Traversing is that type of survey in which a number of connected survey lines form the framework and the directions and lengths of the survey lines are measured with the help of an angle measuring instrument and a tape or chain respectively.
TYPES OF SURVEYING
There are two types of traverse surveying. They are:
- Closed traverse: When the lines form a circuit which ends at the starting point, it is known as closed traverse.
- Open traverse : When the lines form a circuit ends elsewhere except starting point, it is said to be an open traverse.
The closed traverse is suitable for locating the boundaries of lakes, woods, etc and for survey of large areas. the open traverse is suitable for surveying a long narrow strip of land as required for a road of canal or the coast line.
DIFFERENCE BETWEEN CHAIN AND TRAVERSE SURVEYING
Traverse survey differs from chain surveying in the arrangement of the survey lines is not limited to any particular geometric figure as in chain surveying, where a system of connected triangles forms the fundamental basis of the skeleton. Also, check lines etc. are not necessary in traverse as the traverse lines may be arranged near the details. The details etc. are directly located with respect to the survey lines either by offsetting or by any other method.
METHODS OF TRAVERSING
There are several methods of traversing, depending on the instruments used in determining the relative directions of the traverse lines. The following are the principal methods:
- Chain traversing
- Chain and compass traversing
- Transit type traversing a)By fast needle method b)By measurement of angles between the lines
- Plane table traversing
The method in which the whole work is done with chain and tape is called chain traversing. No angle measurement is used and the directions of the lines are fixed entirely by linear measurements Angles fixed by linear or tie measurements are known as chain angles. The method is unsuitable for accurate work and is generally used if an angle measuring instrument such as a compass, sextant or theodolite is available.
2.CHAIN AND COMPASS TRAVERSING
In chain and compass traversing, the magnetic bearings of the survey lines are measured by a compass and the lengths of the lines are measured either with a chain or with a tape. The direction of magnetic meridian is established at each traverse station independently. The method is also known as tree or loose needle method.
3.TRAVERSING BY FAST NEEDLE METHOD
The method in which the magnetic bearings of traverse lines are measured by a theodolite fitted with s compass is called traversing by fast needle method. The direction of the magnetic meridian is not established at each station but instead, the magnetic bearings of the lines are measured with reference so that direction of the magnetic meridian established at the first station. There are three methods of observing the bearings of lines by fast needle method.
- Direct method with transiting
- Direct method without transiting
- Back bearing method
TRAVERSING BY DIRECT OBSERVATION OF ANGLES
In this method, the angles between the lines are directly measured by a theodolite and the magnetic bearing of other lines can be calculated in this method. The angles measured at different stations may be either
- included angles and
- deflection angles
TRAVERSING BY INCLUDED ANGLE
An included angle at a station is either of the two angles formed n\by two survey lines meeting there and these angles should be measured clockwise. The method consists simply in measuring each angle directly form a back sight on the preceding station. The angled may also be measured by repetition. The angles measured from back station may be interior or exterior depending on the direction of progress.
TRAVERSE BY DEFLECTION ANGLES
A deflection angle is the angle which a survey line makes with the prolongation of the preceding line. It is designated as right (R) or left (L) according as it is measured clockwise or anti-clockwise from the prolongation of the previous line. This type of traversing is more suitable for survey of roads, railways, pipe-lines etc where the survey lines make small deflection angles.
ERRORS IN TRAVERSING
The errors involved in closed traversing are two kinds:
- linear and
Travers by included angles:
- The sum of measured interior angles should be equal to (2N-4), where N=number of sides of the traverse.
- If the exterior angles are measured, their sum should be equal to (2N=4)p/2
- Travers by definition angles: The algebraic sum of the deflection angles should be equal to 360°, taking the right hand and deflection angles as a positive and left hand angles as negative.
- Traversing by direct observation of bearings: The force bearing of the last line should be equal to its back bearing ±180° measured from the initial station.
CHECKS IN OPEN TRAVERSE
No direct checks of angular measurement are available. So indirect checks can be made.As illustrated in the Fig(a) the addition to the observation of bearing of AB at station A, bearing of AD can also be measured., if possible. Similarly, at D, bearing of DA can be measured and check applied. If the two bearings differ by 180°, the work may be accepted as correct.
Another method, which furnishes a check when work is plotted is shown as in Fig (b) and consists reading the bearing to any prominent point P from each of the consecutive stations. The check in plotting consists in laying off the lines AP, BP, CP, etc and noting whether the lines pass through one point.
PLOTTING A TRAVERSE SURVEY
There are two principal methods of traverse survey:
- Angles and distance method: This method is of three types. (a)By protractor (b)By the tangent of the angle (c)By the chord of the angle.
- Co-ordinate method.
In the figure, the latitude and the departure of the line AB of length l and reduced bearing q are given by
L= + l cosq and D=+ l sinq
To calculate the latitude and departure of the traverse lines, it is first essential to reduce the bearing in the quadrant system. The signs of latitude and departures will depended upon the reduced bearing of the line. The following table gives signs of latitudes and departures.
R.B and quadrant
0° to 90°
90° to 180°
180° to 270°
270° to 360°
Thus, latitude and departure co-ordinates pf any point with reference to the preceding point are equal to the latitude and departure of the line joining the preceding point to the point under consideration. Such co-ordinates are also known as consecutive co-ordinates or dependent co-ordinates.