The first step in the field is to set the board. To do so it is only necessary to set a line a.b. on the board parallel to A.B. on the ground. The alidade is laid on the line a.b. and—standing at A.—the planetabler revolves his board until he sees B. through the sight vanes. Lines can then be drawn from a. towards any other pcints on the landscape. Similar lines from b. inter secting those from a. will then fix the positions. This intersection is an important factor in plane tabling. Even though the topog rapher may be given many control points fixed by the theodolite, he must amplify that control for the detail surveying he has to do. As a general rule then intersection is used today in providing the minor control for tomorrow. The actual mapping is based mainly on the process known as interpolation, resection, or "making the point." This is simply to find, from the positions of three or more control points, the position at which the table is set up. There are several methods of resection, the simplest of which will be described briefly: The board is roughly set and rays are drawn backwards from three control points towards the observer; the alidade being aligned on each in turn, so that it touches its plotted position. If the board has been truly set these rays will all pass through a point. If not, the true position of the observer will be nearest to the ray from the nearest point, furthest from the ray from the most distant. Again if the three control points lie round and outside the observer his position will be within the triangle of error. If the observer is outside his points his position is outside the triangle of error.
At each point so fixed the topographer sketches in the detail im mediately around him, on directions drawn along the alidade, and at distances which are measured tachymetrically or by pacing or estimation. Heights are fixed by observing angles of elevation or depression and multiplying their tangents by the distances.
In average country a topographer will survey a square mile or so per diem on the one inch scale, but his rate will depend greatly upon his transport. Even at such scales as 3 inches to the mile, where the amount of detail to be shown demands in tensive surveying rather than fast movement, a bicycle or some other transport is advisable.
The scales suitable for plane tabling lie between a quarter inch and three inches to the mile. At larger scales the plane table may still be and often is used, but more as a record of instrumental measurement and less purely graphically.
The lengths or "legs" of the traverse may be measured in many ways. In control traversing invar or steel tapes are used in cate
nary (as in base measurement) or laid flat in both cases under tension. For topographical surveying legs are measured with tapes or chains, ropes or rattans, by cyclometer or by pacing. They may also be measured optically with rangefinder, telemeter, subtense bar, or tachymeter. In geographical surveying, distances may be determined by observed differences of latitude on observed azi muths, or estimated from the time taken to travel over them on foot, horse, camel or motor car. Any class of precision may in fact be obtained, varying in fractional error from to A.
The angles of a traverse may be measured with theodolite or compass, may be obtained graphically on the plane table, or estimated from the direction of a sound, generally in the form of a prearranged call or whistle from a forward observer.
In the topographical survey of a new country various classes of traversing may occur. In the Federated Malay States and in West African Colonies, precise traverses with a linear error of been used as a framework or control. In the actual detail survey of forest regions minor traverses fill up the gaps between more precise and costly control traverses and afford opportunity of plotting detail. Examples of Topographical Traverses which will explain the procedure are given below.
In a recent three inch to one mile survey in Johore, the trigo nometrical control consisted of a number of points assumed as errorless. Between them were run traverses with the compass and chain. The course of the traverse was cut as straight as possible.
The observer plotted his bearings and lengths on squared paper at 6 inches to the mile, including the mapping of detail and contours. On reaching the closing point the traverse was adjusted graphically to its correct length, reduced to 3 inches to the mile and plotted on the plane table. Errors were, in general, of the order of The triangle of traverses was then cut up by minor traverses run ning straight and parallel at 200 feet intervals. These traverses were compass and "rattan" (a long creeper, marked to length, and compared each day with a standard chain). The detail and con tours were mapped during the course of the traverse which was plotted direct on the plane table and seldom showed need of adjustment.
As explained above traverses which are graphic in principle are adjusted graphically and proportionally. In adjusting instru mental (or booked) traverses the normal rule is to adjust as follows : As the arithmetical sum of all x's (or y's) is to any one x (or y), so is the whole error in x's (or y's) to the correction to the corre sponding x (cr y).
An adjustment on these lines gives equal weight to angular and linear measurement and admits the fact that there is, in general, no evidence on which to give preference among (i.e., to weight) the measurements.