Mapping with Plane Table and Alidade
| Field | Geologic field mapping |
| Went Obsolete | (seemed to be moribund in the late 1960's, probably earlier.) |
| Made Obsolete By | Surveying transits, and now GPS. |
| Knowledge Assumed | Drafting, Geometry, Trigometry. In adverse conditions. (think wind, rain, mud, bugs, snakes, …) |
| When useful | As an exercise for the student. |
A piece of paper was taped to a 3 X 3 ft board, mounted on a tripod waist high. This “table” was leveled with a bubble on a Brunton compass (perhaps another obsolete skill), or on the alidade itself.
The alidade is/was a straight-edge ruler with a small sighting telescope mounted on it. The eyepiece was at an angle so you could look down on it while bending over the table (careful, don't bump it!). The telescope would be adjusted up and down to measure vertial angles too. While sighting at a rock outcrop or at your helper holding a tall marked rod, you would draw a precise line with a 6H pencil along the straight edge from your position to the “target”, and write down how much higher or lower it was, in degrees, than the table.
After a number of points had been marked, the whole business was moved to one of the points and set up again. Now the lines would be drawn to the old points (triangulation), and extended to new points.
An so, a map was drawn, fairly accurately, and very slowly.
