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along the bank. One may avoid this problem by setting up the study area at a fairly low flow, so that the tape can be stretched along the streambed and not along the bank. Some error may be introduced by sag in a tape when stretched across a transect. In many cases, the error is small enough that it can be neglected. However, for long transects, the magnitude of the sag error may be large enough to warrant correction. The correction factor for sag can be calculated by: 3 G 24 pT where, C = Difference between length of curve and straight line from one support L = unsupported length of tape, in feet W = weight of tape, in pounds per foot P = pull on tape, in pounds (due to sag) to next, in feet (2Z) The correction factor, C, is always negative and is added algebra- ically to L to determine the corrected total distance across the channel. Errors due to sag are systematic in that they always cause the recorded distance to be greater than the true distance. A correction for sag is necessary only if the magnitude of the error is significant to the level of precision with which the distance measurements are made. For example, a 250-foot cable weighing 0.032 pounds per foot, with a pull of 500 pounds, would have a correction of - 3-33 feet. This would equate to an incremental adjustment of 0.04 feet every 10 feet. Thus, with a precision requirement of measuring to the nearest foot, the correction factor is negligible. However, if only 50 pounds of pull were applied, the. correction factor would be -13.3 feet, or about 0.5 feet per 10 feet. In this case, correction would be justi- fied. It should be obvious that one way of reducing errors due to sag would be to use a tightener to apply a large amount of pull to the tag line or tape. Using a tightener, such as a winch or come-along, it may be possible to tape across channels 500 to 1000 feet wide (the upper limit to successful taping has not been established). Without benefit of such equipment, the upper limit for taping across channels is about 400 feet. Use of Stadia Horizontal, straight line distances can be measured directly with many levels. These levels contain two small horizontal cross hairs above and below the main horizontal cross hair. The distance between the telescope and rod is found by subtracting the rod reading for the lower stadia cross hair from that of the upper stadia hair and multi- plying by a constant (usually 100). Figure A-12 shows a view through the telescope on a stadia rod 93 feet away. 56