What is Ranging in Surveying? Direct and Indirect Ranging

What is Ranging in Surveying?

Ranging is a surveying technique that is used to measure distances between objects. It is typically used to measure the distances between points on the surface of the Earth and targets in space. Ranging can be used to calculate the altitude or direction of an object.

Ranging is the process of establishing an intermediate point on a straight line between two end points. This is often done by measuring distance with a straight edge. In its broadest sense, it is used to measure any distance.

The chain or tape must be stretched along the survey line that connects two terminal stations while measuring the survey lines.

Ranging, in general, can be defined as the process of establishing several intermediate points to measure the survey line.

When the length of the line to be measured is less than the length of the chain, the measurement is smooth. If the length of the line is greater, the survey lines must be divided by specific intermediate points before the chaining process can begin. This is known as ranging.

Types of Ranging

Direct Ranging

When the two survey stations are inter-visible, i.e., the two endpoints of the survey line are visible, direct ranging is used. Direct ranging can be done by eye or with the assistance of an eye instrument. It can also be of two types.

Direct ranging by eye

This type of ranging is carried out, when the survey stations are visible to the rangers or surveyors.

The surveyor observes and records the angle or bearing of the line to be measured from his end point (base point) and then walks towards his other end point (terminal station) until he gets another view of the line.

He also notes down the angle or bearing at this point. He walks back to his base station with a set of angles recorded for two points on the survey line. The process is repeated until the required length of line is measured.

For instance;

Let A and B be the two inter-visible points at the ends of the survey line
The surveyor then stands with a ranging rod between points A and B.
Then, using the ranging rod, another person fixes or establishes an intermediate point between the lines AB. Let C be the established intermediate point.
It should be noted that the distance between the intermediate point and the first point A of the survey line is not greater than one chain length.
The surveyor at point A then signals another person at the intermediate points, resulting in the ranging rod being perfectly aligned with endpoints A and B.
As a result, the intermediate points are identified.

Direct ranging by Ranger

Ranging with a line ranger: In this case, intermediate points are fixed directly using a line ranger. The line ranger is an instrument that has either two plane mirror arrangements or two isosceles prisms that are placed one over the other.

The prism’s diagonals are arranged and silvered in such a way that they reflect incident rays.

For instance:

Consider two survey line endpoints, A and B.
The surveyor begins by placing two rods at points A and B.
Let C be the fixed intermediate point.
The surveyor then places the line ranger at the intermediate point C based solely on visual judgment.
Similarly, the rays from point B are received by the ranger’s upper prism. The diagonal then reflects this ray back to the observer.
As a result, the observer can see both images of the ranging rods at points A and B.
Initially, the images of the ranging rods are not in the same vertical line.
As a result, the surveyor moves the line ranger until the two images of the ranging rods form the same vertical line.
Once the images are aligned, the plumb bob is used to transfer point C to the ground.

The main advantage of this method over the eye judgment method is that it can be easily performed by one person. This method also achieves a higher level of accuracy.

Indirect Ranging

It is also known as reciprocal ranging. It is used when the survey line’s two endpoints are either not visible to each other or are separated by a large distance.

For example, consider A and B to be the points that are not visible. The following steps are now being taken.

A1 and B1 are thought to be in such a way that A and B1 are visible from A1 and A1 and B are visible from B1.
A1 is moved to position A2, so that A, A2, and B1 are all in the same line.
Similarly, B1 is moved to B2 so that A2, B2, and B are all in the same line.
The process is repeated until all of the points are in the same line, at which point intermediate points are found.

Errors in Ranging

Incorrect adjustment of ranging rods between two survey line points- This occurs when the ranging rods are not vertically aligned with the survey line and form an angle in their correct alignment.
Inaccurate reading in mountainous areas due to change in elevation of the ground- In these areas, the survey line’s angle is not correctly determined by eye or by light reflection from mirrors.
Loss of sight of one survey station- This occurs when a surveyor loses sight of one survey station, usually due to certain trees being between two stations.
Wandering – This is mainly caused by the inaccuracy of the instrument used for ranging and also by the surroundings, especially if trees or buildings are present in close proximity. The possibility of wandering increases if it is a cloudy day or under artificial light.

Instruments Used

The instruments used in Ranging are:

Ranging Rods

Ranging rods are made of wood or metal. They are usually between 1 and 2 meters in length. The surveying instruments that they support depend on the type of ranging to be performed. For instance:

Apart from a ranging rod, a level prism is also required to be used with the rod to determine whether the rod is inclined at an angle, or if it is horizontal to the ground.

The rod can be graduated for measuring angles (in degrees) but it cannot be read easily due to its short length.

Offset Rods

In cases where the base station is far from the terminal point, and it is difficult to observe the offset from the base station, a ranging rod can be placed at ground level.

The offset rod is then moved along the route of the survey line until it is visible to a surveyor.

The surveyor can then read and record this offset on his ranging rods.

Ranging Poles

A pole is placed on the ground at the base station, and a sighting line is extended from the base station to the point on which it is desired to fix or establish an intermediate point.

Rules for Ranging

The surveyor should never locate the baseline himself because he cannot be sure that the baseline is perfectly horizontal.
A ranging rod should never rest on anything but the ground, or on a self-leveling instrument such as a level rod or prism set atop a tripod.
Never look through the ranging rod’s diagonals while a prism is being employed, unless the rod is silvered on both sides.
Never allow anyone to stand in front of a ranging rod while it is in use.
Never set up a ranging rod when it is likely to be struck by lightning.
Be sure that the offsets are recorded on the field notes and are not affected by any later movement of instruments or personnel, or corrected for any non-adjustments between the base station and terminal point.
In case a prism is used, haze should be avoided as it can interfere with the vision of the ranging rod’s image and make them difficult to observe.
If the base station is on high ground, and viewing the terminal point from there causes a loss of sight of one or more intermediate points, then such intermediate points shall be located by direct ranging.

Purpose of Ranging in Surveying

The main purpose of ranging is to provide a means for ensuring accuracy in determining the position of points on a survey line.

A secondary purpose is to determine the position of points between two survey line points when these intermediate points are not visible from each other or from the base station.

In summary, the purposes are:

To determine the position of points between two survey line points in order to complete a triangle or a quadrilateral.
To determine the distance between two survey line points when it is not possible to do so using triangulation.
To determine the elevation of objects from a known point, such as short towers, etc.
To verify the accuracy of a compass and of instruments being used in triangulation or it may be needed for quality control checks on instruments being used for triangulation
To detect any changes in the position of points when they are compared with triangulation or topographic maps

History of Ranging

The history of ranging goes back to ancient times. The Romans used an instrument called a specula which was a rod or pole, used along with a measuring device called a gnomon to measure the angle between two objects.

A similar instrument was first described by Campanus (1602-1678) of Vienna, Austria. Campanus was a noted astronomer, mathematician, and surveyor. In 1630 he introduced his instrument for the measurement of angles in surveying.

The modern ranging rod or staff as it is called in the US was invented by Sir William Emerson (1805-1881) and W J Mudge (1786-1868), who were great pioneers of modern surveying techniques. Sir Emerson and W J Mudge made great strides in making accurate survey possible.

They also introduced the use of instruments for determining distances rather than sighting measurements.

It is important to note that using triangles for measuring distances was neglected by the Romans, who used a map and a measuring stick.

Sir William Emerson, who along with Mudge was credited with creating the modern technique of triangulation, applied the principle of ranging when surveying England in 1839.

A starting point was set at Dover, then distance and angles were measured in all other directions until it met up with its opposite at Calais.

The survey was carried out by instruments, including a prism and theodolite, which were used to make an estimation of the distance between points.

Ranging FAQs

What is ranging and its type?

Ranging-the process of establishing an intermediate point on a straight line between two end points is known as ranging. There are two methods for ranging.

1) Direct ranging 2) Indirect ranging or (reciprocal ranging)

What is do you mean by ranging in surveying?

Ranging is the process of establishing an intermediate point on a straight line between two end points.

How can we range out the survey line?

Before a survey line can be chained, it must be rang. When the chain line is much longer, it may be necessary to establish a number of intermediate points before chaining.

Ranging can be done directly with the naked eye, with a line ranger, or with a Theodolite.

What is the difference between direct and indirect ranging?

Direct ranging is only possible when the end stations are visible to each other. The process of interpolating intermediate points by using reciprocal ranging or running an auxiliary line.

Indirect ranging is used when the endpoints are not visible and the ground is high.

How we can find the intermediate point by using ranging?

To find the position of an intermediate point between two endpoints, with the aid of a compass and a Theodolite.

What is ranging rod?

It is a pole that has a plumb bob at the top and is used to locate an intermediate point between two end points.

What is the meaning of ranging?

Meaning of ranging: a surveying method for establishing one or more points between two known or sighted points on the ground by measurement of distance and angle.

Ranging can be done visually, for example with a Theodolite, to observe the difference in elevation between stations.

Which is the best ranging rod?

A ranging rod can never be the “best” because there are no two rods that are the same, even though they may look the same.

The level of accuracy required by a project will determine whether a wooden or metal ranging rod would be needed.

What is used in ranging a tower?

A prism of 10x magnification is used in ranging towers so as to give a wider field of view, which makes observation easier.

What is a ranging line?

This is a line that is fixed at a given distance from the base station. The ranging line is used to extend the range of a surveyor’s compass.

What is the difference between ranged and measured?

When you are surveying, you take measurements, which are based on comparison with instruments. When you are ranging, you measure distances by observation only; no instrument has been used.

How the ranging is done in plane and hilly areas?

Hold a ranging rod vertically at arm’s length at P. Thumb and forefinger should be used to hold the rod lightly.

The surveyor now instructs his assistant to move the ranging rod to the left or right. Until all three ranging rods are in the same straight line.

The line angle is then measured and the distance between the two ends of the line is noted. The surveyor takes a few steps along the base station, observing the end stations from time to time.

Is ranging safe?

Ranging is a safe procedure when a number of safety precautions are taken.

What are the advantages of ranging?

It is accurate, quick, and inexpensive. It requires no instruments and can be used to range any terrain. It can be used from any distance from the end stations.

What is the procedure for ranging in-plane areas?

The first step is to set up two known points P1, P2 and then to set out a line at right angles to them.
A surveyor locates the range line and then moves the ranging rods in a straight line.
The surveyor notes the position of P1 and P2 on the base station, including direction and distance.
Then another rod is moved along its base station at right angles to the first, ending up with rods in a horizontal plane as shown.
This procedure is followed till all three rods are in the same horizontal plane.
Each of the positioning rods is then moved along its base station to obtain a measure for the angle.
The distance between P1 and P2 is now recorded, then P1 and P2 are read again from the base station. The angle is adjusted if necessary to obtain the correct measurement.

Where do you place ranging rods?

Ranging rods are normally placed at every 10 m along a survey line.

Use a prism which will give a wider-angle view and make observations easier.

How do you use ranging rods?

A ranging pole should be held loosely between the thumb and index finger, about 10 cm above the soil. The assistant loosens the ranging pole when the observer indicates that it is in the proper position.

What is the minimum number of ranging rods required for ranging?

The minimum numbers of ranging rods required for ranging depend upon the type of terrain being surveyed, the width of the base station, and the length of the survey line.

What is the distance between ranging poles?

The spacing between ranging poles, in direct ranging, will be:

For each side of a square – Surveyor’s chain length.

Can we use the metric rod for ranging?

Yes, if required. But the rods used for ranging should not be mounted at any height above the ground and should not touch any object while taking sighting.

Ranging may be done by using an artificial ground.