Adjustments Of Plane Table
For a plane table survey to be accurate and adequate, subsequent precautions shall be observed and adjustment made accordingly.
(a) The table board surface shall be as perfectly plane as probable; or else the rays drawn on the drawing map will experience various obstructions and therefore will not be accurate and straight. The planeness of the board could be checked through using the straight edge in various directions and erasing ridges and higher spots through sand papering. Planing could be undertaken if roughness is acute.
(b) Further to being smooth and plane, the board surface should also be horizontal, i.e. perpendicular to the vertical axis of rotation of the table. The horizontality is checked through using the spirit level. The level is placed frequently in perpendicular positions and bubble checked for center. Any deviation from horizontality is right using washer/packing among the board and the supports. The bubble of the level should keep central even while the table is slowly revolved by 360o.
(c) In sequence to draw the rays in perfect straight lines, a ruling edge of the alidade shall be managed straight. Any deviations shall be straightened through filing accurately.
(d) The sight vanes given on the alidade should be normal to the ruler base. This will ensure which the line of sight of the instrument is parallel to the ruling edge so in which the rays drawn represent the direction of the line of sight.
This requirement can be tested through hanging a plumb line at a distance from the instrument and bisect it along with the sight vanes of the alidade. The error will be denoted through hair line of vane tilting with respect to the plumb line. Essential adjustment could be made either through filling or placing some packing under the alidade base as needed.
(e) The spirit level axes should be parallel to the alidade base axes.
Same accuracy requirements and adjustment are essential for telescopic alidade.