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sectional or water surface profiles the headstakes for which elevations have been determined are used as benchmarks. The instrument is set up at each cross section and a backsight taken on the headstake to deter- mine HI. When the height of instrument has been determined, elevations are measured at various distances across the transect (foresights). There is no way to check the foresights, other than to rerun the profile; therefore, extreme care must be taken to prevent mistakes in reading and recording the rod readings and.in calculating the elevations at all stations. Figure A-10 shows a typical instrument setup for obtaining the water surface elevation and cross sectional profile of transect 0+00 of figures A-5 and A-8. Note that the precision of ground elevations across the transect is to the nearest 0.1 ft, but the water surface elevation is measured to 0.01 foot. A typical set of field notes for the profile is shown in Figure A-11. Note that the field notes for each profile contains a description of*the substrate as provided by the rodman as he traverses the cross section. MEASURING DISTANCES Distances are measured at several times during-a field survey; dis- tances between headstakes, thalweg distances, and distances across the profiles of the cross sections. Distance measurements are also used for locating and re-locating the benchmark or heads-takes. The most commonly used instruments for measuring distance are the tape, stadia, and elec- tronic distance meters (EDM). Tap in Taping is the most commonly used technique for measuring distance. Several types of tapes are available, but the steel surveyor's and the tagline are the most popular. The surveyor's tape is graduated in one- foot increments, with the last foot subdivided into tenths. Taglines may be small cable, cotton rope, or polypropylene, and are usually marked in one-foot increments, with special markings at all 10- and 100- foot marks. The only constraint in selecting a tape., besides personal preference, is that the tape should be incremented in the same units as the level rod. If the rod is metric, use a metric tape; avoid tapes marked in feet and inches. If measurement units are mixed, the field notes will be subject to a great deal of conversion, which is a poten- tial source of error. The distance to be measured is the horizontal distance between objects, whether headstakes, transacts or positions on a transect. One advantage of using a tape is that it is possible to measure distances along a curve. This is particularly useful when measuring between tran- sects along a meandering stream. This procedure introduces some error, and can be eliminated by placing transects close enough that the dis- tance between can be approximated by a straight line. Greater errors are introduced by stringing the tape up, over, and around obstructions 83