measuring the area. measuring the area - considerations all eastings are 2 cm (1 km) apart all...
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MEASURING THE AREA
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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
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How do we measure the area?
• Count the number of grid squares (n)• Area = n sq. km
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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
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Visual method
6324 25 26 27 28 29 30
62
61
60
59
58
57
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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
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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
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So, what is the correct solution?
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Visual method
90 42 43 44 45 46 47 48
89
88
87
86
85
84
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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
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TOTAL AREA
• Total areaf X 1
+h X ½
+m X 2/3
+l X 1/3
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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
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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
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REPRESENTING HEIGHTS ON
TOPOGRAPHICAL MAPS
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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
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Types of heights
1. Triangulated Height2. Spot Height3. Relative Height
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Triangulated Height• Determined using principles of trigonometry• Accurate• Expressed on maps using a ∆• For example, ∆ 224
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Prominent Surveyed Tree
• Triangulated height written on tree bark• Tree is shown in black colour- • For example
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Bench Mark
• Triangulated height written on nearby rock or wall
• Shown using BM• For example, BM 403
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Spot Height
• Height estimated using the value of adjacent contours
• Shown with a dot• For example, .544
560
540
.544
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Relative Height
• Height (depth) of a feature relative to surroundings
• Shown using r• For example, 20r
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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
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End of Presentation