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How does a plane table locate map features using an alidade?
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surveying
Figure 1: Photogrammetric photographs from two short, overlapping flight strips arranged for supplying mapping details. Photo-control points are shown on only one photograph; shading indicates a typical terrain feature such as a lake (see text).
In civil engineering and land surveying, a plane table is a device used to plot map features directly from field observations. It consists of a flat drawing board mounted on a tripod that can be leveled and rotated.12
To locate features, the surveyor follows these steps:
- Orientation: The table is set up over a known framework point or baseline end. It is rotated until the line on the paper representing the baseline is parallel to the actual baseline on the ground.1
- Sighting: An alidade—a straightedge equipped with sights—is placed on the table. The surveyor aligns the alidade with a target feature and draws a pencil ray along the rule toward that point.31
- Intersection: This process is repeated from a second known framework point or the other end of the baseline. The map position of the feature is determined by the intersection of the two rays drawn on the paper.1
This graphical method allows for the direct recording of data in the field, which is particularly efficient in open or mountainous terrain.1