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Visual Impacts

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).

Perspective views of the road cut through Sideling Hill in western Maryland. The top image is derived from the NED and the bottom image is derived from the SRTM data (colors range from blue for the lowest elevations to red for the highest elevations within the scene). The viewing direction is toward the southeast. The arrow indicates the location of the cut through the prominent ridge. Without any adjacent terrain features to block the view, the road cut can be seen from many locations over long distances. Perspective views are useful to qualitatively portray the visual impacts of topographic surface changes.

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.

Viewshed for the Sideling Hill road cut in western Maryland. The viewshed (orange) and the topographic change polygon and its centroid (yellow) are overlaid on SRTM shaded relief. The circle, with a radius of 12.5 km, shows the extent of the viewshed calculations. The area shown is about 32.7 kilometers east-west by 26.4 kilometers north-south.

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.

Viewshed for a quarry located near San Juan Capistrano, California. The viewshed (orange) and the topographic change polygon and its centroid (yellow) are overlaid on SRTM shaded relief. The area shown is about 32.7 kilometers east-west by 26.4 kilometers north-south.

Viewshed for a mine located near Worthington, Pennsylvania. The viewshed (orange) and the topographic change polygon and its centroid (yellow) are overlaid on SRTM shaded relief. The area shown is about 32.7 kilometers east-west by 26.4 kilometers north-south.

The figure below shows a comparison between the Maryland road cut and three other features in terms of their viewsheds.

Comparison of the viewsheds for the Sideling Hill road cut in western Maryland and three large mine features. The planimetric area of the road cut is very small compared to the mines, but its topographic setting results in a very large viewshed area. Consequently, the viewshed-area-to-change-area ratio for the road cut is much higher than the mine ratios. The size of each area shown is about 32.7 kilometers east-west by 26.4 kilometers north-south.

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.

Area, volume, and viewshed information for a comparison among several topographic 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.

Combined viewshed of 24 topographic change features in Webster County, West Virginia. The viewshed (orange), county boundary (black outline), and the topographic change polygons (yellow) are overlaid on SRTM shaded relief. The combined viewshed is more than 60 times the area of the change polygons. The area shown is about 52.7 kilometers east-west by 58.1 kilometers north-south.