Topographic maps and sketch mapping . o Chestnut Hill is 3| miles onthe map. Now a line of sight from Wirts just passing overHill 712 will rise 712-558 or 154 feet, or a rise of 154 feet inone mile. Hence at a distance of S-2 miles the line of sight willrise in proportion 154x3j-=-l, or 540 feet, and its elevationwill be 558+540, or 1098 feet, which is higher than ChestnutHill (931) and the hill is therefore invisible. This method isillustrated in Fig. 36. Note that the distances A to C and Ato B need not be scaled in miles, but can be measured in anyconvenient unit. Indeed this problem can be
Topographic maps and sketch mapping . o Chestnut Hill is 3| miles onthe map. Now a line of sight from Wirts just passing overHill 712 will rise 712-558 or 154 feet, or a rise of 154 feet inone mile. Hence at a distance of S-2 miles the line of sight willrise in proportion 154x3j-=-l, or 540 feet, and its elevationwill be 558+540, or 1098 feet, which is higher than ChestnutHill (931) and the hill is therefore invisible. This method isillustrated in Fig. 36. Note that the distances A to C and Ato B need not be scaled in miles, but can be measured in anyconvenient unit. Indeed this problem can be solved mentallywith sufficient accuracy to answer the question. VISIBILITY PROBLEMS 65 It is obvious that Chestnut Hill would have been visibleif the line of sight at B had been 931 feet or lower. Anotherproblem which is solved in a similar manner is to determinethe height necessary for a tower at Wirts in order that wemay see Chestnut Hill. Graphically we simply draw a lineof sight back from B just touching C and find that it comes. .El. 712 ^31.;558 Fig. 36.—Intervisibility Computation. at D about 60 or 70 feet above A, giving the required heightas shown by the dotted line in Fig. 35. Mathematically, weknow that this line of sight must have a slope of 931 — 712,or 219 feet in 2\ miles (the distance from Hill 712 to Chest-nut Hill), hence between Hill 712 and Wirts it will drop219Xl-^2f, or 88 feet and its elevation at Wirts would be Horizontal Line - Tangent to levelline at 2 Miles 3 Miles 1 Miles
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectcartogr, bookyear1920
