AlidadeThe alidade-planetable survey is a means of plotting a traverse as it is made, thus avoiding the likelihood of mathematical blunders and recording errors that arise while working with the theodolite.
The lack of precision inherent in graphical plotting owing to the grossness of the tools (pencil, paper and scale) can be minimized by careful attention to detail. The optics of the telescopic alidade are similar to those of the theodolite, hut the accuracy of measurement of both horizontal and vertical angles is obviously lower, although a vernier is provided for the Beaman arc.
|
Inst. Sta |
Rod Point |
Stradia Int. |
Beaman |
Beaman |
Ht Stadia Int. |
Vector |
V factor stand. int. |
Cross-hair reading |
Net diff | Elev. at point |
|
Station A |
A-1 | 2.3 | 50 | 100 | 230 | 0 | 0 | -8.3 | -8.3 | 378.1 |
| A-2 | 9.7 | 52 | 94 | 911.8 | 2 | 19.4 | -12.4 | 7 | 393.4 | |
Plane TableGeorge was often "Instrument Man" when a plane table-alidade survey was carried out. His job then involved plotting graphically at a scale of 1:1000 the distance to and the direction of the next turning point using a very hard pencil on heavy paper milled for stability and the straightedge at the base of the telescopic alidade. He oriented the plane table using the magnetic compass, and insured precision in succeeding azimuths by exact alignment of the vertical hair in the reticule with the target. Intercepts in the horizontal subdivisions of the reticule and the stadia rod corrected for the angle of sight gave precise measurements of distance, (in meters) then scaled off on the penciled traverse line. He determined elevations (in feet) from one of the three scales on the Beaman arc or calculated them using the formulas given in the discussion of the theodolite, or both. The locations and elevations of measured sections were tied in as spurs to the main traverse.
Sometimes two rodmen-geologists worked at the same time keeping George fully occupied for the duration of the survey.