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April, 2002 SURVEYING a site has always been the most cumbersome part of a civil engineering job. The engineers have to explore such tracts of land that are inaccessible, full of mud or slush, strewn by reptiles, having neck-high thorny bushes and mountain-like sand dunes. Knee-high gum boots, tough denim clothes, dark glasses and a felt cap may be helpful in protection of the body but the long hours consumed in carrying out an accurate survey work may leave their tell-tale signs on the body and the face of an engineer. At such times, availability of such precision instruments that may be helpful in quick and accurate survey work may prove to be a boon to the surveying team. Chain-links, compasses, steel tapes and plane tables have been the common survey instruments much in use and explained in engineering colleges even today. A Theodolite and A Level are quite frequently used these days but these also involve a lot of manual calculations and field-book recordings. Constant efforts have therefore continued to make them easier and faster to operate, more versatile, more accurate and disturbance-proof. A host of precision instruments have now been developed that are so helpful in survey work that surveying may look like a cakewalk in future. A few of them are discussed here: Automatic Levels: Dumpy Levels, used now to find the level difference between two points and to mark points at different heights with respect to a given point suffer from the problems of setting up the instruments as it takes a long time to set up these levels. Any sort of vibrations in the vicinity of these Level stations disturb their setting. Automatic Levels now produced level themselves automatically, once these are roughly levelled. Even the vibrations of the heavy machinery moving nearby don’t cause any disturbance as these Levels keep applying correction to themselves as per the variation in the vibrations produced by the machinery. In addition, these Levels provide a sharp and continuous laser beam which ensures full visibility to the user at a given grade. The grade for the beam can be adjusted by the user. A continuously rotating beam with a given grade is also provided, thereby giving a plane in the same grade. A single touch button sets the Level ready to measure, display and record the readings. These Levels have very high degree of accuracy, measuring up to 0.4mm dimension with time taken for a measurement less than 4 seconds. Magnification is 24 times and the Staff used is of fibre glass. Electronic Theodolites: The conventional theodolites being used these days for survey work need to be operated manually as all their components are mechanical and take time for adjustment. These theodolites threaten a large chance of human error. Generally the accuracy in survey work is checked by the closing of the traverse. In doing so, the error does not get eliminated but gets divided. Electronic Theodolites now developed are very easy to use, fast in measurement and highly accurate, to a degree of 0.1 second. Their output can be seen directly on the computer or can be recorded on an electronic field book. Magnification provided by these theodolites is 30 times. Their weight is quite less, not more than 3.5 kg. Selection of the electronic theodolite can be made with respect to the accuracy required. Normally, theodolites having an accuracy of 20 to 60 seconds, if used, give reasonably good results. Global Positioning Systems: Global Positioning Systems, known in short as GPS, are portable instruments that receive signals from satellites that are specially launched for survey work. These instruments are very useful in surveying for tunnels, parking garages or in dense forests where the signals get obstructed due to dense growth of trees. By using GPS, data can be collected for preparation of Topographic maps. Even seismic and geodetic control surveys can be performed by use of GPS. Global Positioning Systems use reflector-less Electronic Distance Meter technology. By use of this technology, distances can be measured at a very high speed and to utmost accuracy. Measurements take a fraction of a second. A single touch button gives the distances ranging from the fraction of a metre to many kilometres. Total Stations: A Total Station is a combination of an electronic theodolite and an Electronic Distance Metre. It is an extremely useful survey instrument. It carries out the survey work to determine the angles between two points in a horizontal plane and between the true vertical and a given point. It processes the data immediately and produces all calculations in a fast manner. It can measure the distance between two points that are not accessible because of existence of a river or a large building between them. It can also measure the height of a very tall structure such as chimneys, cooling towers, multi-storey building or the H.T. wires above ground level. It can also set out distances up to an accuracy of millimeters. In general, while a theodolite measures the angles and bearings and the tapes or chains measure the linear distances, the recordings thus made are put to manual calculations to work out the ( X, Y, Z ) coordinates of a target-point. In a Total Station, there is an in-built software that itself analyses the recordings and gives the three coordinates directly. Leica Total Station instruments have almost caused a revolution in the field of surveying by having a memory of 4000 data points and taking readings up to 3 kilometre distances from a single station. Contour plans and topographical surveys can also be easily prepared with the help of Total Stations. Laser Tracking Systems: A laser based tracking system that can measure creep in tunnels, sway of tall buildings and can plot the movement of resonant structures to a high degree of accuracy has now been developed. Called Laser Tracking System, it can record up to 1000 readings per second ! Tall chimneys are designed to tolerate a sway of 1/500th of their height at the top. Thus a 220-metre high chimney for a power plant can sway up to 440 cm either way. It is very important to measure this sway in actual as any increase in it may not be good for the health of the chimney. Laser Tracking System becomes very useful here. Not only the sway of a tall structure, a LTS can even measure the movement of a number of moving objects. It projects an eye-safe laser beam towards the target reflector attached to the object to be measured. The LTS detects the beam when it is at the centre of the reflector and automatically locks the reading. It keeps following the target and readings are recorded themselves. Digital Planimeters: To work out the area of a map or an irregular figure or a drawing or a photograph, Digital Planimeters have now been developed. These are battery operated, handy instruments consisting of an electro shaft encoder that generates pulses with respect to the rotation of a measuring wheel which is moved over the outline of the area to be measured. The pulse recordings are processed by large scale integrating circuits and the area is displayed on the Planimeter’s LCD display. Portable Weigh Bridges: While the above instruments help in survey work and in making linear, angular and sway measurements, another invention to make weight measurements is worth mentioning here. It is the Portable Weigh Bridge which uses electronic technology and is cordless, flexible and portable. Portable weigh bridges are fully mobile and can be installed within a few minutes. These give printouts of the axle loads, gross weight, tare weight and net vehicle weights. Even moving vehicles, having a speed up to 6 km /hour, can be weighed by these bridges.
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