Learn something new every day
More Info... by email
Maps, charts and field drawings related to surveying and other related pursuits are made on what is known as a plane table, a table with a solid surface that can be leveled easily by means of its height-adjustable legs. The table’s surface is mounted on a tripod set of legs that swivels 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 usually either 18 inches by 24 inches (45.7cm by 61cm) or 24 inches by 31 inches (61 cm by 78.7 cm), and height is adjustable. Criteria for a site for a plane table station include having a line of sight that takes in as many of the main points of the terrain or building as possible.
The plane table top can be oriented by sighting on a visible point already plotted 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 gives a plotted direction from the original point to the desired point for plotting distance and direction. The alidade is a straight edge with a telescopic sight attached or actual telescopes. A stadia arc is a vertical and horizontal measuring device mounted atop the telescopic lens. Plane table surveying uses stadia arc hairs and cross hairs on the telescopic lens that are read and multiplied by a stadia interval factor.
Elevations in sight can be measured and mapped in lines on paper clamped on the plane table surface. Distances and differences in elevation measurements can thus be taken on-site in plane table surveying to produce finished scale maps. On larger construction project sites, it is possible to set up multiple plane table stations for surveying different aspects of a project.
As shorelines are used in nautical mapmaking, the National Oceanic and Atmospheric Administration (NOAA) needed to survey the territorial and marine limits of the U.S. coastline for charts and as a necessary reference in its administration of coastal natural resources. Not only the coast, but also hydrographic surveys had to be made of the coastal waters and any known hazards located off-shore. Starting in 1834, elevations of natural features above the seas were mapped using plane tables on stations towed by ships. The plane table mapping stations were used for the next 20 years to complete all the coastline topographical maps.
One of our editors will review your suggestion and make changes if warranted. Note that depending on the number of suggestions we receive, this can take anywhere from a few hours to a few days. Thank you for helping to improve wiseGEEK!