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6.B. Topographic Detail and Accuracy

Differences in topographic detail among the sources are evident in GLOBE. This change in level of topographic information is especially evident at the boundary between areas derived from DTED and DCW in regions of higher relief. The mosaicking techniques that were used resulted in a smoothing of the transition areas, but the change in detail between the two sources remains noticeable. Seams within data from "a single source" are sometimes apparent. Examples include DMA/USGS data in the Seward Peninsula of Alaska and DTED coverage near Cordoba and Mendoza, Argentina.

Even if the same topographic feature (ridge, stream valley, lake, etc.) is represented in the data derived from the two sources, the elevations across the feature may change somewhat abruptly due to the varying accuracy of the sources. Derived products (such as slope maps) for the source transition areas also emphasize the differences in topographic information derived from the varying sources.

Users are reminded that the accuracy levels described above are estimates, and that the accuracy for specific locations within the overall area derived from any one source can vary from the estimate. For instance, approximately half of the 1o x1o DTED cells (the production and distribution unit for full resolution DTED) have an absolute vertical accuracy worse than the product specification of plus or minus 30 meters at 90% confidence. Also, the actual accuracy for some areas derived from the vector contour sources may be better or worse than the estimate. When the map source had multiple contour intervals, the largest interval was used for a conservative estimate. In contrast, some areas may be worse than the estimate because no contour coverage was available for those specific locations.


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