A turning point (TP) is a temporary bench mark that is used to extend the survey a greater distance. They are used anytime it is necessary to move the instrument. Whenever the instrument is moved to a different location, a backsight is taken on a TP to establish the new instrument height. Turning points are usually used wherever the starting station and ending station can not be seen from one instrument position.
The number of turning points used during a survey is controlled by the maximum distance for the instrument, the distance traveled and the variations in the topography, see Figure 15.4. They should be selected with care and must not be moved until the survey is complete. The structure used for a turning point should be dimensionally stable just like a bench mark. A stake, or permanent structure such as a curb must be used. Do not set the rod on the ground when making a turning point; doing so could result in a serious error.
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