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Distance measurment

الكلية كلية الهندسة     القسم  الهندسة المدنية     المرحلة 2
أستاذ المادة كاظم نايف كاظم اليساري       26/12/2017 19:54:06
Distance measurement
Distance is one of the fundamental measurements in surveying. Although frequently measured as a spatial
distance (sloping distance) in three-dimensional space, usually it is the horizontal component which is
required.
Distance is required in many instances, e.g. to give scale to a network of control points, to fix the position
of topographic detail by offsets or polar coordinates, to set out the position of a point in construction
work, etc.
The basic methods of measuring distance are, at the present time, by taping or by electromagnetic
(or electro-optical ) distance measurement, generally designated as EDM. For very rough reconnaissance
surveys or approximate estimates pacing may be suitable.
For distances over 5 km, GPS satellite methods, which can measure the vectors between two points
accurate to 1 ppm are usually more suitable.
4.1 TAPES
Tapes come in a variety of lengths and materials. For engineering work the lengths are generally 10 m,
30 m, 50 m and 100 m.
Linen or glass fibre tapes may be used for general use, where precision is not a prime consideration.
The linen tapes are made from high quality linen, combined with metal fibres to increase their strength.
They are sometimes encased in plastic boxes with recessed handles. These tapes are often graduated in
5-mm intervals only.
More precise versions of the above tapes are made of steel and graduated in millimetres.
For high-accuracy work, steel bands mounted in an open frame are used. They are standardized so that
they measure their nominal length at a designated temperature usually 20?C and at a designated applied
tension usually between 50 N to 80 N. This information is clearly printed on the zero end of the tape.
Figure 4.1 shows a sample of the equipment.
For the most precise work, invar tapes made from 35% nickel and 65% steel are available. The singular
advantage of such tapes is that they have a negligible coefficient of expansion compared with steel, and
hence temperature variations are not critical. Their disadvantages are that the metal is soft and weak, whilst
the price is more than ten times that of steel tapes. An alternative tape, called a Lovar tape, is roughly,
midway between steel and invar.
Much ancillary equipment is necessary in the actual taping process, e.g.
(1) Ranging rods are made of wood or steel, 2 m long and 25 mm in diameter, painted alternately red and
white and have pointed metal shoes to allow them to be thrust into the ground. They are generally used
to align a straight line between two points.
(2) Chaining arrows made from No. 12 steel wire are also used to mark the tape lengths (Figure 4.2).

المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .