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Chapter 2. Literature and Research
Viewshed Analysis
For wireless telecommunications providers, a GIS viewshed analysis has great potential to help plan network extensions by siting transmitter towers. A viewshed is the result of a function that determines which areas of a map can be seen from a given point, line, or area. Wireless telcoms provide telecommunications access by using portions of the radio spectrum to transmit data. If the wavelength used is a Line-Of-Sight frequency,then a viewshed should show the regions that can receive a signal from a potential tower site. So, given the LOS properties of the wavelength, a viewshed would show the zones able to receive data, called a communications viewshed or ‘commshed’. From that information, further viewshed analyses would aid in creating a chain of towers to cover a desired region.The viewshed algorithm does provide sufficient accuracy for communication coverage at smaller scales, such as a county, when the data being collected are of a general nature, i.e. total population covered or percentage of businesses covered. This is true because the resultant viewshed grid is at a finer resolution than the census data with which it is combined, thus making the census blockgroup the limiting factor for data accuracy, not the viewshed. However, when the scale is increased to the size of a typical communication cell (a 1-3 km radius from a transmitter) and the data gathered become more specific (can this particular home or business receive a signal?), the viewshed grid size becomes the limiting factor because data are needed on specific buildings which are smaller than a grid cell of the viewshed. Therefore, the algorithm and its resulting grid must be examined more closely to determine if the inherent errors are still small enough to allow for accurate prediction of a commshed. To create a viewshed the algorithm takes a user-defined viewing point (a potential tower site for a commshed) and a Digital Elevation Model (DEM) and computes which cells of the DEM can be seen from the viewing point and which cells cannot be seen. The viewshed is then displayed as a grid of cells coded with 1’s for visible cells and 0’s for non-visible cells. In order to produce this grid, the algorithm must proceed through several steps. Once the viewing point has been defined, the algorithm determines lines of sight to the cells adjacent to the viewing point, or origin cell. These lines of sight are represented as turquoise lines in Figure 1.1 and the adjacent cells or horizon cells are represented by the red box. Next, the algorithm checks the visibility of the adjacent cells by comparing the elevations of these cells to the elevation of the viewing point cell. If the elevation of a target cell is equal to or greater than the origin cell then it is considered in view and coded with a 1. The elevation angle between the origin and the target cell is then stored. The view horizon is expanded to the next set of neighboring cells.Lines of sight are computed to these cells, shown as yellow lines in Figure 1.1. (Note:not all possible lines of sight are represented in the diagram) To determine visibility for these cells, two angles are computed and compared. First, an elevation is inferred at the point where the line of sight intersects the previous view horizon (where yellow lines intersect red lines in Figure 1.1) and the angle between the origin point and the inferred
elevation point is then recorded. The second angle computed is between the origin point and the target point for the green box horizon. If the second angle is larger than the first angle, then the target cell is coded as visible and the larger second angle is stored in place of the previous horizon angle. If the first angle is larger than the second, then the target cell is coded as non-visible. The view horizon is again expanded, elevations are inferred,angles are computed and the largest is stored to compute visibility. This process is repeated until all cells of the grid are coded (Clayton Crawford, ESRI)
Acknowledgments
Chapter 1: Introduction
Chapter 2: Literature and Research
Chapter 3: Methodology
Chapter 4: Results and Analysis
Chapter 5: Conclusions
Bibliography
Appendix A
