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| Figure :1 Azimuths |
Azimuths are horizontal angles observed clockwise from any reference meridian. In plane surveying, azimuths are generally observed from north, but astronomers and the military have used south as the reference direction. Examples of azimuths observed from north are shown in Figure 1. As illustrated, they can range from 0° to 360° in value. Thus the azimuth of OA is 70°; of OB, 145°; of OC, 235°; and of OD, 330°.
Azimuths may be geodetic, astronomic,magnetic, grid, record, or assumed, depending on the reference meridian used.To avoid any confusion, it is necessary to state in the field notes, at the beginning of work, what reference meridian applies for azimuths, and whether they are observed from north or south. A line’s forward direction can be given by its forward azimuth, and its reverse direction by its back azimuth. In plane surveying, forward azimuths are converted to back azimuths, and vice versa, by adding or subtracting 180°.
For example,
If the azimuth of OA is 70°, the azimuth of AO is 70°+180° = 250°
If the azimuth of OC is 235°, the azimuth of CO is 235° - 180° = 55°
Azimuths can be read directly on the graduated circle of a total station instrument after the instrument has been oriented properly. This can be done by sighting along a line of known azimuth with that value indexed on the circle, and then turning to the desired course. Azimuths are used advantageously in boundary, topographic, control, and other kinds of surveys, as well as in computations.
Bearings
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| Figure :2 Bearings |
Bearings are another system for designating directions of lines. The bearing of a line is defined as the acute horizontal angle between a reference meridian and the line.The angle is observed from either the north or south toward the east or west, to give a reading smaller than 90°.The letter N or S preceding the angle, and E or W following it shows the proper quadrant.Thus, a properly expressed bearing includes quadrant letters and an angular value.An example is N80°E. In Figure 2, all bearings in quadrant NOE are measured clockwise from the meridian. Thus the bearing of line OA is N70°E. All bearings in quadrant SOE are counterclockwise from the meridian, so OB is S35°E. Similarly, the bearing of OC is S55°W and that of OD, N30°W.When lines are in the cardinal directions, the bearings should be listed as “Due North,” “Due East,” “Due South,” or “Due West.”
Geodetic bearings are observed from the geodetic meridian, astronomic bearings from the local astronomic meridian, magnetic bearings from the local magnetic meridian, grid bearings from the appropriate grid meridian, and assumed bearings from an arbitrarily adopted meridian. The magnetic meridian can be obtained in the field by observing the needle of a compass, and used along with observed angles to get computed magnetic bearings.
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| Figure :3 Forward & Back Bearings |
In Figure 3 assume that a compass is set up successively at points A, B, C, and D and bearings read on lines AB, BA, BC, CB, CD, and DC. As previously noted, bearings AB, BC, and CD are forward bearings; those of BA, CB, and DC, back bearings. Back bearings should have the same numerical values as forward bearings but opposite letters.Thus if bearing AB is N44°E, bearing BA is S44°W.
Comparison of Azimuths & Bearings
Because bearings and azimuths are encountered in so many surveying operations, the comparative summary of their properties given in Table 1 should be helpful. Bearings are readily computed from azimuths by noting the quadrant in which the azimuth falls, then converting as shown in the table.
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| Table :1 Comparison of Azimuths and Bearings |