Agenda - 09-29-1999 - 9e - Laserfiche WebLink

Browse Search

APPENDIX II CALCULATING THE CONTEXTUAL RATING FOR A TARGET TRACT BY MEANS OF THREE BUFFER ZONES AROUND THE TRACT The procedure we suggest here is easy to calculate in ArcInfo and produces an intuitively under- standable rating for the context of a target tract of forest. THREE BUFFERS AROUND THE TARGET TRACT First, ArcInfo finds three buffer zones around a target tract. These three buffers extend outward 0.5, 1, and 2 miles from the target tract. Within each buffer, we consider all tracts of forest and non - forest (non - forest is a tract with a Maturity value of 0, as explained in Table 2). The IR for each tract is computed, the resulting numbers are summed, and the sum is divided by the total acreage in the buffer. The result is an average IR for each acre in each buffer. Notice that the IR for each tract in a buffer includes the acreage of the tract. When we divide by the total acreage in the buffer, we end up with an average IR for the buffer or an IR per acre in the buffer. To visualize what an average IR means, consider an extreme case. If a buffer were completely forested with undisturbed Hardwood Forest, with no bodies of water or inventoried natural areas, the average IR in the zone would equal about 6 (3 for Hardwood Forest, times 2 for no disturbance). TWO COMPLICATIONS There are two complications when ArcInfo calcu- lates an average IR in this way. First, ArcInfo can 24 quickly identify buffers around a target tract, but it cannot quickly identify concentric doughnut - shaped zones. A buffer with a narrower width is thus completely included within a buffer with a wider width. We actually want the information for concentric doughnut - shaped zones, rather than for buffers. This problem is easily solved. The average IR of a doughnut - shaped zone is quickly obtained by subtracting the values for a narrower buffer from the values for a ' wider buffer. For instance, the average IR for the zone from 1.0 to 2.0 miles from a target tract equals �y (total IR for 2 -mile -wide buffer) - (total IR for I -mile -wide buffer) (total area for 2- mile -wide buffer) - (total area for I -mile -wide buffer) There is a second complication. ArcInfo can easily determine the set of all polygons that overlap a buffer, but it cannot easily determine the set that are entirely included within a buffer. Consequently, in computing the average IR for any buffer, we have included tracts that overlap that buffer. The buffer "gets credit" for some forest outside the buffer. Tracts of forest that overlap two doughnut- shaped zones get "counted" in the nearer zone. This problem, in most cases, is not a large one. In examing the maps of Contextual Ratings for Orange County forests, we have not noticed distortions in ratings as a result of this complica- ton. From a biological point of view, this situa- tion is not unreasonable. Once an animal finds a patch of suitable habitat, it might easily move throughout it. Although more complicated calcu- lations could remove this problem, they do not seem justified on the basis of our present knowl- edge.