):

  1. 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.

  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 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.

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.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement