Explanations to Topographic Map
- Topographical maps, also known as general purpose maps, are drawn at relatively large scales.
- These maps show important natural and cultural features such as relief, vegetation, water bodies, cultivated land, settlements, and transportation networks, etc.
- These maps are prepared and published by the National Mapping Organisation of each country. For example, the Survey of India prepares the topographical maps in India for the entire country.
- The topographical maps are drawn in the form of series of maps at different scales. Hence, in the given series, all maps employ the same reference point, scale, projection, conventional signs, symbols and colours.
- The topographical maps in India are prepared in two series, i.e. India and Adjacent Countries Series and The International Map Series of the World.
SCALE
Scale is the ratio between the distance of any two points on the map and the actual distance of the same points on the ground.
REPRESENTATIVE FRACTION (RF)
It is the ratio between the distances on the map to its corresponding distance on actual ground. The RF on this map is 1:50,000.
1:50,000
1 cm on paper is 500m (50,000cm) on ground
Survey of India
- The Survey of India is India’s central engineering agency in charge of mapping and surveying.
- Set up in 1767 to help consolidate the territories of the British East India Company
- It is one of the oldest Engineering Departments of the Government of India.
- The Survey of India’s distinguished history includes the handling of the mammoth Great Trigonometric Survey under William Lambton and George Everest.
METHODS OF RELIEF DEPICTION
- Hachure
- Hill Shading
- Layer Tinting or Colouring
- Contours
- Form lines
- Spot Heights
- Bench Marks
- Trigonometrical Stations
- Relative Height
Hachures: Hachures are strokes (short line segments or curves) drawn in the direction of the steepest slope (the aspect direction). Steeper slopes are represented by thicker, shorter strokes, while gentler slopes are represented by thinner, longer and farther apart strokes.
Hill shading : Hillshading is the process of adding light and dark areas or shading to a map to highlight the location of hills or mountains. Hillshading uses light and dark areas to highlight where sunlight would hit and where shadows would form in the presence of hills and mountains.
Layer tinting or colouring : Layer tinting uses different colours (or shades) to represent different heights. It is a mapping convention for darker colours to signify greater height. When using layer tinting, green is often used for low land, yellow for higher land and brown for the highest land.
Contour : Contours are imaginary lines drawn on maps, joining all places with the same height above sea level.
Contour interval: The interval between two consecutive contours is called contour interval
Index contour : Contour lines are thickened at regular intervals to make it easier to read contours. For example at every 100 mts the contour line is made darker. The darker lines are called Index Contours.
Vertical interval (v.i.) and horizontal equivalent (h.e.)
The difference in height between two adjacent contour lines is known as the Vertical Interval V.I. Whereas the distance between any two adjacent contour lines is called the Horizontal Equivalent H.E..The Horizontal Equivalent is calculated with the help of the scale of the map.
Gradient : Gradient is a measure of how steep a slope or a line is. Gradients can be calculated by dividing the vertical height by the horizontal distance.
Form lines : These are broken lines. This method is used in association with contours to show the hilly or mountainous country. They are drawn without any precise measure units. They indicate the minor details which are not shown by
contours.
Spot height : The height of random places between contours shown with a dot.
Eg-.425
Bench mark : Height of a place actually marked on a stone pillar, rock or shown on a building as a permanent reference. It is written as BM 200 m.
Triangulated height : It is the height of a place which has been calculated using trigonometry, represented by a small triangle
e.g. ∆540
Relative height : Relative height is the height of a feature with reference to the height of the surrounding land and NOT to sea level. It is represented by the
height with a small ‘r’
e.g. –12r.
METHODS OF RELIEF REPRESENTATION
The earth’s surface is not uniform and it varies from mountains to hills to plateaus and plains. The elevation and depressions of the earth’s surface are known as physical features or relief features of the earth. The map showing these features is called a relief map.
Conical Hill
It rises almost uniformly from the surrounding land. A conical hill with uniform slope and narrow top is represented by concentric contours spaced almost at regular intervals.
Plateau
A widely stretched flat–topped high land, with relatively steeper slopes, rising above the adjoining plain or sea is called a plateau. The contour lines representing a plateau are normally close spaced at the margins with the innermost contour showing wide gap between its two sides.
‘V’-shaped Valley
It resembles the letter V. A V-shaped valley occurs in mountainous areas. The lowermost part of the V–shaped valley is shown by the innermost contour line with very small gap between its two sides and the lowest value of the contour is assigned to it. The contour value increases with uniform intervals for all other contour lines outward.
‘U’ – shaped Valley
U–shaped valley is formed by strong lateral erosion of glaciers at high altitudes. The flat wide bottom and steep sides makes it resemble the letter ‘U’. The lowermost part of the U–shaped valley is shown by the innermost contour line with a wide gap between its two sides. The contour value increases with uniform intervals for all other contour lines outward.
Gorge
In high altitudes, gorges form in the areas where the vertical erosion by river is more prominent than the lateral erosion. They are deep and narrow river valleys with very steep sides. A gorge is represented by very closely-spaced contour lines on a map with the innermost contour showing small gap between its two sides.
