The Plane Table

Surveying is about the measurement of land and its surface. Cartography is the creation of maps based upon measurements of the earth. Typically, in the past, they were two separate operations, with the measurements were recorded in a field book with distances, direction bearings or angles and then plotted onto a drawing or a map. The plane table combined these operations by allowing the drawing to be made as the measurements were taken.

Plane tables have been around since the 16th century according to some sources, but the simplicity of the device suggests that it likely was used long before that time in some form. The plane table eliminates much of the need to compute angles and slope distances that using a transit and tape would require.

A simple table mounted to a tripod could be set up in most places and leveled. A simple sighting device mounted to a straight edge would allow the user to draw a straight line parallel to that line of sight directly on a piece of paper. A distance then measured to a point of interest such as a hill top point, a fence or a building corner could be measured along that line and plotted. Simple, the method required no calculation and it allowed the mapmaker to see the drawing develop as objects were plotted.

A Plane Table with Various Sighting Devices

The sighting device is called the alidade. The earliest instruments used only a straight edge with sight vanes attached to it. These types continued to be used by the military for quick mapping and similar, (usually larger) devices were used by navigators on ships to plot the ship’s location by sighting on known points along a coast and plotting lines on coastal charts. Later alidades used telescopes with cross hairs and stadia lines. A simple compass needle in a box set on the table allowed the map to be oriented with respect to North.

The telescope with stadia cross hairs variety of alidade allowed the plane table user to obtain the distance to the object he was plotting on the map. Sighting through the scope at a scale on a vertical level rod, the operator could read where the top and bottom stadia lines crossed the rod. With the distance between the top and bottom cross hairs measured on the rod, along with the vertical angle on the alidade, a simple calculation would give the distance to the object which could be plotted directly on the map. Slide rules were developed to do this calculation.

Plane tables were used extensively for topographic mapping. The limited accuracy of the method sufficed for such purposes. The plane table method to produce maps was frequently used by geologists when mapping rock formations and soil types. Many of the surviving plane table alidades have markings of ownership by the US Topographic and Geological Survey. Military operations used plane tables for field maps during operations.

Sighting Devices used with a Plane Table

My own experience with plane tables was limited to a field exercise in college where we had to map a portion of the campus. It was not a great experience as I recall, because it was easy to bump the table off level working around and on it, humidity made the paper expand and wrinkle and wind could be a problem. It just seemed easier to measure angles and distances and record them in a field book and make the map indoors on a drafting table, but those with proficiency with plane tables would likely disagree.

Now, the plane table is another obsolete technology. Global Positioning Systems (GPS), Aerial photography and scanning and satellite imagery have replaced the need to set up a table out in a field with a sheet of paper and draw a map.

Ron Blauch, Member, EAIA