Significant Topographic Changes in the United States
When contrasting the general nature of anthropogenic processes versus other geomorphic agents, the visual impact of human activity generally is greater than that of natural processes.
With the ability to detect specific areas of topographic surface change from multitemporal elevation datasets, and the analysis and visualization capabilities available in GIS software packages, progress is being made on better understanding the visual effects of significant landscape disturbances. Perspective views derived from pre- and post-event elevation data are a simple qualitative method of presenting the visual effects of a vertical land transformation (see figure below).
Viewshed, or intervisibility, analysis also is a common GIS technique used to quantify spatially the nature of how terrain may be perceived by an observer on the ground. A viewshed identifies the areas on the landscape that can be seen from a specific location, or stated conversely, the areas on the landscape from which a specific location can be seen. For assessing the effects of a topographic surface change, the relative size of the viewshed of the disturbed area is one way to quantify the impact of the earth-moving operations. For instance, a large amount of material may be displaced during surface mining, but if the operation occurs in a rugged high-relief area, the viewshed for the disturbed area may be relatively small (the mine area can only be seen by observers close to it because of adjacent terrain). Alternatively, if surface mining occurs on an isolated upland area surrounded by flat ground, its viewshed can be large, even if the actual amount of displaced material is relatively small. The examples presented below demonstrate how the visual impacts of human geomorphic activities detected with multitemporal geospatial data can be quantitatively assessed.
The figure above shows the viewshed for the Sideling Hill road cut in western Maryland. For computational efficiency, the viewshed was limited to a radius of 12.5 kilometers from a point at the centroid of the topographic change polygon, with the point having been assigned the maximum SRTM elevation within the polygon. It has been shown that using representative terrain points for viewshed calculation greatly reduces computation time without any significant reduction in visibility information. The planimetric area of the cut itself is relatively small at 0.08 square kilometers, but because of its location on a very prominent topographic feature in the area the cut can be seen from many locations over long distances. One way to characterize the magnitude of an individual topographic change feature (cut or fill) is to compare its planimetric area to the volume of displaced material. Such a comparison will indicate the relative shallowness or steepness of the vertical land transformation. The volume-to-area ratio for the Sideling Hill road cut is 43.22, which is slightly higher than the average ratio of 29.73 for all features in the topographic change inventory. A more useful metric may be the viewshed-area-to-change-area ratio, especially for quantifying and comparing the visual impacts of specific topographic change features. The viewshed-area-to-change-area ratio for the Sideling Hill road cut is 1242.0, which indicates that the viewshed is very large relative to the area of the topographic change polygon.
The following two figures show the viewsheds for two features that have about the same planimetric area and volume as the Sideling Hill road cut. Note that in each case the viewshed-area-to-change-area ratio is much less than the ratio for the Maryland road cut.
The figure below shows a comparison between the Maryland road cut and three other features in terms of their viewsheds.
These three features are the largest mines by area in each of the following ecoregions: Northern Lakes and Forests (upper right on above figure), Wyoming Basin (lower left), and Northwestern Great Plains (lower right). In each case, the area and volume of the mine greatly exceed that of the road cut, but the viewshed is smaller for the mine. Also, the viewshed-area-to-change-area ratios for the three mines are roughly the same, but each is significantly less than the ratio value for the road cut. These ratios illustrate their usefulness to quantify and compare the visual impacts of individual topographic change features.
The table below lists further information for the topographic change features shown in the figures above. Note that the volume-to-area ratio provides little, if any, discrimination among the features, whereas the viewshed-area-to-change-area ratio clearly distinguishes the variable visual impact of the change features.
The next figure illustrates how the combined viewsheds of a group of topographic change features can be used to assess the visual impacts over a larger area. Webster County, West Virginia contains 24 topographic change polygons for a total of 1.95 square kilometers (0.14 percent of the county). The combined viewsheds for these 24 features cover 118.71 square kilometers, which is more than 60 times the area of the change polygons. Most, but not all, of the combined viewshed area falls within the county, with 98.93 square kilometers covered (6.84 percent of the county) that represent an area more than 50 times the area of the change polygons. The remainder of the combined viewshed area falls into an adjacent county, which is an example of the concept of externality that is important in land use planning. A viewshed of any topographic change feature could be considered to be an expression of externality, but in this case the effects actually fall into a neighboring jurisdiction.