It is necessary to check whether the accessories satisfies some basic conditions and if required, necessary adjustments are to be done before starting any plane table surveying work. The operations involved in this are known as temporary adjustment of plane table. The conditions needed to be tested and subsequent adjustments are as follows:
1. The surface of the board should be a perfectly plane.
Test : It is tested by placing a straight edge on the top surface of the plane table in different directions. If there is no gap between the base of the straight edge and the surface of the plane table then the surface is perfectly plane. Otherwise, the surface is not perfectly plane.
Adjustment : If the gaps are minute, those are removed by rubbing with sand paper and for more gaps, the table should be replaced.
2. The fiducial edge of the alidade should be straight.
Test : It is tested by drawing a fine line on the paper along the fiducial edge of the alidade. Then, by reversing the alidade, end for end, and placing against the line drawn, a line is to be drawn again along the fiducial edge. If the two lines coincide, the edge is straight. Otherwise, the edge is not straight.
Adjustment : The fiducial edge of the alidade is to be made straight by filing and then test is repeated till satisfactory outcome.
3. In fully opened condition, the sight vanes of the alidade should be perpendicular to its base.
Test : Hang a plumb bob at a distance of about 5 to 10 m from the plane table. Bisect the string of the plumb bob through the alidade placed on properly leveled plane table. If the sighting slit, the object vane hair and the plumb bob string lie the same vertical line, the vanes of the alidade are perpendicular to the base of the alidade. Otherwise, it requires adjustment.
Adjustment : is being carried out by inserting packing under the base of the sight vanes or by filing the base, as required. The test and adjustment get repeated till satisfactory outcome is achieved.
4. The telescopic alidade if used should be in perfect adjustment.
The testing and adjustment of the telescopic alidade are to be carried as discussed in case of theodolite in Module 6.
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