Key Points
Key Points: Vertical Interval and Horizontal Equivalent
- Vertical Interval (V.I.) – It is the difference in height between two successive contour lines.
- Unit of Measurement – V.I. is measured in metres or feet and remains constant on a given contour map.
- Example of V.I. – If the vertical interval is 100 metres, contours will be drawn at 0, 100, 200, 300 metres, and so on.
- Horizontal Equivalent (H.E.) – It is the horizontal distance between two successive contour lines.
- Relation with Slope – H.E. is small on steep slopes (contours close together) and large on gentle slopes (contours far apart).
Key Points: Contours – Precautions, Interpolation & Profile
- Precautions for Drawing Contours – Contours are continuous curved lines, drawn at a constant interval. They do not break and usually form closed curves.
- Spacing of Contours – Contours are close together on steep slopes and far apart on gentle slopes. They generally do not cross each other (except in special cases like cliffs).
- Interpolation of Contours – It is the method of drawing contour lines between known height points (spot heights, bench marks, etc.) using equal contour intervals.
- Fixing Contour Interval – The contour interval is decided based on the range of elevation and is usually taken in round figures (10 m, 20 m, 50 m, 100 m, etc.).
- Profile or Cross-Section – A cross-section is drawn from contours to show the vertical shape of the land along a line, helping to understand slopes and landforms clearly.
Key Points: Methods of Showing Relief on Maps
- Relief Representation – Maps are two-dimensional, so special methods are used to show three-dimensional landforms like mountains and valleys.
- Colour Method – Different colours show different heights (e.g., green for plains, brown for hills, white for snow areas). It gives a quick overall view but may overlap.
- Hachures – Short broken lines drawn along slopes. Close thick lines show steep slopes; thin, spaced lines show gentle slopes. They show shape but not exact height.
- Spot Heights, Bench Marks & Trigonometrical Stations – These give exact height above sea level at specific points but do not show the full relief of an area.
- Hill Shading – Slopes are shaded to show light and shadow effect. It gives a natural look but does not show exact heights clearly.
- Layer Tints (Hypsometric Tints) – Different colour shades are used between contour lines to show height differences (blue for sea, green for lowlands, brown for highlands).
- Contours – Imaginary lines joining places of equal height above sea level. They are the most accurate and commonly used method of showing relief.
Key Points: Methods of Drawing a Profile
- Two Methods – A profile (cross-section) can be drawn by:
1. Drawing perpendiculars
2. Using a paper strip - By Drawing Perpendiculars – A base line is drawn equal to the section line on the map. Perpendicular lines are erected and contour heights are marked according to the vertical interval.
- Joining Points – After marking all heights, the points are joined smoothly to form the profile of the land.
- Vertical Scale – The horizontal scale remains the same as the map, but the vertical scale may be slightly exaggerated (5–10 times) for clarity.
- By Paper Strip Method – A paper strip is placed along the section line, contour points are marked, transferred to a graph with a suitable vertical scale, and then joined smoothly to complete the profile.
Key Points: Slopes & Relief by Contours
- Steep slope – Contours close together.
- Gentle slope – Contours far apart.
- Uniform slope – Contours evenly spaced.
- Concave & Convex slopes – Spacing changes from bottom to top.
- Conical hill – Circular contours, height increases towards centre.
- V-shaped valley – ‘V’ points upstream; Spur – ‘V’ points downhill.
- Cliff/Waterfall – Contours very close or touching (very steep).
