Practical 2 Chain and Compass Surveying - Computation of areas.ppt

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1. Soil and Water Conservation Engineering
SWE 112 (1+1)
Practical 2
Computation of Area by Chain and Cross
Staff Survey

2. Computation of Area by Chain and Cross Staff Survey
Aim
To locate the boundaries of the given field and also to
determine the area (by chain and cross staff survey)
Instruments required
Chain, cross staff, arrows, ranging rods

3. Computation of Area by Chain and Cross Staff Survey
Procedure
In order to calculate the area of any piece of land which
is irregular in shape it is necessary to divide the area into
number of right angled triangles and trapezoidal in
shape.
Corners along the boundary of the field should be first
identified and named as A,B,C,D,E,F and G. any two
stations located in opposite sides should be selected in
such a way that distance between them is the longest of
other stations and almost equal number of corners
(stations) are located on both sides.

4. Computation of Area by Chain and Cross Staff Survey
Procedure
Chaining should be started along the base line and off set
distance of the corners on both sides are to be measured
simultaneously.
All the details should be entered in the field observation
book.
Care should be taken that no offset is over looked before
the chain is moved forward.

5. Computation of Area by Chain and Cross Staff Survey
Procedure
To check the accuracy of the field, work distance
between any two corners should be measured directly
and compared.
After the field work is over the survey may be plotted to
a suitable scale.
The area enclosed by the boundary lines is divided into a
number of triangles and trapezoids.

6. Computation of Area by Chain and Cross Staff Survey
Procedure
The area of each segment may be plotted to a suitable
scale.
The area enclosed by the boundary lines is divided into a
number of triangles and trapezoids.
The area of each segment may be computed by the
suitable formulae.
The computation of the areas should be written in the
tabular form.

7. Computation of Area
'area'inthecontextofsurveyingreferstotheareaofa
tractoflandprojecteduponthehorizontalplane,and
nottotheactualareaoftheland.
Areamaybeexpressedas
I.Squaremetres
2.Hectares(1hectare=10,000m
2
)
3.Squarefeet
4.Acres(1acre=4840sq.yd.=43,560sq.ft.).

8. Computation of area from field notes
Thisisdoneintwosteps.
Step1
Incross-staffsurvey,theareaoffieldcanbedirectly
calculatedfromfields.
Duringsurveyworkthewholeareaisdividedinto
somegeometricalfigures,suchastriangles,squares
andtrapeziums,andthentheareaiscalculated.
Step2
Theareaalongtheboundariesiscalculated.

9. Computation of area from field notes
1.Areaoftriangle
Wherea,bandcarethesides,and
•AreaoftriangleA=1/2xbxh
Whereb=baseh=altitude
2.AreaofrectangleA=axb
whereaandbarethesides.
3.Areaofsquare=a
2
whereaisthesideofthesquare.
4.AreaoftrapeziumA=1/2(a+b)xd
whereaandbaretheparallelsides,anddisthe
perpendiculardistancebetweenthem.

10. Problem 1
Thefollowingisasamplechainagereadingofachain
survey.Drawamapbasedonchainagereadingand
Calculatetheareaofthefieldusingtabularmethod.

11. Problem –1 –Solution
Drawamapbasedonthechainagereadinggivenbelow

12. Problem –1 –Solution
S.
No.
(1)
Figure
(2)
Chainage
(m)
(3)
Base
length
(m)
(4)
Offsets
(m)
(5)
Mean
offset
(m)
(6)
Area
(m
2
)
(6) x (4)
1∆ACG 0 and 20200 and 42(0+42)/2 =
21
420
2Trap.GCEI20 and 654542 and 5850 2250
3∆IEB 65 and 1104558 and 029 1305
4∆BFJ 90 and 110200 and 6030 600
5Trap.FJHD40 and 905060 and 2040 2000
6∆DHA 0 and 404020 and 010 400
Total Area = Sum ofthe individual area6975

13. Problem 2
Thefollowingisachainagereadingofachainsurvey.Draw
amapbasedonchainagereadingandCalculatetheareaof
thefieldusingtabularmethodandtriangulationmethod.

14. Problem 2 -Solution
A
B
C
D
E
F
G
15
30
60
88
98
20
30
22
15
14
H
I
J
K
L
D
G
15 20
3014
60 30
8815
98 22

15. Problem 2 -Solution

16. Problem –2 –Solution
S. No. Figure Base length
or
Parallel side
(a)
Offset length or
perpendicular
distance
(b)
Area
(m
2
)
1∆ DHC 15 20 150
2Trap.HJBC
3Trap.ABJL
4∆ALG
5∆FKG
6Trap.EFKI
7.∆EID
Total Area 3611

17. Problem 3
Plot the following cross staff survey of a field and calculate
its area (all dimensions are in m)
A
0
15
30
48
72
96
30 B
45 C
E 48
F 36
D

18. Problem 4
Plot the following oblique offset of survey of a field and
calculate its area (all dimensions are in m)
5
10
0
20
7
9
10
8
7
8
10
12
6
5

19. Problem 4
Plot the following oblique offset of survey of a field and
calculate its area (all dimensions are in m)
Areaofunequalsidesoftriangle
Wherea,bandcarethesides,and