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4 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS The system of vertical control corresponds to what is sometimes described as "reciprocal leveling." This method maintains balanced ele- vations throughout the <lOurse of the survey in moving from one transit setup to another. A long sight and a short sight are taken from each of two successive hubs (stakes over which transit is set). The short sight consists of measuring up from the hub to the level of the eyepiece of the instrument, using either a level rod, an engineer's folding rule, or a tape. The meas- urement can 'be made within 0.01 foot and is therefore equal in accuracy tJ other observa- tions. The method of "reciprocal leveling" is equivalent to making a "peg test" between each of two successive hubs. The differences in elevation obtaiued (when averaged) are there- fore a eontinuous record of the error in adjust- ment of the telescope level. If the differences are plotted against the (doubled) distances be- tween hubs and an average line drawn, the elevation correction for any distance thus deter- mined may be used to adjust the elevations on side shots. If the error is over 0.03 foot per 100 feet, the instrument should be adjusted. The field notes shown on figure 1 illustrate the start of a survey using the prescribed method of vertical control. The differences in the eleva- tion of H.I.'s represent random, not instru- mental, errors. Figure 2 is a replica of the same set of notes, with an instrumental error of 0.03 foot per 100 feet in the rod reading (rod read- ings are too low). Note that despite the instru- mental error, the elevations of the H.I.'s and of the hubs used under each transit setup are exactly the same as in the first set of figure 1. The method of determining the corrections to elevations is illustrated on figure 3. In the notes, the H.I. determined from the preceding hub is always entered first, then the H.I. com- puted from the hub on which the instrument is set. The second H.I. is subtracted from the first, and the difference plotted against the sum of the two distances read between the hubs, as on figure 3. An average straight line is drawn through the plotted points, starting from the origin. [N OTE.- The line should go through the origin unless a systematic error is being made in measuring up from the hub.] Correc- tions based on this line are applied to elevations of only the side shots, using the algebraic sign as determined from the correction curve. Note that these balanced elevations agree with cor- responding elevations of the notes of fignre 1. Elevations of hubs, reference marks, and high-water marks are read to hundredths of a foot; elevations of cross sections are generally read to tenths of a foot, except those of dam crests, culverts, and paved highways, where hundredths are used. Stadia readings with ver- tical angles should not be used for determining elevations, except in unusual cases for cross sec- tions. If used, the adjustment of the vertical circle should first be checked. Where the rod held on high-water marks or other features is above or below the horizontal line of sight, or where a reading of the hori- zontal crosshair is obstructed, time may be saved with no appreciable loss of accuracy by use of the "interval" or "stepping" method. Whole or half stadia intervals may be used, for as many as 3 interva'ls. By holding the number of intervals to a maximum of 3, the error from this source will be a maximum of 0.002 foot vertically per 100 feet of horizontal distance. The method is usually limited to side shots, but with e,,'treme care it may, if necessary, be used between hubs. Where a small fall in water surface is in- volved, every effort should be made to keep the instrument in good adjustment and to adjust the elevations of high-water marks. If the area covered by the survey is small, and all shots are made from one instrument setup, no evidence of instrument error is available; a peg test should then be made and shown in the notes, or a peg test made on the same day should be referred to. An alteruati ve would be to use a minimum of two hubs on each survey, so that the notes would automatically contain a test of the instrument. . Peg test Establish two points, A and B, near ground level, 200-300 feet apart. The test may be run between these points or stakes in either of two ways. .