Ranging and measuring over a hill PDF

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Module 2 LINEAR MEASUREMENTS Learning Outcomes On the completion of this module you should be able to:    

Describe the principle of ranging and measuring over a hill and through depression Understand and describe measuring around a pond, across a river or busy road Describe measuring when a building obstruct vision Understand survey methods

2.8 Ranging and measuring over a hill Ranging: Method of locating or establishing intermediate points on a straight line between two fixed point or two survey stations is called as ranging. Types of ranging There are two methods of ranging: 

Direct Method (Two ends of survey line or stations are inter-visible)



Indirect Method (Two ends of survey line or stations are not inter-visible) 2.8.1 DIRECT METHOD: This method is used when two ends of survey stations or survey lines are inter-visible. 2.8.2 INDIRECT OR RECIPROCAL RANGING: This method is used when two ends of survey stations or survey line are not inter-visible.  Ranging and measuring over a hill

Let A and B be the two stations which are not inter-visible. So to proceed in straight line between A and B process of indirect ranging is applied. Two intermediate points C and D are located in such a way that person standing with ranging rod at D can see C and A whereas person with ranging rod at C can see D and B. Now person at D will guide the person at C to come in line with D and A on a new position D1. Now the person at D1 will guide the person at D to come to a new position D1 such that C1, D1 and B are on same line. Ranging rod is fixed at C1 & D1 and chaining is continued along the hill.

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Figure 2.13

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2.9

OBSTACLES IN CHAINAGE A chain line may be interrupted in the following situations: i. When chaining is free, but vision is obstructed ii. When chaining is obstructed, but vision is free and, iii. When chaining and vision are both obstructed 2.9.1 When chaining is free, but vision is obstructed Such is a problem arises when a rising ground or a jungle area interrupts the chain line. Here the end stations are not intervisible. There may be two cases: Case 1: The end stations may be visible from some intermediate points on the rising ground. In this case, a reciprocal ranging is resorted to. Case 2: the end stations are not visible from the intermediate points where a jungle area comes across the chain line. In this case the obstacle may be crossed over using a random line as explained below:

Let AB be the actual chain line which cannot be ranged and extended because of interruption by a jungle. Let the chain line be extended to R. A point P is selected on the chain line and a random line PT is taken in a suitable direction. Point C,D and E are selected on the random line, and perpendiculars are projected from them. He perpendicular at C meets the chain line at C1 Theoretically, the perpendiculars at D and E will meet the chain line at D1 and E1, Now, the distances PC, PD, PE and CC1 are measured (fig. 2.14) From triangles PDD1 and PCC1

Figure 2.14

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From (1) and (2), the lengths DD1 and EE1 are calculated. These calculated distances are measured along the perpendiculars at D and E. Points D1 and E1 should lie in the chain line AB, which can be extended accordingly. Distance =

√ PE 2+ EE2

2.9.2 Chaining obstructed but vision free Such a problem arises when a pond or river comes across the chain line. The situations may be tackled in the following ways. when a pond interrupts the chain line, it is possible to go around the obstruction

Suppose AB is the chain line. Two points C and D are selected on it on opposite banks of the pond. Equal perpendiculars CE and DF are erected at C and D. The distance EF is measured.

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Here, CD=EF

(fig 2.15 a)

The pond may also be crossed by forming a triangle as shown in (fig2.15(b)). A point A and C is selected on the chain line. The perpendicular CE is set at C, and a line ED is suitably taken. The distances CE and ED are measured. So, CD =

=

√ ED2 −CE2

Case II: Sometimes it is not possible to go around the obstruction. (a). Imagine a small river comes across the chain line. Suppose AB is the chain line. Two points C and D are selected on this line on opposite banks of the river. At C a perpendicular CE is erected and bisected at F. A perpendicular is set out at E and a point G is so selected on it that D, F and G are in the same straight line. From triangles DCF and GEF, GE = CD This distance GE is measure, and thus the distance CD is obtained indirectly (fig 2.16)). (b) Consider the case when a large river interrupts the chain line. Let AB be the chain line. Points C, D and E are selected on this line such that D and E are on opposite banks of the river. The perpendiculars DF and erected on the chain line in such a way that E, F and G are on the same straight line. The line FH is taken parallel to CD. Now, from triangles DEF and HFG,

ED DF

ED =

=

=

FH HG

FH HG

where, FH= CD

x DF

CD CG − DF

CH=DF

x DF

HG = CG - CH

Therefore, HG =CG-DF

The distances CD, DF and CG are measured. Thus the required distance ED can be calculated (fig.2.16(b)).

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2.9.3 Chaining and vision both obstruct Such a problem arises when a building comes across the chain line. It is solved in the following manner. Suppose AB is the chain line. Two points C and D are selected on it at one side of the building. Equal perpendiculars CC 1 and DD 1 are erected. The line C 1D1 is extended until the building is crossed. On the extended line, two pints E 1 and F1 are selected. Then perpendiculars E1E and F1F are so erected that E1E =F1F =D1D = C1C Thus, the points C, D E and F will lie on the same straight line AB. Here, The distance D1C1 is measured, and is equal to the required distance DE (fig. 2.17)

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2.10

LINEAR SURVEY METHODS

Linear surveying involves measurement of distances between points on the surface of the earth. There are various methods of linear surveying and their relative merit depends upon the degree of precision (accuracy) required. Linear surveying methods can be broadly divided into three heads: 1. Direct measurement 2. Measurement by optical means 3. Electronic methods 

Direct Measurement

In this surveying method, distances are actually measured on the surface of the earth by means of chains, tapes, etc. 

Measurement by Optical Means

In this method, observations are taken through a telescope and distances are determined by calculation as in tachometer or triangulation. The typical example is the theodolites. 

Electronic Methods

In these linear surveying methods, distances are measured with instruments that rely on propagation, reflection and subsequent reception of either radio or light waves. The various instruments that are used under the electronic methods are: (i) Geodimeter An instrument measuring the distance between two points by means of a laser. The distance measured in the case of geodimeter is based on the propagation of modulated light waves. The other three instruments use radio waves for distance measurement.

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Figure 2.18 Geodimenter (ii) Tellurometer System of Distance Measurement is primarily designed to meet the requirements of Geodetic accuracy over useful Geodetic distances. It may, however, be used for any measurement purposes for which its characteristics make it suitable.

Figure 2.19 Tellurometer (iii) Decca navigator A hyperbolic radio navigation system which allowed ships and aircraft to determine their position by receiving radio signals from fixed navigational beacons.

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Figure 2.20 Decca navigator

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REFERENECES 1. N5 Building and Structural Surveying by Chris Brink 2. http://www.cegyan.com/blog/ranging-in-surveying 3. http://www.civilprojectsonline.com/surveying-and-levelling/types-of-rangingchain-surveying/ 4. https://www.slideshare.net/gauravhtandon1/linear-measurements-25265965

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