Applications of Viewshed Analysis (Line-Of-Sight Operation, Raster-Based Viewshed Analysis, TIN-Based Viewshed Analysis, and Cumulative Viewshed) and Accuracy of Viewshed Analysis (Especially for GATE-Geospatial 2022)

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Diagram Shows Functionality of Viewshed Analysis

Line-Of-Sight Operation

The line-of-sight operation is the basis for viewshed analysis. The line of sight connects the viewpoint and the target. If any land, or any object on the land, rises above the line, then the target is invisible to the viewpoint. If no land or object blocks the view, then the target is visible to the viewpoint.

Viewshed analysis expands the line-of-sight operation to cover every possible cell or every possible TIN facet in the study area. Because viewshed analysis can be a time-consuming operation, various algorithms have been developed for computing viewsheds

Figure Shows Visible and Invisible Portion

In figure, a sightline connects two points on a TIN in (a) . The vertical profile of the sightline is depicted in (b) . In both diagrams, the visible portion is shown in white and the invisible portion in black.

Raster-Based Viewshed Analysis

Deriving a viewshed from an elevation raster follows a series of steps. First a sightline is set up between the viewpoint and a target location. Second, a set of intermediate points is derived along the sightline. Typically, these intermediate points are chosen from the intersections between the sightline and the grid lines of the elevation raster, disregarding areas inside the grid cells. Third, the elevations of the intermediate points are estimated. Finally, the computing algorithm examines the elevations of the intermediate points and determines if the target is visible or not.

TIN-Based Viewshed Analysis

Deriving viewshed from a TIN is not as well defined as from an elevation raster. Different rules can be applied. The first rule determines whether a TIN triangle can be divided into visible and invisible parts or whether an entire triangle can be defined as either visible or invisible. The latter is simpler than the former in computer processing time. Assuming that an entire triangle is to be either visible or invisible, the second rule determines whether the visibility is to be based on one, two, or all three points that make up the triangle or the center point of the triangle. The one-point rule is not as stringent as the two-or three-point rule.

Cumulative Viewshed

The output of a viewshed analysis, using either an elevation raster or a TIN as the data source, is a binary map showing visible and not-visible area. Given one viewpoint, a viewshed map has the value of 1 for visible and 0 for not visible. Given two or more viewpoints, a viewshed map becomes a cumulative viewshed map. Two options are common for presenting a cumulative viewshed map. The first option uses counting operations. For example, a cumulative viewshed map based on two viewpoints has three possible values: 2 for visible from both points, 1 for visible from one point and 0 for not visible.

Accuracy of Viewshed Analysis

The accuracy of viewshed analysis depends on the accuracy of the surface data, the data model, and the rule for judging visibility. According to Maloy and Dean, the average level of agreement is only slightly higher than 50 percent between GIS-predicted raster-based viewsheds and field-surveyed viewsheds. A more recent study finds the level of agreement ranging between 66 and 85 percent, depending on the GIS software and the DEM resolution.

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