測繪工程畢業(yè)設計外文翻譯---水準尺和水準儀-文庫吧
2025-04-16 21:50 本頁面
【正文】 as. It may be necessary to provide some special support for the instrument, such as stakes or a platform. The tripod legs should be well spread apart and adjustde so that the footplate under the leveling screws is approximately level. The insatrumentman walks around the instrument and pushes each leg frimly into the ground. On hillsides it is usually convenient to place ong leg uphill and two downhill. After the instrument has been levelde as much as possible by adjusting the tripod legs, the telescope is turned over a pair of opposite leveling screws if a fourscrew instrument is being the bubble is roughly centered by turning that pair of screw in opposite directions to each other. The bubble will move in the direction of the left thumb. Next, the telescope is turned over the other pair of leveling screws and the bubble is again roughly centered. The telescope is turned back iver the first pair and the bubble is again roughly centered, and so on. This process is repeated a few more times with increasing care untill the bubble is centered with the telescope turned over either pair of screws. If the level is properly sdjusted, the bubble should remain centered when the telescopeis turued in any direction. It is to be expected that there will be a slight maladjustment of the instrument that will result in a slight movement of the bubble。 however, the precision of thework should not be adversely affected if the bubble is centered each time a rod reading is taken. The first step in leveling a threescrew instrument is to turn the telescope untill the bubble tube is parallel to two of the screws. The bubble is centered by turning these two screws in opposite directions. Next, the telescope is turned so that the bubble tube is perpendicular to a line through screws. The bubble is centered by turning screw . These steps are repeated untill the bubble stays centered when the telescope is turned back and forth. Electronic Distance Measurements A major advance in surveying in recent years has been the development of electronic distancemeasuring instruments (EDMIs). These devices determine lengths based on phase changes that occur as eletromagic energy of known wavelength travels from one end of a line to the other and returns. The first EDM instrument was intronduced in 1948 by Swedish physicist Erik Bergstrand. His device, called the geodimeter(an acronym for geodetic distance meter), resulted from attempts to improve methods for measuring the velocity of light. The instrument transmetted visible light and was capable of accurately measuring distances up to about 25 mi (40km) at night. In 1957 a second EDM apparatus. the tellurometer. Designed by and introduced in South Africa, transmitted invisi
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