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 3 The Surveyors LevelThree basic types of level can be distinguished. (1) Dumpy levelIn a Dumpy level the telescope and vertical spindle are cast as one piece. The levelling head shown consists essentially of two plates, the telescope being mounted on the upper plate while the lower plate screws directly on to a tripod. The two plates are held apart by three levelling screws or foot screws, and adjustments to these enable accurate levelling of the instrument to be carried out. When this has been effected, using the bubble attached to the telescope, the instrument should remain level no matter in which direction it is pointed(Fig 1). (2) Tilting levelIn this level the telescope is not rigidly fixed to the vertical spindle, but is capable of a slight tilt in the vertical plane about an axis placed immediately below the telescope. This movement is governed by a fine-setting screw at the eyepiece end, and the bubble is brought to the center of its run for each reading of the level. Thus the line of collimation need not be perpendicular to the vertical axis, as it must in the Dumpy level.Tilting levels are robust and capable of the highest accuracy, and many modern levels, whether for precise work or for ordinary levelling, incorporate the principle(Fig 2).(3) Automatic levelThere is now a great range of levelling instruments available with no precise bubble attached. The telescopes of these instruments need only be approximately levelled and then a compensating device, usually based on a pendulum system inside the telescope, corrects for the residual mislevelment. These levels are very popular because of their ease of use, but they do suffer from instability as mentioned later. The tilting levelFigure 3 shows a current model of a simple tilting level supplied by Hall and Watts Limited. The upper part of the instrument consists of a telescope and a sensitive bubble. An inclined mirror enables the bubble to be viewed from the eyepiece end of the telescope and this, when folded down, protects the bubble.Using the fine-setting screw mounted just below the eyepiece of the telescope, the bubble assembly can be tilted vertically to centre the bubble and so define a horizontal line of sight.The telescope may be directed along any particular line of sight by means of a clamping screw, slight lateral movements to the left or right of this line being made with the slow motion or tangent screw. A small circular bubble set in the lower part can be used to approximately level the instrument using a quick-release ball and socket base, and this is why the instrument may be referred to as a quick set level.There are many possible variations in design. Figure 4 shows another Watts level, mounted in a more conventional three-screw base and provided with a prismatic coincidence reader.Prismatic coincidence bubble readerThis refinement, designed to eliminate excessive movement around the instrument, is the coincidence bubble reader, and the device is now fitted to many, if not most, modern instruments. An arrangement of prisms reflects an image of both ends of the bubble into the eyepiece. The image is split down the centre, and one-half of each end is seen next to the other half in the bubble reader eyepiece.As the telescope is tilted, the two halves appear to move in opposite directions. This magnifies the actual movement twofold and, in the Watts model for example, this is further magnified by the optical system, which makes very exact setting possible. Consequently, bubbles of lower sensitivity than would otherwise be necessary can be employed, and this greatly improves stability and ease of setting. Setting upThe following sequence of operations is required to bring a tilting level ready for use. (a) Screw the lower plate of the instrument on the head of the tripod, whose legs have been opened and firmly fixed on the ground. (b) The circular bubble should be brought to its central position, using the foot screws or ball and socket assembly. If using an instrument fitted with foot screws, the best procedure is as follows. Referring to the Figure, by rotating foot screws 1and 2 in opposite directions at the same time the bubble can be set on the line 12. Now, by rotating foot screw 3 only, the bubble can be centred in the target ring. (c) It is essential that parallax between the cross-hairs and the image of the levelling staff be eliminated, for reasons already explained. There is no doubt that failure to do so is responsible for much error in levelling. To eliminate parallax: (i) turn the telescope to the sky, or hold a piece of white paper in front of the objective, and focus the eyepiece so that the cross-hairs appear clear and distinct. This is usually achieved by turning the eyepiece, which is threaded into the telescope barrel. It must be realized that the eyepiece setting depends on the characteristics of the surveyors eye, so that it will vary from one person to another; for one given operator, the setting will not vary; (ii) now sight the levelling staff and focus its image with the focusing screw so that when the eye is moved slightly there is no relative movement between the image and the cross-hairs.(d) Center the sensitive bubble using the tilting screw before every reading. (b), (c) and (d) can be referred to as temporary adjustment. New Words and ExpressionsLevel：n 水平；水准器，水准仪；水平面，水平线；水准测量；级，电平，能级；均衡；高度，程度； vt 置平，抄平，使水平，瞄准，对准；vi 变平，拉平Dumpy level：定镜水准仪telescope：望远镜vertical spindle：垂直轴，竖轴levelling head：三角架头plate：版，板，片；底片，感光板；刻图片，印刷版，印张；整页插图；电镀screw：螺旋，螺钉，螺丝tripod.：三角架levelling screw：倾斜螺旋（水准仪的），安平螺旋foot screw：脚螺旋adjustment：调整，调节，校正；平差carry out：完成，实现，贯彻，执行bubble：气泡，水准器tilting level：倾斜水准仪rigidly：坚硬地，严格地tilt：（使）倾斜，（使）翘起vertical plane：铅垂面，竖面fine-setting screw ：微调螺旋eyepiece：目镜collimation：视准，瞄准，准直，平行校正collimation axis：视准轴collimate：照准，视准，准直，平行校正；确定象主点perpendicular：垂直的，正交的vertical axis：竖轴，纵轴robust：精力充沛的，健壮的，稳健的automatic level：自动安平水准仪available：可利用的，可得到的compensate：偿还，补偿compensating device：补偿装置pe