Transversing in Forest engineering
A series of straight lines connecting a series of established points along a route of purvey is termed as a traverse. Traverse may be : (1) open or unclosed traverse (Fig. 4.35 a), (ii) closed traverse (Fig. 4.35 b) or (iii) partially closed and partially open traverse
The more common and accurate method of chain survey is triangulation, which has been described in detail already. However, it is possible to carry out traverse survey also with chain, in which directions of successive chain lines are established by laying out . chain angles. However, chain traverse is not Ordinarily recommended as it runs counter to the fundamental principle of survey, viz., to work from the whole to the part and not from part to the whole. Errors progressively increase at each successive station due to cumulative effect.
CHAIN TRIANGULATION
There are two general methods of surveying : i.e., Triangulation and Traversing.
Chain triangulation or chain surveying is a system of surveying in which sides of various triangles are measured directly in the field and no angular measurements are required. The underlying principle is to divide the area to be surveyed into a frame-work of triangles, since triangle is the only figure which is determinate in size and shape if the lengths of the sides are known. The location and disposition of the triangles will depend on configuration of the ground, natural obstacles, if any, and the extent and nature of details desired to be plotted.
Survey Stations
A Survey Station is a point where main, base or any other chain or tie line begins or terminates ; or it is apoint on the system of triangulation and check lines that is measured to or from. 'the one at the beginning or the end of a chain line is quite an important one. There are two kinds of stations, viz., main stations and subsidiary or tie stations. Main stations are the principal corners of the main lines which command the boundaries of the survey. Tie or subsidiary stations are the points selected on the main survey lines where it may be required to run subsidiary line-s to locate interior details such as fences, buildings, etc., when these are distant from the main lines. Stations are usually indicated with a small circle round the station which is lettered or numbered ; capital letters are used to denote main stations,
e.g., (A), (B), etc. •; and small letters generally for tie stations, e.g., (a), (h), etc., (as in Fig. 4.1). Letters may be written inside or outside the circles. A, Et C, D in Fig. 4.1 are main stations and p, q, r the subsidiary or tie stations.
Selection of Stations
For a proper frame-work of survey, following points should be borne* in mind during reconnaissance, before finalising the plan of the survey : — (1) The number of main lines should be as few as possible, and sliould run as close and parallel to the outer boundaries as possible. Lesser the number of lines easier will be the plotting and lesser the chances of error in plotting. (2) Flags at the main stations should be intervisible. 43) Main lines should form well conditioned triangles. (A( The main principle of surveying, viz., working from the whole to the part, should be strictly observed. (5) A long line should be run through the centre and whole length of the area (called the base line) on level ground, as far as possible. The entire frame-work of the survey rests on the base line. (6) Each triangle should be provided with at least one check line, to check the measurements of all the main lines. Tie lines should be run to obtain offsets conveniently to all details that are to be plotted. (7) Chain lines should be so located as to avoid obstacles in chaining, as far as possible.
Station Lines—
Straight lines joining the station points are known as station lines or chain lines ; these may be called main or survey lines if these join main stations, and subsidiary or tie lines if these join subsidiary or tie stations. Survey lines are denoted by the corresponding letters of the stations, e. g., AB, BC, etc. In Fig. 4.1, lines AB, BC, etc., arc main survey lines, pq and Ar are subsidiary lines.
Skeleton or Frame--
work of Survey 'The system of lines and triangles constituting the area to be surveyed is called skeleton or frame workof survey. Details of survey are determined by measure¬ments with reference to these lines forming the main and subsi¬diary triangles. In Fig. 4.1, ABCKD forms the frame-work of Survey.
Base Line—
A base line is usually the longest and the most important of all chain lines, running through the centre and lengthof the area to be surveyed, e.g., line BD in Fig. 4.1. It should ht carefully selected, on a level ground, and accurately measured horizontally twice or thrice, as the accuracy of the entire survey will ultimately depend on the accuracy and straightness of this line. It fixes up the directions of all other lines, as it is on this'll! base line that the frame-work of survey is built up. If necessary, two base lines, in the form of 'X' should be laid out. Since the liability to inaccuracy in triangulation increases with the distance from the base, it is necessary to locate the base line in the centre of the survey, which avoids accumulation of errors in the system of triangles built thereon.
Check or Proof Line -
Its function is to check the accuracy of measurements and plotting by making cross measurements, 0,., the measured length of a line—called check or proof line in the field should tally with the length scaled on the plan. It is generally a line joining the apex of a triangle to some fixed point on the opposite side, or a line joining fixed points on any two sides of the triangle, e.g., lines Ar and pq in Fig. 4.1. A check or proof line is not essential for plotting the survey—it is primarily, meant for checking the accuracy of plotting and, incidentally, provides au additional chain line for filling up interior details also.
Tie Line—
A tie line serves dual purpose. It provides a check on the accuracy of the frame-work and also enables the surveyor to pick up interior details, which happen to be quite far away from the main- chainlines. In Fig. 4 1, the line pq is a tie line joining fixed points p and q, termed as tie stations, on the main survey lines.
