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Op <br />positive (+). There is only one backsight for each setup of <br />the instrument. Please note that the term "backsight" has <br />nothing to do with the direction in which the instrument is <br />pointed. <br />2. Freight of Instrument <br />The height of instrument (HI) is the elevation of the line of <br />sight when the instrument is level. It is found by adding the <br />backsight rod reading to the known elevation of the point on <br />which the back-sight was taken. Figure A-3 shows a backsight <br />on a benchmark to. determine height of instrument. <br />BS = 7.35 <br />HI = 107.35 - <br />B M =100.00 <br />Figure A-3: Determination of the height of instrument by taking a <br />backsight on a bench marls. <br />J <br />3. Foresight <br />A foresight (FS) is a rod r, <br />elevation of which is to be <br />the foresight is negative (, <br />HI to find the elevation of <br />foresight. on a headstake to <br />headstake. <br />wading taken on any point, the <br />determined. The algebraic sign of <br />-). The FS is subtracted from the <br />the point. Figure A-4 shows a <br />determine the elevation of the <br />r=S = 5.35 <br />HI =107.35 <br />ELEVATION =101.50 <br />Figure A-4. Oetermi^ation of an unknown leadStale ei?va-ion by taking <br />a foresight from a known height of ins1-runent.