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the output will automatically default to the user-defined cell size. For example, ifroads have an influence distance of 15m and a cell size of 30m is used, the cellsize will override the system and the road influence distance will be 30m. For thetutorial example, we will use a cell size of 30 which corresponds with the scale ofthe input data. You can experiment using different cell sizes to get an idea of howthe resolution and file size correspondingly change.f. Specify the Overlay Function. This is where to specify the map algebra functionthat you want used when combining all of the output risk element groups. A riskelement group means all of the risk elements created by a single input layer (e.g.roads, towns, quarries). Later you will be given the option of specifying theoverlay function within a risk element group (e.g. types of roads). For the tutorialexample, we will use the default SUM function, which is the more commonlyused function, since it aggregates all risk element intensity values.MAXIMUM – Takes the maximum grid value for each cell in allcomputed intensity layers. For example, if cell values betweenlayers are 35, 78, and 21, the final cell number would be 78.SUM - Takes the sum of the grid values for each cell in allcomputed intensity layers. For example, if cell values betweenlayers are 35, 78, and 21, the final cell number would be 134.Other less used overlay functions available for use include:MEAN: Takes the arithmetic average of the values between input rasters on acell-by-cell basis. The mean provides a measure of the center of the distribution ofthe values.MINIMUM - Takes the minimum grid value for each cell in all computedintensity layers. For example, if cell values between layers are 35, 78, and 21, thefinal cell number would be 21. This is not a common option to choose since avalue of 0 (no risk) in any location in any risk element input will produce a valueof 0 in that location in the final output grid regardless of any of the other inputvalues. In other words in areas of no overlap between risk elements the riskassigned will be 0.TNC Protected Area Tools (PAT) Version 3.0The Nature Conservancy, August 200921