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<br /> <br />-36.:. <br /> <br />Observer's errors in snow depth frolll aerial markers are almost <br />exclusively a result of misreading the marker from the passing aircraft. <br />)Jsually they are detected easily by comparison with adjacent courses or <br />markers unless the depth is relatively great, say more than about 8 feet. <br />Correctio:rt of the record usually is not justified unless photographs <br />were taken to verify the reading. ' <br /> <br />. <br /> <br />Data of "snow:on the ground" at climatologic statiOns, especially <br />that collected by non-technical cooperative observers, seem to contain <br />SOllle 'substantial errors. At least in part, the seeming discrepancies are <br />identified with severe and prolonged storms during which the total depth <br />of snow at a given point may change ,rapidly as new snow falls, as snow <br />drifts with the wind; and as the psck changes in density and texture. <br />,Observations made under conditions ao unstable may beaccura,te at the <br />moment, but not obviously consistent from one point to another. When <br />conditions are stable, however, "snow on theiground" can afford a fairly <br />consistent measure of the snowpack.' "Isochion" (depth-of-snow) maps and <br />profiles oan be constructed to suggest whether ,a particular set of data <br />is representative and reliable. <br /> <br />Defective equipment <br /> <br />Faulty or improper equipment may cause errors in snow-course surveys. <br />The tubular spring scales used geneJ!'allyfor weighing the tube and snow <br />core are not entirely dependableJ they should be checked periodically with <br />standard weights, and replaced when'necessary. " Le,ssconJreruently carried, <br />:there is anilable a dial-type scale that is highlyrelia,b16 and that may <br />be read to O.l ounce (0.1 inch water equivalent). In its common form, <br />this scale is suitable for snowpacks of less than about 30 inches water <br />aquiv!l1ent. A modified fol'JJL, available on special order, compensates <br />all or most of the empty-tube weight and extends the capacity to about <br />60 inches water equivalent. <br /> <br />Teeth of the snow-tube cutter ahoe may become bent, or dented and so <br />may differ from the diameter required for translating weight in ounces to <br />water equivalent in inches. Such damage to cutter teeth usually results <br />from earlier striking rock or boulders. If teeth are dull, the cutter may <br />not take full-diameter core frOlll lenses 'of ice or compact frozen snow. <br /> <br />Errors due to defective equipment usually are infrequent, systematio, <br />alld relatively small. For the most 'part theY are avoidable if equipment <br />is inspected periodioally. <br /> <br />I <br />