Viewsheds from Key Observation Points for El Camino Real de los Tejas National Historic Trail  (ELTE)

Visibility viewsheds incorporate influences of distance from observer, object size and limits of human visual acuity to define the degree of visibility as a probability between 1 - 0. Average visibility viewsheds represent the average visibility value across all visibility viewsheds, thus representing a middle scenario relative to maximum and minimum visibility viewsheds. Average Visibility viewsheds can be used as a potential resource conflict screening tools as it relates to the Great Plains Wind Energy Programmatic Environmental Impact Statement. Data includes binary and composite viewsheds, and average, maximum, minimum, and composite visibility viewsheds for the NPS unit.  Viewsheds have been derived using a 30m National Elevation Dataset (NED) digital elevation model. Additonal viewshed parameters: Observer Height (offset A) was set at 2 meters. A vertical development object height (offset B) was set at 110 meters, representing an average wind tower and associated blade height. 
	  
	  A binary viewshed (1 visible, 0 not visible) was created for the defined NPS Unit specific Key Observation Points (KOP). A composite viewshed is the visibility of multiple viewsheds combined into one. A visible value in a composite viewshed implies that across all the combined binary viewsheds (one per key observation pointacross the nps unit in this case), at a minimum at least one of the sample points is visible. On a cell by cell basis throughout the study area of interest the numbers of visible sample points are recorded in the composite viewshed. Composite viewsheds are a quick way to synthesize multiple viewsheds into one layer, thus giving an efficient and cursory overview of potential visual resource effects.
	  
	  To summarize visibility viewsheds across numerous viewsheds, (e.g. multiple viewsheds per high priority segment) three visibility scenario summary viewsheds have been derived:  1) A maximum visibility scenario is evaluated using a "Products" visibility viewshed, which represents the probability that all sample points are visible.  Maximum visibility viewsheds are derived by multiplying probability values per visibility viewshed.  2) A minimum visibility scenario is assessed using a "Fuzzy sum" visibility viewshed.  Minimum visibility viewsheds represent the probability that one sample point is visible, and is derived by calculating the fuzzy sum value across the probability values per visibility viewsheds.  3) Lastly an average visibility scenario is created from an "Average" visibility calculation.  Average visibility viewsheds represent the average visibility value across all visibility viewsheds, thus representing a middle scenario relative to the aforementioned maximum and minimum visibility viewsheds.  Equations for the maximum, average and minimum visibility viewsheds are defined below:  Maximum Visibility: Products Visibility =(p1*p2*pn...), Average Visibility: Average Visibility =((p1*p2*pn)/n), and Minimum Visibility: Fuzzy Sum Visibility =(1-((1-p1 )*(1-p2 )*(1-pn )* ...).
	  
	  Moving beyond a simplistic binary viewshed approach, visibility viewsheds define the degree of visibility as a probability between 1 - 0. Visibility viewsheds incorporate the influences of distance from observer, object size (solar energy towers, troughs, panels, etc.) and limits of human visual acuity to derive a fuzzy membership value. A fuzzy membership value is a probability of visibility ranging between 1 - 0, where a value of one implies that the object would be easily visible under most conditions and for most viewers, while a lower value represents reduced visibility. Visibility viewshed calculation is performed using the modified fuzzy viewshed equations (Ogburn D.E. 2006).  Visibility viewsheds have been defined using: a foreground distance (b1) of 1 km, a visual arc threshold value of 1 minute (limit of 20/20 vision) which is used in the object width multiplier calculation, and an object width value of 10 meters.

Tags: ahprc, areas-of-high-potential-for-resource-conflict, composite-viewshed, key-observation-points, renewable-energy, resource-condition-analysis, resource-conflict-analysis, viewsheds, visibility-viewsheds, wind-energy
Organization: Department of the Interior