The task of determining the horizontal distances between two existing points and of setting a new point at a specified distance from some other fixed position are fundamental surveying operations. The surveyor must select the appropriate equipment and apply suitable field procedures in order to determine or set and mark distances with the required degree of accuracy. Depending on the specific application and the required accuracy, one of several methods may be used to determine horizontal distance. The most common methods include pacing, taping, and EDM (Electronic Distance Measurement). Taping has been the traditional surveying method for horizontal distance measurement for many years. It is a direct and relatively slow procedure, which requires manual skill on the part of the surveyors.
One of the basic measurements in surveying is the determination of the distance between two points on the earth’s surface for use in fixing position, set out and in scaling. Usually spatial distance is measured. In plane surveying, the distances measured are reduced to their equivalent horizontal distance either by the procedures used to make the measurement or by applying numerical corrections for the slope distance (spatial distance). The method to be employed in measuring distance depends on the required accuracy of the measurement and this in turn depends on purpose for which the measurement is intended. Horizontal distances may be determined by many methods.
Rough Distance Measurement
In certain surveying applications, only a rough approximation of distance is necessary; a method called pacing or the use of a simple measuring wheels, may be sufficient in these instances.
Example - Locating topographic features during the preliminary reconnaissance of a building site, searching for the property corners etc.
1) Pacing
Pacing consists of counting the number of steps or paces in a required distance. This is used to provide distance estimates when no measuring device is available or precision is not required. Distances obtained by pacing are sufficiently accurate for many purposes in surveying. Pacing is also used to validate survey work and eliminate any taping blunders. Measuring your pace length requires a measured 100 feet distance. You then walk this distance and count the number of steps. It is best to repeat the process four times and average the results. In this method, distances can be measured with an accuracy of about 1:100 by pacing.
Distance = Unit Pace × Number of Paces
It is possible to adjust your pace to an even three feet, but this should usually be avoided. It is very difficult to maintain an unnatural pace length over a long distance. Accurate pacing is done by using your natural pace, even if it is an uneven length such as 2.6 feet. It is difficult to maintain an even pace when going uphill or downhill. Using your natural pace will make this easier.
Another error can occur if you are not consistent in starting with either the heel or toe of your shoe. If you place your toe at the start point, then also measure the end point with your toe. Starting with the heel and ending with the toe is a common mistake. Some surveyors prefer to count strides. A stride is two steps or paces. This reduces the counting but often requires using part of a stride to determine the total distance. Pacing is a valuable skill for surveyors. It requires some practice and concentration.
2) Odometer
Vehicle odometers are helpful in determining long distances such as for layout or checking vision at intersections. Precision of 1/20 is reasonable. Based on diameter of tires (no of revolutions x wheel diameter), this method gives a fairly reliable result provided a check is done periodically on a known length. During each measurement a constant tyre pressure has to be maintained.
3) Measuring Wheel
A simple measuring wheel mounted on a rod can be used to determine distances, by pushing the rod and rolling the wheel along the line to be measured. An attached device called an odometer serves to count the number of turns of the wheels. From the known circumference of the wheel and the number of revolutions, distances for reconnaissance can be determined with relative accuracy of about 1:200. This device is particularly useful for rough measurement of distance along curved lines. It is commonly used to record distances such as curb length or paving quantities and can also be helpful for determining distances along a curve.
Distance = Odometer Reading x Circumference of the Wheel (𝜋D)
Where D is the diameter of the measuring wheel
4) Tape
This method involves direct measurement of distances with a tape or chain. Steel tapes are most commonly used. It is available in lengths varying from 15m to 100m. Measuring horizontal distances with a tape is simple in theory, but in actual practice, it is not as easy as it appears at first glance. It takes skill and experience for a surveyor to be able to tape a distance with a relative accuracy between 1:3000 and 1:5000, which is generally acceptable range for most preliminary surveys.
5) Tachometry
Distance can be measured indirectly by optical surveying instruments like theodolite. The method is quite rapid and sufficiently accurate for many types of surveying operations.
6) Electronic Distance Measurement (EDM)
These are indirect distance measuring instruments that work using the invariant velocity of light or electromagnetic waves in vacuum. They have high degree of accuracy and are effectively used for long distances for modern surveying operations. This quickly provides very precise measurements but requires experienced personnel and relatively expensive equipment.
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