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Holding the Rod Normally, the duties of the rodman are a relatively simple routine, and border on being boring. However, when collecting data for hydraulic simulation the rodman's life is more exciting. Basically, the rodman must keep the rod plumb over the point to be measured. This task of handling a long rod in three feet of fast water is easier said than done. The.rodman's job can be made easier by equipping the rod with a rod level. A rod level is a small bull's-eye spirit level, mounted on an L- shaped bracket which can be attached to the rod. When the bull's eye bubble is centered, the rod is plumb in both directions. This is especially important when dipping the water surface. In the absence of a rod level, the levelman makes certain the rod is plumb in a lateral direction by its coincidence with the vertical cross hair. The rodman then rocks the rod forward and backward, through the plumb line. The levelman watches through the telescope, noting the minimum rod reading. The minimum rod reading will occur when the rod is stan `-nid g plumb. As a related issue, the rodman must be aware of tree branches which tend to deflect the top of the rod. This is particularly true when the long fiberglass rods are used. They may be perfectly plumb at the rodman's level, but bowed away from plumb up where the readings are being made. This is a point to keep in mind-when placing headstakes. Errors can be reduced significantly by avoiding areas of low canopy. If these areas cannot be.avoided, the rodman may have to serve as a part- time tree pruner. LEVELING Two different types of leveling are commonly used for collection of hydraulic simulation data. Differential leveling is the process of de- termining the difference in elevation between two points. Differential leveling is used for determining headstake and water surface elevations. Profile leveling, the process of determining the elevation of points at measured distances along a selected line, is used for measuring the cross section profiles, and determining the water surface profile. Several definitions are commonly used in all types of leveling: 1. Backsight A backsight (BS) is a rod reading taken on a point of known elevation. It is the vertical distance between the line of sight and the point of known elevation on which the rod is held. The line of sight is always (except in extremely unusual cases) higher than the benchmark or turning point. Therefore, the algebraic sign of the backsight rod reading is -7 5