What Is Ranging?
Ranging is the process of determining locations of intermediate points to enable accurate measurements of survey lines. It involves the use of ranging rods, offset rods and ranging poles to accurately locate these points. This method of surveying helps in obtaining more precise measurements and results in higher accuracy.
Ranging Out Survey Lines
Measuring Survey Lines
When conducting a survey, it is important to accurately measure the survey lines. This can be done by stretching a chain or tape along the desired line, joining two terminal stations. If the length of the line is shorter than the chain, then the measurement process is relatively straightforward. However, if the length of the line is greater, the survey lines must be divided by using intermediate points, a process known as ranging.
Direct Ranging
Direct ranging is a process used to accurately measure survey lines of greater length. During this process, the chain or tape is divided into multiple sections and each section is measured separately. This allows for a greater accuracy of measurement, as each individual section can be measured with precision.
Indirect Ranging
Indirect ranging is an alternative method for measuring survey lines of greater length. This method involves measuring the angle between two sides of a triangle and then calculating the length of the third side. This method is helpful when the line to be measured is curved, as it allows for a more accurate measurement of the line’s length.
1. Direct Ranging
What is Direct Ranging?
Understanding Direct Ranging
What is Direct Ranging?
Direct ranging is the process of measuring distances between two points when the intermediate points are visible from each other. It can be done with the naked eye or with the help of an eye instrument.
Understanding Direct Ranging
Direct ranging is a technique used to measure distances between two points where the intermediate points between them can be seen from each other. This technique can be done by the naked eye or with the help of an eye instrument. This method is useful in surveying and mapping, as it allows for accurate and efficient measurements.
Ranging by Eye
Achieving Intervisibility between Two Points
In order to establish intervisibility between two points, A and B, a surveyor is necessary. The surveyor stands at point A, and holds a ranging rod at point B. The ranging rod is kept at approximately half a metre in length. Figure-1 illustrates this process.
How to Establish an Intermediate Point
- The assistant takes the ranging rod and establishes a point in between AB, close to the line of AB.
- The distance from the point A should not exceed one chain length.
- The surveyor gives signals to the assistant to move the traverse until the rod is in line with points A and B.
- By this method, other intermediate points are determined.
Ranging by Line Ranger
Figure 2: Line Ranger
Overview
Figure 2 shows a line ranger that is composed of two plane mirrors or two isosceles prisms placed one over the other. The diagonals of the prisms are silvered and arranged in such a way that they can reflect incident rays.
Mirror/Prism Arrangement
The line ranger in Figure 2 consists of either two plane mirrors or two isosceles prisms placed one over the other. The diagonals of the prisms are silvered and arranged in a particular way to enable the reflection of incident rays.
Reflective Properties
The silvered diagonals of the prisms in the line ranger of Figure 2 are designed to reflect incident rays in order to produce the desired effect. This reflective property enables the line ranger to perform its intended functions.
Using a Line Ranger to Range a Point
The line ranger is a surveying instrument used to measure the distance between two points. It consists of a handle with a hook, which is used to attach a plumb bob for transferring the point to the ground. In order to range the point ‘P’, two rods are fixed at points A and B.
Transferring the Point to the Ground
The lower prism abc receives the rays coming from A which is then reflected by the diagonal ac towards the observer. The upper prism dbc receives the rays from B which is then reflected by the diagonal bd towards the observer. As a result, the observer can see the images of the ranging rods A and B, which might not be in the same vertical line. To transfer the point P to the ground, the surveyor must move the instrument until the two images come in the same vertical line. With the help of the plumb bob, the point P can then be accurately transferred to the ground.
Advantages of Using the Line Ranger
The line ranger is a useful instrument for surveying as it enables one person to locate the intermediate points without having to go to the other end of the survey line. This makes the surveying process much more efficient and eliminates the need for multiple people to be involved.
2. Indirect Ranging
Indirect Ranging: A Necessary Technique
Indirect ranging is a technique used when two points are not intervisible or are located at a long distance. It involves the use of two intermediate points, M1 and N1, located very near the chain line, such that from M1 both N1 and B are visible, and from N1 both M1 and A are visible. This procedure is shown in Figure 3.
Process of Finding Points M and N in Survey Line AB
At M1 and N1, two surveyors stay with ranging rods to find the points M and N in the survey line AB. The person standing at M1 directs the person at N1 to move to a new position N2. The new position N2 must be inline with M1B.
Using Ranging Rods to Fix Points M2 and N2
Next, a person at N2 directs the person at M1 to move to a position M2. The position M2 must be inline with N2A. With the two persons at M2 and N2, the process is repeated until the points M and N are in the survey line AB.
Fixing Points M and N Through Direct Ranging
When the person standing at M finds the person standing at N in line with NA and vice versa, points M and N are fixed. The other points can then be fixed by direct ranging.