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Show The location of the starting spots and the percentage of the path that slide are very important in pinpointing areas of potential trouble. There is a tendency to consider a slide path safe once it has avalanched, and this is often the case if the entire path slid. If, on the other hand, only part of the path avalanched, the remaining snow poses a serious threat that might be overlooked unless complete data are recorded. The height of the fracture line, vertical fall distance and the proportion of the total path that slid all contribute objectivity to avalanche si ze. Figure 10 also has space for entering avalanche control action that produced no avalanches. These are facts often not recorded, but left to memory. Since well- executed control action that produces no avalanche is an indication of stability, these data are too important to be left to memory alone, even on smal1 areas. One of the essential items of information about slab avalanches is the fracture line height. For on- site investigations, this can be measured with the tape or folding rule already described under Section k ( A8 a £ b). When inspected from a distance, the fracture line height must be estimated. One aid to accurate estimation is a reticle scale in binoculars, which provide a direct measure of objects if their distance is known. In the case of an active ski area where large- scale contour maps are available ( as they now are for several Class A areas), a table of distances from regular observation points can easily be prepared. Consult a local supplier of optical instruments for information about reticles. Surplus military binoculars sometimes come with built- in reticles graduated in mils'". A table can easily be constructed to give object size ( fracture line height) in terms of distance and mils. Another aid to estimating fracture lines is to mark an object of known size adjacent to the fracture zone which will remain visible all winter. Rock outcrops, lift towers, or other existing structures can be measured and a list of sizes compiled for ready reference. Even better is the installation of reference marks especially for measurement purposes. For instance, a pole with short cross- arms of known spacing ( say 1 meter) may be installed adjacent to the fracture zone of an active avalanche path. Comparison through binoculars of a fracture line with the cross- arm spacing * An artillery mil is 1/ 6^* 00 of a circle. An infantry mil is the angle subtended by 1 yard at 1000 yards distance. 100 artillery mils equal 98.2 infantry mils. Reticles come in both kinds. 32 |