FUNDAMENTALS OF SURVEYING : Chaining on Uneven or Slopping Ground

Chaining on Uneven or Slopping Ground

Generally, for all plotting works horizontal distances between the points are required. So it is necessary either to measure directly the horizontal distance between the points or to measure sloping distance and reduce it to horizontal distance.

There are two methods

1.         Direct method or method of stepping

> Distance is measured in small horizontal stretches.

> The follower holds the zero end ofthe tape at A while the leader selects the any suitable length L1 ofthe tape and moves forward. Then follow directs for ranging. Leader pulls the tape tight, makes it horizontal and point 1 is then transferred to the ground by plumb bob. Similarly distance is measured till B.

> In case ofirregular slopes this is only suitable method.

> It is always convenient to measure down the hill.

2.         Indirect method

> In case of regular or even slopes, the sloping distance can be measured and the horizontal distance

can be calculated.

> In such case angle of slope or difference in elevation (height) between the two points are also

measured.

Method 1. Angle measured using clinometers

Horizontal distance D = distance along the slope X cos0

0 = angle of slope with horizontal.

AC  D 1  D 2  l ₁ cos  1  l ₂ cos  2

Method 2: Difference in level measured

D l  h

2            2

D = horizontal distance,

l = distance along slope,

h = difference in the levels.

Method 3: Hypotenusal Allowance

When the chain is stretched on the slope, the arrow is not kept at the end of chain but it is placed

in advance of the end, by an amount which allows slope correction AA’

The distance AA’ is of such magnitude that the horizontal equivalent ofBA in equal to 1 chain.

BA =100sec 0 links for 20m chain

BA’ = 100 links

So AA’ =100 (sec 0-1) links for 20m chain AA’= 50 0 links When 0 is in radian

2

If θ is in degree,

AA’ = 0.015 0 links

2

1.5 2

AA’ =              0 links for 20m chain.
100

If the slope is measured by leveling, then it is given as 1 in n 50

So AA’ =___ links

n2         for 20m chains.

Errors

> Precision is the degree of perfection used in the instruments the methods and the observations. > Accuracy is the degree of perfection obtained in the field.

> The difference between the true value and the measured value ofa quantity is called true error. > A discrepancy is the difference between two measured values of the same quantity; it is not an error.

Sources of error

1.    Instrumental

> Arise due to imperfection or faulty adjustment ofthe instrument.

> Can be eliminated by precision and by applying proper correction.

> Example: tape may be too long or too short, inadjustment of angle measuring instrument, etc.

2.    Personal

Arise due to want ofperfection ofhuman sight in observing and oftouch in manipulating instruments. Example: wrong reading, wrong noting, etc.

3.    Natural

> Arise due to variations in natural phenomenon such as temperature, humidity, gravity, wind, refraction and magnetic declination, etc.