The National Elevation Dataset (NED), maintained by the United States Geological Survey (USGS), is a collection of multiple datasets using different collection techniques that form digital elevation models(DEMs). The various techniques used to collect the data for the DEMs include electronic image correlation (Gestalt Photo Mapper (GPM) instrument), manual profiling (MP) on stereoplotters, contour-to-grid interpolation (CTOG), and an improved contour-to-grid interpolation known as “LineTrace+” (LT4X).
To verify the accuracy of this combined information we need to look at both the absolute and relative vertical accuracy. We first have to take the reference control point data set and partition it into subsets according to the production method of the quadrangle on which each point was located. The MP and GPM are less accurate than the CTOG and LT4X DEMs.
To find the relative vertical accuracy, we use the equation RV = |delta-ref - delta-NED| where delta-ref is the absolute value of the reference elevation distance and delta-NED is the absolute value of the NED elevation difference. The overall finding of the RV is that it is accurate to at most 22.07 meters off.
As new technology for data collection and higher-accuracy data become more abundant, the NED will become more accurate over time.
Gesch, D., Oimoen, M., Greenlee, S., Nelson, C., Steuck, M., & Tyler, D. (2002). The national elevation dataset. Photogrammetric engineering and remote sensing, 68(1), 5-32.
It's interesting how most of the geodetic control points are located East and in particularly mountainous states such as Colorado or Virginia. However, there doesn't seem to be that much variance in point density in Texas despite the fact that half the state is predominantly low lands and the other half mostly hilly.
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