NSDI framework data represent "the most common data themes [that] users need" (FGDC, 1997, p. 3), including geodetic control, orthoimagery, elevation, hydrography, transportation, governmental unit boundaries, and cadastral reference information. Some themes, like transportation and governmental units, represent things that have well-defined edges. In this sense we can think of things like roads and political boundaries as discrete phenomena. The vector approach to geographic representation is well suited to digitizing discrete phenomena. Line features do a good job of representing roads, for example, and polygons are useful approximations of boundaries.
As you recall from Chapter 1, however, one of the distinguishing properties of the Earth's surface is that it is continuous. Some phenomena distributed across the surface are continuous too. Terrain elevations, gravity, magnetic declination and surface air temperature can be measured practically everywhere. For many purposes, raster data are best suited to representing continuous phenomena.
An implication of continuity is that there is an infinite number of locations at which phenomena can be measured. It is not possible, obviously, to take an infinite number of measurements. Even if it were, the mass of data produced would not be usable. The solution, of course, is to collect a sample of measurements, and to estimate attribute values for locations that are left unmeasured. Chapter 7 also considers how missing elevations in a raster grid can be estimated from existing elevations, using a procedure called interpolation. The inverse distance weighted interpolation procedure relies upon another fundamental property of geographic data, spatial dependence.
The chapter concludes by investigating the characteristics and current status of the hydrography, transportation, governmental units, and cadastral themes. You had the opportunity to access, download, and open several of the data themes using viewers provided by USGS as part of its National Map initiative. In general, you should have found that although neither the NSDI or National Map visions have been fully realized, substantial elements of each is in place. Further progress depends on the American public's continuing commitment to public data, and to the political will of our representatives in government.