Flat tables are used for surveying and mapping tasks, with a solid surface mounted on a tripod and an alidade for telescopic sighting. The table can be oriented using a compass or visible point, and multiple stations can be used for large projects. Flat tables were used by NOAA to map the US coastline and offshore hazards beginning in 1834.
Maps, graphs, and field drawings related to surveying and other related tasks are made on what’s known as a flat table—a table with a solid surface that can be easily leveled by means of its height-adjustable legs. The table surface is mounted on a tripod of legs that rotates in any direction for its use as a mounting base for an alidade, which is a surveying instrument with a telescopic sight. Table surfaces are typically 18 cm by 24 cm (45.7 inches by 61 inches) or 24 inches by 31 cm (61 inches by 78.7 inches), and the height is adjustable. The criteria for a site for an air table station include a line of sight encompassing as many major terrain or building points as possible.
The table top can be oriented by aiming at a visible point already traced or by using a compass to arrive at a north-south orientation. A straight line along the edge that is parallel to the line of sight provides a tracked direction from the original point to the desired point to track distance and direction. The alidade is a slide rule with an attached telescopic sight or real telescopes. An arc stadia is a vertical and horizontal measuring device mounted on the telescopic lens. Plane table sensing uses stadia arc hairs and reticles on the telescopic lens which are read and multiplied by a stadia range factor.
The odds in sight can be measured and mapped into lines on paper affixed to the flat surface of the table. Distances and differences in elevation measurements can then be taken in situ in the flat table survey to produce finished scale maps. On large job sites, multiple flat table stations can be set up to survey different aspects of a project.
Because coastlines are used in the creation of nautical maps, the National Oceanic and Atmospheric Administration (NOAA) needed to map the territorial and marine limits of the United States coastline for maps and as a necessary reference in its stewardship of coastal natural resources. Not only the coast, but hydrographic surveys also had to be made of the coastal waters and any known offshore hazards. Beginning in 1834, the elevations of natural features above the seas were mapped using flat tables on ship-towed stations. Planar mapping stations were used for the next 20 years to complete all topographic maps of the coast.
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