Transit Traverse by Deflection Angles

This type of survey applies primarily to the control traverse of the route or area survey where accuracy is required. Distance between traverse points may be determined by stadia or chaining. Chaining is recommended when a high degree of accuracy is required. The transit is used to keep the chain handler on line and to obtain any vertical angles necessary to correct inclined distances to true horizontal. Angles are measured with the transit. Checks aremade by doubling the angles or repeating the angular measurement enough times to obtain the desired precision. Angles are measured in several ways:

(1)by turning to the right; (2) by measurement of the interior angles; (3) by measurement of the deflection angles; or (4) by measurement of the azimuths.

Most traverse surveys used in soil conservation work consist of establishing a route using a deflection-angle traverse that provides a means of locating points for either a closed or continuous traverse.

A deflection angle is the angle between a line and the extension of the preceding line. Deflection angles are recorded as right or left. In figure 1-31 the angle at B is 29"101 left because the angle was measured counterclockwise from the extension of the preceding line (A-B). The angle at C is 48'30' right because the angle was measured clockwise from the extension of line B-C. This method of angular measurement readily lends itself to the calculation of bearings if one of the lines is known.

In figure 1-31, the bearing of line A-B is given as S 82"E; therefore, bearing of line B-C= 180" - 82"- (29 "lo') =N 68'50' E bearing of line C-D = 180" - (68 "50') - (48 "30')

= S 62'40' E. Follow these procedures to take deflection angles:

1. Set transit on point E and set index of vernier at 0" (fig. 1-32).

2. With lower clamp loose, sight on point D and tighten clamp. With lower slow-motion screw, sight point D exactly. (Telescope is direct.)

3. Reverse telescope, loosen upper clamp, and rotate transit about its vertical axis in the direction(to the left) of point F. Tighten upper clamp and, using the tangent screw, sight point F exactly. Then read the angle and record it (58'00' L).

4. Leaving the telescope in a reversed position, loosen the lower clamp and rotate the transit about its vertical axis and sight point D again by use of tangent screw and lower clamp.

5. Rotate telescope (now in direct position), turn angle (to the left), and sight point F by means of upper clamp and tangent screw. The angle has been doubled (116'02' L),and one half of the total recording is the correct deflection angle to use. For example, in figure 1-32, point E: First angle =58'001 L

Double angle = 116'02' L

Mean deflection angle = 58 "01' L

6. Deflection angles are obtained in this manner for each point around the traverse.

Lesson 26

Transit-Stadia Traverse

The transit-stadia survey (sometimes called a stadia-azimuth survey) is frequently used for

preliminary work requiring the locations of boundaries, or the position of points, objects, lines, and elevations. This method of surveying is rapid and sufficiently accurate for many types of soil conservation work.

Stadia surveys usually consist of two parts: (1) the traverse, and (2) the taking of topography. The first is the horizontal control, and the second is the vertical

control. The controls provide for the taking of elevations for preparing topographic maps, locating details, or taking side shots. The two parts of a transit-stadia survey may be done together or separately. For small areas, the traverse and the topography surveys frequently are done together. For large areas, it is advisable to run the vertical and horizontal controls and check them before taking the topography. This will detect any errors in the traverse before the topographic shots are taken and plotted on the map being prepared.

Lesson 27

Horizontal Control Only

The fieldwork in this survey consists of determining horizontal angles (azimuths) to points or objects and obtaining distances between points by stadia. When a traverse is run by the transit-stadia method, all directions and angles are referred to a reference line. This may be an established line from a previous survey, a true meridian, magnetic meridian, or an assumed meridan. The azimuth angle is always measured clockwise from the zero azimuth. On any given survey the position of zero azimuth should always be the same-usually north (fig. 1-33).

When starting the transit-stadia survey, orient the instrument at the first station as follows:

Set the instrument on station A (assume azimuths will be taken from magnetic north).

Set horizontal circle on 0".

Point telescope toward magnetic north and release needle. Place 0"of magnetic circle at north point, and tighten lower clamp.

With the instrument oriented:

1. Loosen upper clamp.

2. Measure all angles from magnetic north, turning and reading all angles clockwise.

3. Complete all work at station A.

4. Sight on station B and read azimuth of line A B.

5. Keep this azimuth reading set on the circle, and move instrument to station B.

To carry the direction of the meridian from one transit station to the next (station A to B),use one of two methods.