There are various techniques of making linear measurements, and their corresponding merit depends upon the degree of precision required. These methods are three types.
- Direct measurements
- Measurements by optical means
- Electronic methods
The distances are measured actually in the case of direct measurements. On the ground with the help of a chain or a tape or any other instrument. In the optical methods, a telescope is used for taking observations and calculations are done for measuring the distances, such as in tachometer or triangulation. In the electronic methods, the distances are measured with instruments that rely upon on distribution, absorption and subsequent reception of either light waves or radio ray. The instruments that usually use in the electronic methods are
- The lambda position fixing system and
- the Decca Navigator
In the case of Geodimeter, the method of measurement is based on the propagation of modulated light waves. But for distance measurements, the other three instruments use radio waves. The various methods of measuring the distances directly are as follows:
- Measurement with pedometer
- Measurement with pedometer
- Measurement by odometer and speedometer
Pacing Measurements of distances by pacing is chiefly confined to the preliminary surveys and explorations where a surveyor is called upon to do a rough survey as quickly as possible. It may also be used to check the distances measured by other means roughly.
The method consists in counting the number of paces between the two points of a line. The length of the line can then be computed by knowing the average length of the pace. The length of the pace varies with the individual and also with the nature of the ground. The inclination of the region and the speed of packing. A length of pace more nearly that of one’s natural step is preferable. The length of one’s natural step may be determined by walking on fair lap level ground over various lines of known lengths. One can soon learn to pace distances along the level. Unobstructed ground with a degree of accuracy pacing equivalent approximately to 1 in 100. However pacing Over rough ground on the slope may be difficult.
Passometer is an instrument shaped like a watch and is carried in the pocket or attached to one leg. The motion of the body operates the mechanism of the instrument, and it automatically registers the number of paces, thus avoiding the monotony and strain of counting the paces, by the surveyor. The number of paces registered by the passometers can then be multiplied by the average length of the pace to get the distance.
Pedometer is an instrument similar to the passometers except that, adjusted to the length of the pace of the person carrying it, is registers the total distance covered by any number of paces.
(4) Odometer and Speedometer:
The odometer is a device for displaying the number of revolutions of a wheel. The well-known speedometer works on this system. The odometer is suited to a wheel which is rolled along the line whose length is required. The number of revolutions registered by the odometer can then be multiplied by the circumference of the wheel to get the distance. Since the instrument registers the length of the surface passed over, its readings obtained on the undulating ground are inaccurate. If the route is smooth, the speedometer of an automobile can be used to measure the distance approximately.
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