![]() The instrument is set up at point 1 (it does not have to be exactly over the point).Ī short BS reading is taken on point A and a long FS reading on point B, Figure F-7 Points 1 and 2 are instrument locations and are marked with wooden stakes Points A and B are elevation points and are marked with stable pins. ![]() The points should be on an approximately straight line and placed at a uniform interval of 75 to 100 ft, Figure E-6. ft/ft) or angle above (+) or below (-) horizontal.Ī baseline which consists of four roughly evenly spaced points is established. ![]() If there is a collimation error, the two elevation differences will not be the same.īecause collimation error is a function of distance, the sight length for each reading is needed. The elevation difference between two points is determined using a short BS / long FS condition followed by a long BS / short FS condition. Check (1) Procedureīecause collimation error cancels if BS and FS distances are balanced, checking for it purposely uses unbalanced sight distances. A LoS that is inclined or depressed from horizontal, Figure F-5, introduces a rod reading error.Ĭollimation error is caused by the vertical position of the crosshairs: if they are too low then the LoS is inclined if they are too high, the LoS is depressed. A non-horizontal LoS means the instrument has a collimation error. On an adjusted instrument which is correctly set up, the LoS should be horizontal and perpendicular to the instrument's VA.
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