is 5.5 feet. The bench mark elevation, added to thebacksight reading, gives a instrument height of 105.5feet.NOTE: Since the backsight (BS) reading isadded to the elevation of the bench mark (BM) toobtain the instrument height, it is usually called aplus (+) sight.Once the height (HI) of the instrument isestablished, you can determine the elevation of anyTo determine the elevation of a point after theheight (HI) has been established, place the level rod onthe point in question and take a reading through thelevel. This sighting is called the foresight (FS) readingand is subtracted from the height to obtain the elevationof the point. The formula for determining elevation isas follows: HI - FS = ELFor example, as shown in figure 15-50, the height(HI) of the instrument is 105.5 feet. The foresight (FS)reading is 2.3 feet. The instrument height minus theforesight reading gives that point an elevation of 103.2point within the instrument range.feet.Figure 15-49.—Backsight reading to determine instrument height.Figure 15-50.—Foresight reading to determine elevation.15-29
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
