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Different Methods of measuring Distance
Accuracy attained by pacing is sufficient for a great many purposes in surveying. Pacing consists of counting the paces in a distance you are measuring. First the person who will do the measurment must of the length of his/her pace determined.
Figure below shows a surveyer using a roller marker, or odometer, on a paved a street in a new subdivision. An odometer is a wheel which rolls on the surface of ground and converts the number of revolutions into a slope-distance measurement.
There are two basic methods for measuring distance with a tape. These mathods are slope taping and horizontal taping. In the slope taping method , the slope is held as required by the slope of the ground, the slope of the tape is measured and the horizontal distance is computed. In the horizontal taping method, the tape is held horizontally and the required graduation is projected on the ground with a plumb bob.
True D = Cos a (slope D)
Measure alone a level line
Measure increments along a level line using a plumb bob and chaning pins.
Horizontal Sights by Stadia
Shown in figure below, stadia has a focusing screw that causes the objective lens to move, where it is desired to determined the distance D, from the center of the instrument to the rod. When the surveyer looks through the transit scope he can see a vertical and s horizontal line which are called cross hairs. Also, there are two small horizontal lines at the top and bottom of vertical cross hair which are called stadia lines. The difference between those two lines is called stadia interval. Stadia distance equals the stadia interval times 100.
EDM: Electronic Distance Measurement (using laser light pulses and a prism)
Electronic distance measuring devices (EDMs) have some advantages over other measurement methods. They easiy measure lines that are difficult of access , such as across lakes and rivers, highways, standing farm corps, etc.
Precision of various measuring methods
Pacing has a precision of 1:50 to 1:200 and is usually used for reconnaissance and rough planning. The stadia planing has a precision of 1:250 to 1:1000 and it is very common in maping, rough survey, checking of other types of measurement. Ordinary taping has a precision of 1:1000 to 1:5000 and it is used for regular land survey and building construction. Precision taping has a precision of 1:10,000 to 1:30,000 and is employed for excelent land surveys, precise construction work, and city surveys. Electronic distance measuring equipment has a precision of -/+ 0.04 ft (instrument constant), -/+ 1:300,000 of length measured. It has been used in the past for precise government surveys and is now becoming more common for land development and very precise construction work.
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