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detect errors evolving from instrument setup. Another point to keep in mind is that error reduction can be significant if backsights and fore- sights are kept about the same length. Figures A-6 and A-8 show a typical headstake level loop performed on the hypothetical stream reach presented in the section on transect placement. In Figures A-6 and A-8, backsights are indicated by plus signs (++++) while foresights are indicated by minus signs (----). The rod reading for each sight is written directly above the line of sight. Elevations. of headstakes, turning points, and instrument heights are given for each location. Figure A-6 shows the forward (usually upstream) survey of headstake elevations through the study reach. Field notes for the forward survey are shown in Figure A-7. The left hand sheets of surveying field note- books contain 6 columns. Five columns are needed for leveling notes. From left to right, the column headings are station (STA), backlight (BS), instrument height (HI), foresight (FS), and elevation (Elev.). Elevations should be calculated for each headstake or turning point as the survey proceeds, to facilitate finding errors on the return survey. Figure A-8 shows instrument setups, rod readings, and elevations for the return leg of the level loop. It is not unusual for the return leg to be a bit more organized than the forward leg, as the crew becomes more familiar with the study area. Figure A-9 shows the field notes for 0_1 the return survey of the study area. Note that the field notes in Figures A-7 and A-9 contain the same information as the schematics in Figures A-6 and A-8. It is somewhat confusing to the beginner to interpret two readings on the same line, such as the entries for turning points. This makes it especially impor- tant to calculate elevations as the survey proceeds. Simply remember that the elevation of a turning point must be calculated before a back- sight on the turning point can be used to calculate the new HI. Having completed the level loop, we may now determine whether or, not we have "closed" within the allowable range of error. Using equa- tion 20 (page _), the allowable error of closure is: Allowable error = 0_05 X2$0 = 0.018 (Z1) The error of closure at the benchmark for the sample level loop was 100.00 - 99.99 = 0.01 ft. Therefore, closure was obtained within allow- able limits for third order precision. Profile Leveling Profile leveling requires the measurement of both horizontal distances and vertical elevations. When measuring either the cross- J -7,?