1.13. Measurement of Distances

One of the most important activities in surveying is the measurement or computation of horizontal distance between two points. If the points are at different elevations, the distance measured is the slope distance, and a vertical angle of the line joining the point is also required to compute the horizontal distance.

In surveying, the primary goal is often to determine distances between points accurately. When measuring between two points, if they are on different elevations (like a hill and a valley), the distance you measure is known as the slope distance. However, to find out the direct horizontal distance (the one that would be measured on a flat surface), you also need to know the vertical angle of the line connecting the two points. This is crucial for accurate mapping and land measurement.

Imagine two buildings on a hillside. If you want to know the horizontal distance between them, you not only need to measure the slope distance but also how much higher one building is compared to the other. This method ensures that engineers have accurate data to design roads, bridges, or other structures.

Depending on the accuracy desired in measurement, there are two broad methods of measuring horizontal distances: (a) Direct methods, and (b) Indirect methods.

There are two main approaches to measure distances in surveying. The direct methods provide straightforward measurements using physical tools or equipment at the locations of interest. In contrast, indirect methods involve more complicated measurements and calculations that often use angles and other measurements to infer the distance.

Think of direct methods like using a ruler to measure the length of a table directly, while an indirect method would be measuring the shadow of the table and using mathematical calculations to determine its height based on that shadow.

The direct methods can be employed using either of the following equipment: Pacing, Passometer and Pedometer, Odometer, Survey Chains, and Tapes.

Direct measurement methods involve physical tools that help surveyors accurately determine distances between points. For example, pacing is simply walking a distance and counting steps, while using devices like passometers and pedometers can automate this process. Odometers measure distance based on wheel rotations, and survey chains or tapes provide precise measurements on the ground.

Consider how a runner might track their distance with a pedometer that counts steps, or how a car's odometer keeps track of the miles driven. These tools help convert physical movement into measurable distance, similar to what surveyors do with their equipment.

Survey Chains are used to measure distances when great precision is not needed. Metallic chains of fixed lengths, 50 feet or 100 feet, having indicators on every 10 feet may be used. These chains can give errors in measurements for larger distances due to sag and pull.

Survey chains are physical lengths of metal that are useful for measuring ground distances, especially when extreme accuracy isn't critical. Each chain is marked at intervals (like every 10 feet), allowing surveyors to measure long distances by linking several chains together. However, for longer distances, the chains can sag under their own weight or during use, leading to measurement errors.

Imagine trying to measure the length of a large field with a heavy rope. If the rope sags in the middle due to its weight, you might end up measuring a longer distance than what’s really there. This is similar to the issues faced when using survey chains in the field.

Tapes are used to measure the distances much more accurately than chains. Linen tapes, metallic tapes, steel tapes, and Invar tapes are used depending on the precision needed.

Measurement tapes offer more precision compared to survey chains. There are various types: linen tapes are lightweight but not very durable for long distances; metallic tapes are stronger and more resilient; steel tapes provide superior accuracy; Invar tapes are specifically made for high precision tasks due to their stability against temperature changes.

Think of a tailor using a flexible measuring tape to take someone's measurements. Depending on the fabric and purpose, they choose the right type of tape to ensure they get accurate and reliable measurements, much like surveyors choose different tapes based on their measurement needs.

The distance could be measured on level ground or sloping ground with a chain or tape in a similar manner. On uneven or sloping ground, the distance is directly measured in small stretches by keeping the chain/tape horizontal.

When measuring distances using chains or tapes, surveyors place markers to define the start and end points. On level ground, they can lay the chain directly in a straight line. However, on sloping terrain, they need to keep the measuring tool horizontal and use pegs to mark the horizontal projections, carefully adding all measured sections to get the total distance.

Imagine walking up a hill with a tape measure. You place the tape horizontally at every step level to get your distance, rather than following the slope, similar to how surveyors have to adjust their measuring techniques on uneven terrain.

Ranging is a process to establish intermediate points between two stations along the same line so that measurements are made along a straight line. Ranging is carried out as (a) Direct ranging, and (b) Indirect ranging.

Ranging helps surveyors ensure that their distance measurements between two points are as straight as possible. Direct ranging requires visual estimation to place markers accurately along a line, while indirect ranging is used when visibility is obstructed, requiring collaborative adjustments from multiple points.

It's like playing a game of telephone where each person passes a message along a straight line. Each player (or surveyor in this case) has to ensure they can see and correctly communicate to the next in line, maintaining the straight path for the message (or measurement).

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