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6.C. Production Artifacts

Artifacts due to the production method are apparent in some areas of GLOBE. While the magnitude of the artifacts in a local area are usually well within the estimated accuracy for the source, they may affect some applications of the DEM.

Some areas derived from DTED exhibit a striping artifact, most likely due to the production method of the DTED (see Section 5.A.i.a for more information). The artifact is very evident in the full resolution data, but remains noticeable even in the generalized 30 arc-second version. Generally, the pattern is more noticeable in low relief areas, while in higher relief areas it is masked by the actual terrain variation.

Another pattern seen in some areas derived from DTED is a blocky appearance, which is a reflection of the 1o tiling structure of the full resolution DTED from source data with certain characteristics (Section 5.A.i.a). These areas derived from contiguous DTED 1o x1o cells appear blocky because of vertical offsets among the tiles in the original full resolution DTED.

The artifacts in the DTED areas may be visible or obscured, depending on the method used to display the data. For instance, when viewing the DEMs as an image either in shades of gray or color, the artifacts may be subdued or hidden, depending on the number of shades or colors used. If the data are displayed as a shaded relief image, the appearance of the artifacts will vary depending on the direction of illumination, vertical exaggeration applied, and the scale of the display. Generally, none of the artifacts will be visible on a course-scale portrayal of the global data set.

Some production artifacts are also present in the areas derived from the vector sources. Small artificial mounds and depressions may be present in localized areas, particularly where steep topography is adjacent to relatively level areas, and the hypsography data are sparse. Additionally, a "stair step" (or terracing) effect may be seen in profiles of some areas. Here the transition between contour line elevations does not slope constantly across the area but instead is covered by a flat area with sharper changes in slope at the locations of the contour lines. Histograms show sharp peaks at elevations that are multiples of the source’s contour interval. This effect is common in DEMs produced by gridding of contour data, in which the interpolation process favors elevations at or near the contour values, thus leading to a greater frequency of those elevations. Every effort to reduce these effects has been made by careful selection of parameters for the interpolation process. However, some level of these conditions inevitably remain due to the nature of vector-to-raster surface generation.


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