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. . . GENERAL FIELD AND OFFICE PROCEDURES FOR INDIRECT DISCHARGE MEASUREMENTS 7 veying text readily available. Detailed steps in proper sequence are important because of the interrelation of the various adj ustments that might be required. The prescribed method of keeping field notes compenSates for constant instrumental errors. However, keeping errors within appropriate limits reduces the need for note corrections, reduces the chance for mistakes, and saves money. Horizontal control Begin horizontal control by referring the survey to magnetic north. After establishing zero azimuth, observe a distant point as a check point for use later in the surveyor in the future if the survey has to be recovered. Read stadia distance and azimuth for each surveyed point. Read allgles to the nearest minute of arc for all hubs and reference points, to the nearest 5 minutes for high-water marks and other side shots. When moving from one hub to another, read the stadia distance again from the second point to the first; take abacksight (for setting azimuth) on the preceding station either (1) with telescope plunged and upper plate clamped at the forward azimuth, or (2) with a telescope normal and upper plate clamped at the forward azimuth plus 1800. After the first setup, read the magnetic bearing at each successive setup as a check on the computed azimuth. Remem- ber that steeel bridges, powerlines, and other metal rbjects may affect the magnetic bearing. If these procedures are followed, there is ordi- narily no need for closure of the horizontal traverse. At times, however, surveys may cover large flooded areas, and the terram may be so rough that short distances between hubs and many transit points are needed. Under such conditions, the cumulative error in posi- tion may become large enough to require some supplementary means of avoiding large errors of horizontal closure. It may be necessary to use triangulation to establish firmly the principal traverse corners. Locate the site on a map and refer it to the gaging station and to roads, tributaries, or other landmarks in order to define the lOcation. Tie in and describe .the location of permanent or semipermanent marks so that the horizontal control can be recovered some years later, if necessary. Field notes An example of the recommended form of keeping field notes is shown on figures 1 and 4. A step-by-step explanation of the procedure covering both the horizontal and vertical controls, follows: A. Set transit over station 1 (a solidly set stake or the equivalent). 1. Clamp upper plate at zero; with lower plate unclamped, point telescope to maguetic north as indicated by compass needle. Clamp lower plate and loosen upper clamp. Angle readings will now represent azimuth from magnetic north. The azimuth of magnetic north as 0000' is recorded on line 1 (see sample notes, fig. 1). 2. Read azimuth, stadia distance, and rod on reference mark RP2 and record on line 2; eompute R.I. and record on line 4. 3. Measure distance from top of hub at station 1 to telescope horizontal axis as 5.14; record in parentheses (denoting reading not obtained by transit) and compute elevation of stationlas 20.08 (line 5) . 4. Read azimuth, stadia distances, and rod on all side shots; repeat reading on RP2 as check. 5. Read azimuth, stadia distance, and rod on station 2; tighten upper clamp on azimuth to station 2; loosen lower clamp; com- pute preliminary elevation of 11.31 for hub 2 (line 11). B. Set transit over station 2. 1. Check vernier reading to see that no slippage has occurred while moving and that reading checks azimuth from station 1 to 2. Plunge telescope and sight on station 1. (When Zeiss level or theodolite-type instruments are used, telescope cannot be plunged; azimuth at station 2 and succeeding stations is transferred by setting upper plate to read forward azimuth plus 1800, then backsighting on preceding station.) Tighten lower clamp and loosen upper clamp. Plunge telescope back to nor- mal position; read azimuth of magnetic north and record on line 14.