关 键 词：

测绘工程专业英语 测绘工程专业

资源描述：

测绘工程专业英语,测绘工程专业英语,测绘工程专业

内容简介：

Section 6 Determination of True Bearing by Gyro-thodoliteThe gyroscope has been used in navigation as a north-seeking device for a considerable period of time and certain manufacturers of surveying instruments are now producing units which allow the direct establishment of the meridian by theodolite without the need for calculations based on astronomical observations.The development of the gyroscope for the precise transfer of bearings underground in mining surveying dates from about the beginning of the First World War, but an instrument capable of registering bearings to within one minute of arc did not appear until after the end of the Second World War. Even then it was of considerable weight, but advances in the design of gyroscopes for airborne inertial navigation systems have allowed much more compact and portable units. For instance the Wild GAK1, described later, has a total mass of about 13.6kg, and its accuracy is of the same order as that obtained in sun observations.In this instrument a rotor or spinner is driven at speeds in excess of 20 000 rev/min, the axis of spin being in a horizontal- plane. Figure 1 shows the earth rotating about its axis with an angular velocity of ; at a place in latitude an angular velocity ofcos obtains about the meridian. The spinner of a gyroscope set up at this point will try to maintain its initial spatial position provided that its angular momentum, I, is large, but the rotation of the earth itself pulls the spinner out of this plane. There is a consequent reaction in the form of a rotation, or precession, about the vertical (or output) axis of the spinner which holds until the spin axis lies in the meridian at the place. Figure 2(a) shows the three axes of the spinner and Fig. 2(b) shows the spin axis at an angle of a to the meridian. The earths rotation causes an interference torque (ME) equal to lcos, and a consequent precessional couple of ME sin a is induced which forces the spinner into the meridian with an angular rate of cos sin a. When the spinner is suspended (as it is in the GAK1 attachment) it will be seen that the maximum precessional couple occurs when is zero, i.e. at the equator, and that the gyro will float freely at the poles where cos is zero. The precessing gyro does not align immediately on the meridian but oscillates about it. This can be placed on instruments such as Wild TIA, T16 or T2 theodolites which must be suitably modified, and it is then connected to a control unit or converter. The oscillating system (Fig. 3(a), containing the spinner and optical system with an index mark is carried by a tape suspended vertically within the chimney of the supporting case and fastened at the top. The converter contains a nickel-cadmium battery and an electronic unit; the battery is connected by leads to a six-way adaptor on the supporting case, and is controlled by a selector switch. A further switch indicating run and brake is provided with two indicators, one of which, measure, shows a green colour when the spinner is revolving at its working speed. When setting up, the theodolite telescope should be roughly pointed towards north by means of, say, prismatic compass or tubular compass observations, and the gyro attachment can then be fastened on the theodolite with the telescope in the face-left position. The cable from the converter unit is attached and the observer checks that the gyro is clamped by means of a device underneath the supporting case. The battery switch is set to on and when the other switch is set to run the wait indicator remains red for a short period of time before the green color shows at measure, indicating that the spinner is now running at its correct operational speed and that it can be undamped. As the spinner oscillates about the meridian the gyro index mark can be seen moving across an auxiliary scale in an observation tube, and when the spin axis and the line of sight of the theodolite are in the same vertical plane that mark should be centred in the middle of the scale, which is also marked by a V-shaped index (Fig. 3(b).Thus if the mid-position of the oscillations is established, the telescope line of sight at the station is oriented towards the north, and the purpose of all observations is to determine that position. A calibration constant (E), which is the horizontal angle between the direction of the line of sight in the mid-position of oscillation and the meridian at a station, must however be determined for each instrument at frequent intervals by comparing the bearing, AZ, determined by astronomical observations with that, AG, determined by the gyrotheodolite, i.e. E=AZ-AG. The calibration constants of gyrotheodolites can show a systematic drift caused by unrequired torque on the gimbal system which, in the case of the suspended spinner, is the tape and the bearing of the spin axis. Chrzanowski has suggested in an article New techniques in mine orientation surveys, Canadian Surveyor 24 (I) 1970, that checks be made immediately before and after the underground work and on the same day.The form of oscillation is sinusoidal and in the middle of an oscillation the index mark is moving at maximum speed in the relevant field of view but it slows down quite markedly as the turning points are approached and for a very short period is stationary at such points. If the gyro index mark has been kept centred within the V-shaped index by using the horizontal motion of the theodolite alidade, the corresponding horizontal circle reading of the theodolite can be obtained for this turning point.Various methods of orientation are available including quick methods for pre-orientation before the adoption of a more precise method. In one of the quick methods the line of sight should be set within 15of the estimated meridian and by following up the gyro index mark, with the V-shaped index in coincidence, horizontal circle readings are obtained for two successive turning points on opposite sides of the meridian. The mean horizontal circle reading gives the approximate meridian and the line of sight of the telescope can therefore be so established. It is essential that the two index marks be kept in coincidence so that the suspension tape is held free from torsion. Some initial practice is required for this and to facilitate the work some instruments are provided with a wider-range horizontal slow-motion screw. An accuracy of the order of 3 is quoted by Messrs Wild for this method.The methods devised for the precise location of the center of oscillation, can be classified into tracking and non-tracking methods. One of the former group is an extension of the above quick method in that several successive turning points are observed, ensuring that the coincidence previously mentioned is maintained. The corresponding horizontal circle readings have to be obtained very quickly since the gyro index mark, relating to the spin axis, is only stationary for a very short period when it also has to be centred accurately on the zero division of the scale. Knowing the circle readings for pointings on the referring object (M, mean of readings on two faces) and the meridian (N), the two are related to give the geographical azimuth as (M- N + E). Four to six turning points are observed for highest accuracy.New Words and Expressionsbearing：象限角，方位角，方向角；矿层走向；轴承，支承gyroscope：陀螺仪，回转装置navigation：导航north-seeking device：指北设备meridian：子午线，经线，子午圈astronomical observation：天文观测inertial navigation system：惯性导航系统sun observat