Change point is considered when, 1) The survey area is larger to cover the entire area for survey, and to ensure more accuracy. 2) To Access points otherwise invisible due to any obstruction from a single station, necessitating a change point. 3) It is advisable to mark permanent bench marks on Permanent (immovable) objects.
Procedure, 1. Set instrument station at such a location from where maximum points can be covered/observed. 2. Level the instrument by using the 'level tube' attached to the instrument. 3. Take the first reading on a temporary bench mark, if permanent bench mark is not available. thus, first reading at the bench mark will give Height of the instrument (H.I). 4. As now H.I. level is known, Consider taking readings for each plot corners and note down the readings. Use the following formula to find out the Reduced Levels of corner points. Reduced Level (R.L.) at each spot =Height of Instrument - Staff reading.
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