测绘外文翻译外文文献英文文献水准尺和水准仪

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　Level Rods and Lenels

　 There are many kinds of lenel rods

　are in one piece and others (for ease of transporting) are either telescoping or

　rods are usually made of wood and are graduated from zero at the

　may be either selfreading rods that are read directly through the telescope or targetrods where the rodman sets a sliding target on the rod and takes the reading directly. Most rods serve as either self-reading or as target rods. Among the several types of level rods available are the Philadelphia rod,the Chicago rod, and the Florida rod. The Philadelphia rod, the most common one, is made in two sections. It has a rear section that slides on the front section. For readings between 0 and 7 ft, the rear section is not extended; for reading between 7 and 13 ft, it is necessary to extended the rod. When the rod is extended,it is called a high rod. The Philadelphia rod is distinctly divided into feet, tenths, and hundredths by means of alternating black and white spaces painted on the rod. The Chicago rod is 12 ft long and is graduated in the same way as the Philadelphia rod, but it consists of three sliding section. The Florida rod is 10 ft long and is graduated in white an red stripes, each stripe being

　ft wide. Also available for ease of transportation are tapes or ribbons of waterproofed fabric which are marked in the same way that a regular level rod is marked and which can be attached to ordinary wood strips. Once a job is completed, the ribbon can, be removed and rolled up. The wood strip can be thrown away. The instrumentman can clearly read these various level rods through his telescope for distances up to 200 or 300 ft, but for greater distances he must use a target. A target is a small red and white piece of metal attached to the rod. The target has a

　vemier that enables the rodman to take a reading to the nearest

　ft. If the rodman is taking the readings with a target and if the line of sight of the telescope is above the 7-ft mark, it is obvious that he cannot take the reading directly in the normal fashion. Therefore, the back face of the rod is numbered downward from 7 to 13 ft. The target is set at acertain mark on the front face of the rod and as the back section is pushed upward, it runs under an index scale and a vernier which enables the rodman to take the reading on the front. Before setting up the level the instrumentman should give some though to where he must stand in orde to make his sights. In other words, he will consider how to place the tripod legs so that he can stand comfortably between them for the lay-out of the work that he has in mind. The tripod is desirably placed in solid ground where the instrument will not settle as it mose certainly will in muddy or swampy areas. 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 four-screw 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 three-screw 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 distance-measuring instruments (ED-MIs). These devices determine lengths based on phase changes that occur as eletromagnetic 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 invisible microwaves and was capable of measuring distances up to 50 mi (80km) or

　or night. The potential value of these early EDM models to the Surveying profession was immediately recognized: houever, they were expensive and not readily portable for field operations. Furthermore, measuring procedures were lengthy and mathematical reductions to obtain distances from observed values were difficult and time-consuming. In addition. The range of operation of the first geodimeter was limited in daytime use. Continued research and development have overcome all these deficiencies. The chief advantages of electronic surveying are the speed and accuracy with which distances can be measured. If a line of sight is available, long or short lengths can be measured over bodies of water or terrain that is inaccessible for taping. With modern EDM equipment, distance are automatically displayed in digital form in feet or meters, and many have built-in microcomputers that give results internally reduced to horizontal and vertical components. Their many significant advantages have revolutionized surveying procedures and gained worldwide acceptance. The long-distance measurements possible with EDM equipment make use of radios for communication, which is an absolute necessity in modern practice.

　One syetem for classifying EDMIs is by wavelength of transmitted electromagnetic

　energy ; the following categories exist : Electro-optical instruments

　Which transmit either modulatedlaser or infrared light having wavelengths within or slightly beyond the visible region of the spectrum. Microwave equipments

　Which transmits microwaves with frequencies in the range of 3 to 35 GHz corresponding to wavelengths of about

　to

　mm. Another classification system for EDMIs is by operational range . It is rather subjective , but in general two divisions fit into this system : short and medium range .The short-range group includes those devices whose macimum measuring capability does not exceed about 5km . Most equipment in this division is the electro –optical type and uses infrared light . These instruments are small, portable, easy to operate, suitable for a wide variety of field surveying work, and used by many practitioners. Instruments in the medium-range group have measuring capabilities extending to about 100 km and are either the electro-optical (using laser light) or microwave type. Although frequently used in precise geodetic they are also suitable for land and engineering surveys. Longer-range device also available can measure lines longer than 100km,but they are nit generally used in ordinary surveying work. Most operate by trasmitting long radio waves, but some employ microwaves.

　They are used primarily in oceanogaraphic and hydrograpgic surving and navigation.

　In general, EDM equiment measures distances by comparing aline of unkown length to the known wavelength of modulated electromagnetic energy. This is similar to relating a needed distance to the calibrated length of a steel tape.

　Electromagnetic energy propagates through the atmosphere in accordances with the following equation:

　V=fλ

　 (1) Where Vis the velocity of electromanetic energy, in meters per second;f the modulated frequency of the energy ,in hertz, and λthe wavelenth, in meteres. With EDMIs frequency can be precisely controlled but velocity varies with atmophere temperature, pressure,and humidity. Thus wavelength and frequency must vary in conformance with EQ.(1). For accurate electronic distance measuement, therefor., the atmosphere must be sampled and corrctios made accordingly. The generalizedprocedure of measuring distance electronically is depicted in . an edm device, centered by means of a plumb bob or optical plummit over staton A, trasmits a carrier signal of electromagnetic energy upon which a reference frequency has been superimposed or modulated. The signal is returned from staion B to the revevier, so its trvel path is double the slope distance AB. In ,the modulated electromagnetic energy is represented by a series of sine waves having wave-length λ. Any position along a givenj wave can be specified by its phase angle, which is 0°at its beginning, 180°at the midpoint, and 360°at its end. EDM devices used in surveying operate by measuring phase shift. In this procedure, the returned energy undergoes a complete 360°phase change for each even multiple of exactly one-half the wavelength separating the line-s endpoints. If, therefore, the distance is precisely equal to a full multiple of the half-wave-length, the indicated phase change will be zero. In

　example, stations A and B are exactly eight half-wavelengths apart :

　hence, the phase change is zero. ...

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