Figure 3. The alidade.

Plane table mapping is a subject that has received minimum attention over the years, but it played a very important role in the history of surveying. The plane table dates to approximately the beginning of the 17th century. Some of the first references to the plane table are found in early survey textbooks. The early plane table was a mapping tool, as opposed to the compass and chain that was a boundary tool. The early plane table dealt mainly with planimetric features and not with vertical elevation. The use and development of the plane table has gone through four distinct changes since about 1600. I will address each one of these plane tables and its use.

Figure 1. The first alidade was a ruler or scale of brass with fold-down sights.

The First Phase-the "Plain" Table

The first plane table consisted of wooden boards placed together to form a table about 14 x 11" mounted on a tripod. A paper or cardboard sheet was placed on top of this board. The planimetric map or traverse was drawn on the paper. The alidade is the item that has gone through the most changes. The first alidade was a ruler or scale of brass with fold-down sights, about the same configuration as compass sights (see Figure 1). Note also the chain carriers. I do not know why they are such whimsical figures.

The following is a description of the first plain table from The Compleat Surveyor written in 1722 by William Leybourn. In the early text it was called a plain table, not a plane table. I have transcribed this just as it appears in the original text. For some reason, the letter "S" was written as an "F" before 1800:

A Defcription of the Plain Table, how it hath been formerly made, and how it if now altered, it being the moft abfolute Inftrument, of any other, for a Surveyor to ufe, in that it performeth whatfoever may be done either by Theodolite, Circumferentor, Peractor, Perambulator, or any other graduated Inftrument, with the fame eafe and Exactneff.

During its first phase, the early plain table was a tool that used intersection of lines to locate points that were plotted on the paper on the table. The early table did not have a level bubble but used the compass needle to level the table. If the needle was level to the table, this was close enough. The first operation was to orientate the table to north using the compass. The next step was to sight a point that needed to be located with the alidade and draw a line on the board. If the object was close enough to the table, then the distance could be chained. If chaining was not practical, then the point could be sighted from another location. This would give an intersection of lines, resulting in the location. Some of the early chains used with the plain table were only one pole in length-16.5'. The early plain table was used for mapping or cartography. The plain table was used for horizontal location; elevation was not a consideration in most cases. This item remained unchanged until about the middle of the 19th century.

Figure 2. A 19th century plane table.

The Second Phase

The second phase brought about a name change from plain table to plane table. By the middle to the end of the 19th century, the instrument became in shape and size the plane table most surveyors would recognize (see Figure 2). The alidade now had a telescope. The table was about 2' square and was mounted on a tripod with an adjustable mount for leveling and rotating the table to orientate to north. The alidade had a vertical circle to turn a vertical angle and calculate the difference in elevation between points.

This is a description from 1935 by Harry Bouchard, an associate professor of geodesy and surveying at the University of Michigan, on why the plane table is a good choice for topographic mapping: "The plane table is one of the best instruments for topographic surveying, as with it the map is actually drawn in the field where the features can be seen and where the amount of detail to be mapped and the accuracy required can be judged to the best advantage. The topographer, while in the locality, can compare his finished work with the topographic features as they actually appear and thus ascertain if his map represents them properly."

Another good description of the plane table appears in the writings of Charles Davies from 1876: "The plane table is an instrument in which the manufacturer has combined the telescope of the transit with the straight edge and drafting board of the draftsman. When some form of stadia reduction arc is added to the vertical circle, the topographic engineer has an excellent field mapping instrument at his disposal."

During this second phase of the plane table, many different methods were developed to simplify the calculations to convert the slope distance to horizontal distance and the slope vertical angle to a vertical elevation. During the last half of the 19th century, the United States Geological Survey (USGS) put the plane table to use mapping vast areas of the western United States. Many national parks were mapped using a plane table.

Surveyors spent their entire careers bent over a plane table board. During this period, many improvements were made based on suggestions from field surveyors. One of these improvements was the Beaman Arc. The Beaman Arc was a system designed by William Beaman of the USGS. The Beaman Arc is a stadia reduction device, which is attached to the vertical circle of the alidade. Its design was based on the principle that for certain vertical angles the difference in elevation is a whole number multiple of the rod intercept, and the correction to be subtracted from the stadia (slant) distance to obtain the horizontal component is related to the vertical angle. This calculation was a simple multiplication for the horizontal distance using the Beaman "H" index. The vertical difference was a very similar operation using the "V" index on the Beaman Arc. The USGS set up simple standard forms to be used in the field to make these calculations without the use of sine and cosine tables. These improvements were all incorporated into the standard design of the alidades being sold by equipment companies. This alidade design remained unchanged for over 100 years (see Figure 3).

An article on the plane table would not be complete without mention of the stadia board. This was a level rod, about 10' or 12' long and 6" wide with very large letters. Many different styles of letters were tried on the face in search of a blunder-free combination. Many of the later stadia rods built by the survey equipment companies had handles on the back to hold the board and a built-in level bubble in order to plumb the rod. Most folded in half with a hinge in the middle for easy transportation. The faces of many of these rods have an art deco quality and are considered very collectable. One last thing about the stadia board; on a very windy day, the rodman could almost be lifted off the ground by a gust of wind while trying to plumb up the board. Surveyors earned their pay when wrestling a stadia board for eight hours a day.

Figure 4. Plane table mapping in the field.

Phases 3 and 4: The Modern Era

Phase three in plane table design came in the later part of the 20th century when the automatic compensating level was introduced. The automatic compensator was added to the alidade to speed up the leveling process. This greatly enhanced the field speed by which topographic work could be performed. The automatic compensator alidade found great favor in the hands of companies doing mapping and traverse work for oil and gas exploration. During this time period, many surveyors using a plane table were placing a mylar sheet on the table as opposed to the paper sheet. This was a much more stable material and gave the surveyor the option of running the mylar sheet through a copy machine and making an extra copy for field checking. The U.S. Army Corps of Engineers was a great proponent of the plane table, using them to map engineering project surveys and many military bases.

Phase four and the end of the line for the plane table was an alidade that appeared for a short period of time in the 1980s. The alidade was a small EDM that contained automatic slope reduction giving the surveyor a direct read out of the horizontal and vertical difference between the plane table setup and the point being read. Unfortunately-or fortunately-the total station with data collection made the plane table and alidade into museum pieces.

No article would be complete without a mention of the surveyors who utilized plane table boards (see Figure 4). There were only a few days each year suited for plane table mapping. The weather needed to be warm but not hot; sweat dripping from your face and hands could make a mess of the paper drawing. The USGS used an umbrella over the board when mapping.

Surveyors also needed a day with very little wind. And the reflection of the sun off the board would quickly turn your face into old shoe leather. In the winter, your fingers would get stiff from the cold, making it very difficult to draw the map. Bending over the board for eight hours each day without being able to lean on the table took a physical toll on the back. Despite these difficulties, some surveyors worked in ink in the field and produced maps that were nothing short of masterpieces.

Now you have the story of the plane table-gone but not forgotten. Technology is great, and we do need to move forward, but I can't help thinking we may have lost a little of what made a surveyor a surveyor when we lost the plane table.