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MEASURING THE AREA
Measuring the Area - Considerations
• All eastings are 2 cm (1 km) apart• All northings are 2 cm (1 km) apart• Each grid square measures
2 cm X 2 cm
OR 1 km X 1 km
= 1 sq. km
How do we measure the area?
• Count the number of grid squares (n)• Area = n sq. km
Example 1
Calculate the area enclosed within eastings 26 and 29and northings 58 and 62.
Solution• Eastings difference (p) = 29 – 26 = 3• Northings difference (q) = 62 – 58 = 4• Area enclosed = p X q
= 3 X 4= 12 sq km
Visual method
6324 25 26 27 28 29 30
62
61
60
59
58
57
Visual method
6324 25 26 27 28 29 30
62
61 1 2 3
60 4 5 6
59 7 8 9
58 10 11 12
57
Example 2
Calculate the extent of cultivated area enclosed within eastings 43 and 49 and northings 85 and 89.
CWrong Solution• Eastings difference (p) = 49 – 43 = 6• Northings difference (q) = 89 – 85 = 4• Area enclosed = p X q
= 6 X 4= 24 sq km
So, what is the correct solution?
Visual method
90 42 43 44 45 46 47 48
89
88
87
86
85
84
Correct solution
Area enclosed by full grid squares (f)Area enclosed = f X 1
Area enclosed by half grid squares (h)Area enclosed = h X ½
Area enclosed by more than half grid squares (m)Area enclosed = m X 2/3
Area enclosed by less than half grid squares (l) Area enclosed = l X 1/3
TOTAL AREA
• Total areaf X 1
+h X ½
+m X 2/3
+l X 1/3
90 42 43 44 45 46 47 48
89 l m h h l l
88 h f f f f l
87 m f f f m
86 m f f f h
85 l m m m
84
Correct solution - Visual method
TOTAL AREA• Finding the total area
f X 1 = 10 X 1 = 10 sq. km +
h X ½ = 4 X ½ = 2 sq. km +
m X 2/3 = 7 X 2/3 = 4.67 sq. km +
l X 1/3 = 5 X 1/3 = 1.67 sq. km =
Total Area = 18.34 sq. km
REPRESENTING HEIGHTS ON
TOPOGRAPHICAL MAPS
How are heights measured?
• Start from mean sea level• Determine the heights using theodolite
(principles of trigonometry)• Use these heights as bench marks to
determine further heights
Types of heights
1. Triangulated Height2. Spot Height3. Relative Height
Triangulated Height• Determined using principles of trigonometry• Accurate• Expressed on maps using a ∆• For example, ∆ 224
Prominent Surveyed Tree
• Triangulated height written on tree bark• Tree is shown in black colour- • For example
Bench Mark
• Triangulated height written on nearby rock or wall
• Shown using BM• For example, BM 403
Spot Height
• Height estimated using the value of adjacent contours
• Shown with a dot• For example, .544
560
540
.544
Relative Height
• Height (depth) of a feature relative to surroundings
• Shown using r• For example, 20r
Example of relative heightsSymbol Meaning
Relative height of river bank is 7 metres
Relative height of Sand Dune is 11 metres
Relative height of Tank Embankment is 14 metres
22r Relative Depth of Well is 22 metres
14r
11r
7r
End of Presentation
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