Senin, 28 Maret 2011
Types of Survey & Surveying Techniques
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Politeknik Negeri Medan - Teknik Sipil 09' |
The techniques used for surveying and land measurement are dependent upon the type of survey being conducted. In this section there are two choices:
Types of survey
Techniques
Types of Survey
A simple classification is:
1. Land surveys, which fix property lines, calculate land areas and assist with the transfer of real property from one owner to another.
2. Engineering surveys, which collect the data needed to plan and design engineering projects. The information ensures the necessary position and dimension control on the site so that the structure is built in the proper place and as designed.
3. Informational surveys obtain data concerning topography, drainage and man-made features of a large area. This data is portrayed as maps and charts.
Another way to make a simple classification is:
1. Geodetic surveys are precise and over large areas require the curvature of the earth to be considered. Distances and angle measurements must be very, very accurate. A wide variety of techniques are used including triangulation, traversing, trilateration, levelling and astronomical direction fixing.
2. Plane surveys, which consider the surface of the earth to be a plane. Curvature is ignored and calculations are performed using the formulas of plane trigonometry and the properties of plane geometry. These may be considered accurate for limited areas.
Sub-categories of the major classes provide more insight into the various fields of surveying as follows:
· Property surveys determine boundary lines, property corners, rights-of-way provide data necessary for the preparation of land sub-divisions.
· Cadastral surveys are executed by the Federal Government in connection with the disposal of vast areas of land known as the public domain.
· Route surveys are necessary for the design and construction of various engineering projects such as roads, railways, pipelines, canals and powerlines.
· Industrial surveys, or optical metrology, are used in the aircraft and other industries where very accurate dimensional layouts are required.
· Topographic surveys are performed to gather data necessary to prepare topographic maps. These are multicolour contour maps portraying the terrain; and rivers; highways, railways, bridges and other man-made features.
· Hydrographic surveys map the shorelines of bodies of water; chart the bottom of streams, lakes, harbours and coastal waters; measure the flow of rivers; and assess other factors affecting navigation and water resources. The sounding of depths by radar is involved in this type of survey.
· Mine surveys determine the position of underground works such as tunnels and shafts, the position of surface structures and the surface boundaries.
· Aerial surveys use photogrammetry to produce a mosaic of matched vertical photographs, oblique views of landscape and topographic maps drawn from the photographs.
· Construction surveys fix elevations, horizontal positions and dimensions for construction projects.
· Control surveys provide basic horizontal and vertical position data. These are called datum. For most surveying work the vertical position of points in terms of height above a curved reference surface is mean sea level.
Techniques
1. Triangulation
2. Trilateration
3. Traverse
4. Leveling
5. Radiation
Triangulation
Triangulation consists of a series of connected triangles which adjoin or overlap each other, angles being measured from determined fixed stations. Triangulation reduces the number of measures that need to be taped and for this reason is often a preferred method of survey. A known base-line measurement is required. Three examples of triangulation systems are shown below.
A single chain of triangles is a rapid and economical system for covering a narrow strip of land. A chain of quadrilaterals is more accurate with checks being made by various combinations of angles and sides as the survey proceeds. Larger areas use a central point arrangement. A point to note is that all angles should be more than 20°. Angles less than 20° are not considered valid for fixing position. They introduce inaccuracies. This is much the same in navigation where a fix by two bearings requires an angle of intersection of approximately 90°, and for three bearings approximately 60°. Angles less than 30° are not acceptable.
Trilateration
Trilateration uses electronic distance measuring equipment to directly measure the lengths of the sides of triangles from which the angles can be calculated. It is a very useful method for rough terrain where positions can be accurately carried forward and is seen as an alternative method to triangulation.
Traverse
A traverse consists of a series of lines, whose lengths and directions are measured, connecting points whose positions are to be determined. The route of the traverse line can be adjusted for obstacles such as rough or timbered terrain, swampy land, buildings and areas of heavy traffic. A traverse may be either open or closed as shown below.
An open traverse begins at a point of known position and ends at a station whose relative position is unknown. This type of traverse is frequently used for preliminary surveys for highways. A closed traverse begins and ends at the same point whose position is known. An example of this type of traverse is a perimeter survey of a tract of land.
Leveling
Leveling is the operation of determining differences of elevation by measuring vertical distances directly on a graduated rod with the use of a leveling instrument such as a dumpy level, transit or Theodolites. This method is called direct leveling or differential leveling. Indirect leveling can be done using the principle that differences in elevation are proportional to the differences in atmospheric pressure. The difference in elevation between two points can also be determined trigonometrically using vertical angles and horizontal or inclined distances.
Differential leveling
Benchmarks are very important in leveling. They are permanent objects of known elevation located where there is least likelihood of disturbance. They may be a metal or concrete post set close to the ground, a notch in a tree root or a peg or spike in a tree.
Radiation
Radiation is a surveying technique often used in conjunction with a plane table. From a fixed position directly above the corresponding location on the ground bearings are taken to various points at the boundary of the survey area. The bearing lines are drawn on the paper on the table. Distances to the points are measured and then converted to the required scale on the survey sheet. Radiation surveying can be performed in a similar way directly on the ground taking bearings and distances from a fixed position and recording them for later work. Similarly plane table surveying may be used with other techniques such as a traverse or triangulation.
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Surveying
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