measuring the area. measuring the area - considerations all eastings are 2 cm (1 km) apart all...

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