Ranging: Know types and uses

In any construction project, a proper survey of the site is absolutely essential. Surveying the site includes understanding the ground, figuring out relative positions, and preparing a blueprint for the construction.

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Ranging is an important part of the survey procedure, which is basically the act of determining intermediate points to calculate the survey line. Since the survey line is usually longer than the survey chain, fixing intermediate points makes the task easier, which is done through ranging poles or rods.

See also: Total station: Uses of the surveying tool

What is the use of a ranging rod?

A ranging rod or a ranging pole is a surveying instrument commonly used in the construction sector for measurement. It helps mark position while surveying any area. These were initially built of thin, light bamboo that was straight, or exceptionally well-prepared wood like pine, teak or deodar. These days, they are composed of fibreglass, metal, or wood.

Ranging rods typically have a 30mm diameter and are 2 to 3 metres long. These rods have two coats of paint on them. They are used primarily to mark small spaces. Because of this, it is painted with patches of alternating colours that are 200mm, 500mm, and 1 foot long. It makes it practical and simple to use. At the top, coloured flags are connected for clearance in the event of weather or considerable distance issues.

What is an offset ranging rod?

When a ranging rod does not have a flag at the top, it is called an offset rod. They are made of hardwood, and one side of them has an iron shoe. At the other end, a hook is provided. Two thin incisions at the right spots are also accommodated at the height of the eye, using it as a point of fixation and providing them with a precise 90-degree angle.

Ranging rod: Usage

In order to build something, a survey must first be conducted. Through surveying, reference points are marked, and these points serve as a roadmap for the whole construction process. Any survey that must be completed on a construction site must be longer than the chain. The measurements should be in alignment with the chain. Ranging is essentially the method of marking these intermediate points.

Types of ranging

There are two kinds of ranging methods- direct ranging and reciprocal or indirect ranging.

Direct ranging

Direct ranging refers to the placement of a ranging rod through visual observation or the use of any eye instrument. If two points are mutually visible, direct ranging is preferred.

Follow these steps for direct ranging:

Consider points A and B as two ends of the chain line where you have fixed the two ranging rods.
Suppose you need to fix the rod at a point (X) along the chain line to align A and B.
The surveyor needs to stand behind Pole A at a 2 m distance.
The assistant needs to hold the running pole vertically at X, normally at arm’s length and at a safe distance. The thumb and the first finger should be used to gently hold it.
The surveyor directs the assistant to shift the range pole to the right or left, until a precise parallel straight line is formed by all three bars.
The surveyor twists down and peers through the lower portion of the pole to make sure nothing on the pole is vertical.

When three ranging poles come together and seem as one pole, ranging will be perfect. When the surveyor is satisfied that conditions are good, they will indicate to fix the bars to the ground.

Reciprocal or indirect ranging

When the endpoints are not at the same level due to the high ground level between them, a considerable distance between them, or any turn in between, the endpoints cannot be unified. In this case, reciprocal ranging is preferred.

Follow these steps for indirect ranging:

You need to position two surveyors at, say, P1 and P2 near AB.
A person can see P1 and A with a ranging pole at P2, but they cannot see P2 and B at P1.
The person at point P1 will now be directed by person P2 to follow the person at P2 to position P3.
The person at P3 will instruct the person at position P2 to move to position P4 such that P3, P4, and B are in a straight line.

Ranging errors

Potential errors can occur while ranging, such as inaccurate positioning of the ranging rods between two points along the survey line and reading errors in uneven regions like the hills.