Contour line

Also called: Contour, Level line, Contour map · Topic: Topographical Maps: Relief Representation and Interpretation · NCERT: Class 11, Ch 5 "Topographical Maps"; Class 12, Ch 3 "Graphical Representation of Data"

Meaning

A contour line is an imaginary line on a map that joins places of equal height above mean sea level. Mean sea level is the average level of the sea surface, taken as zero height. Contours matter because one set of lines shows three things on a flat sheet of paper: the height of a place, the slope (how steep the land is) and the shape of the land, such as a hill, a valley, a cliff or a hollow.

Explanation

1. What a contour is and how it is drawn

• Contours are also called "level lines". If you walked along one on the ground, you would never go up or down.
• How contours are drawn:
• Earlier: from ground survey and levelling (measuring the height difference between points with instruments in the field).

• Today: from aerial photographs and satellite data.

• Contours do two jobs on a map:

• The value written on the line gives the height of a place.

• The gap between lines shows the slope.

• Contours of different heights do not normally cross.

• One spot on the ground cannot have two heights.
• The rare exception is an overhanging cliff, where rock sticks out over lower ground.

2. Contour interval (VI), horizontal equivalent (HE) and gradient

• Contour interval or Vertical Interval (VI): the fixed height gap between two contours that come one after another.

• It is usually the same across one map, for example 20 m, 50 m or 100 m.

• Horizontal Equivalent (HE): the ground distance, measured flat along the map, between two contours that come one after another.

• It changes with slope.

• How VI and HE work together:

• VI stays the same everywhere on the sheet.
• On steep land you climb one VI in a short distance. So HE is small and the contours sit close together.

• On gentle land you need a long distance to climb the same VI. So HE is large and the contours spread apart.

• Gradient (steepness written as a ratio) = VI ÷ HE.

• Example: VI = 20 m and HE = 400 m gives a gradient of 1 in 20.

3. Types of contour line (USGS practice)

• Index contour: a thick, bold line drawn at every fifth contour and labelled with its height [3].
• Intermediate contour: a thinner line between index contours, drawn at the contour interval [3][4].
• Supplementary contour: an extra line drawn between two regular contours. It shows small landforms that the normal interval would miss [3][4].
• Depression contours: USGS uses three kinds: depression index, depression intermediate and depression supplemental [4].
• On USGS maps, contours are printed in brown [3].

4. Reading landforms and slopes from contours

• Spacing rules:
• Close contours mean a steep slope.
• Wide contours mean a gentle slope.
• Very wide spacing, or no contours at all, means the ground is almost level [3].

• Merging contours mean a vertical face, such as a cliff or a waterfall, where several heights stack on top of one another.

• Closed contours:

• Hill: closed contours whose values rise inward. The highest value sits at the centre.
• Depression (a hollow, pit or basin): closed contours whose values fall inward.

• A depression is often marked with ticks or hachures (short lines drawn at right angles to the contour) [4]. On USGS maps these hashes point towards the lower ground [4].

• Slope types. "Bottom" means the low-value contours and "top" means the high-value contours.

Slope	Contour spacing	Ground	View from the top
Gentle	Far apart, evenly spaced	Rises slowly	—
Steep	Close together, evenly spaced	Rises sharply	—
Concave	Wide at bottom, close at top	Gentle below, steep above. It curves inward like the inside of a bowl	Valley floor can be seen
Convex	Close at bottom, wide at top	Steep below, gentle above. It bulges outward like the outside of a ball	Valley floor is hidden
Undulating / irregular	Close in some stretches, wide in others	Rises and falls in waves	Varies

• Always read the contour values first to find which end is the bottom and which is the top. Spacing alone is not enough.

Where it occurs

• India: Survey of India topographic sheets.
• Contours are the main way these sheets show relief (the ups and downs of the land). The sheets come as Open Series Maps (OSM) and Defence Series Maps.
• Open Series Maps are at 1:50,000 scale. They are free PDF downloads for every Indian state and union territory, including Ladakh, Arunachal Pradesh and the Andaman and Nicobar Islands [2].

• Each OSM sheet carries two numbers: the older sheet number (e.g. 39L/12) and the new OSM number (e.g. H42W12) [2].

• Hill roads and railways in India:

• Engineers read contours to plan routes in the Himalaya and on the ghat sections of the Western Ghats.

• Hill states:

• Contour-based slope maps are used for landslide zoning and for deciding where building is allowed.

• Farms on slopes:

• Contour bunds and terraces are built along contour lines to hold back water and soil.

• World: United States Geological Survey (USGS) maps.

• These maps use index, intermediate and supplementary contours, printed in brown [3][4].

Don't confuse with

• Contour interval (VI) vs Horizontal Equivalent (HE): VI is a height gap and stays constant on one map. HE is a ground distance and changes with slope. It is small on steep slopes and large on gentle ones.
• Concave slope vs convex slope: a concave slope has contours wide at the bottom and close at the top, and you can see the valley floor from the top. A convex slope is the reverse and hides the valley floor. Memory aid: from a con-CAVE slope you can see down, as if you were looking into a cave.
• Hill vs depression: both are shown by closed contours. In a hill, values rise inward. In a depression, values fall inward, and ticks or hachures point towards the lower ground [4].
• Index contour vs supplementary contour: an index contour is every fifth line, drawn thick and labelled [3]. A supplementary contour is an extra line between regular contours, used to show small landforms [3][4].

Prelims Hooks

• A contour line joins points of equal height above mean sea level. Its other name is "level line".
• Close contours = steep slope. Wide contours = gentle slope. Merging contours = cliff or waterfall.
• Gradient = VI ÷ HE. For example, 20 m ÷ 400 m = 1 in 20.
• Trap: "Contours often intersect on steep slopes" is false. Contours of different heights do not normally cross. The only exception is an overhanging cliff.
• Concave = wide spacing at the bottom, close spacing at the top, valley floor visible. Convex = the reverse, valley floor hidden.
• Survey of India Open Series Maps are at 1:50,000 and free to download as PDF. Sheet number 39L/12 corresponds to OSM number H42W12 [2].

Mains Points

• Roads and railways in hill areas (GS-I/GS-III infrastructure):
• Close contours warn of steep gradients.

• Engineers use HE to plan hairpin bends (zig-zag turns), tunnels and cuttings that keep the gradient gentle. This is done in the Himalaya and on the ghat sections of the Western Ghats.

• Disaster risk (GS-III):

• Close or merging contours mark cliffs and steep convex slopes. These are zones prone to landslides and rockfall.
• Contour-based slope maps feed into landslide zoning and decisions on where building is allowed in hill states.

• Free 1:50,000 Open Series Maps make this terrain data openly available for planning [2].

• Agriculture and water (GS-I/GS-III):

• Gentle slopes (wide contours) suit farming and settlement.
• On steeper slopes, contour bunds and terraces built along the contours slow down run-off and reduce soil erosion.
• Depressions shown by contours mark possible sites for ponds and water storage.

Related concepts

• Contour interval
• Horizontal equivalent
• Gentle slope
• Steep slope
• Concave slope
• Convex slope
• Undulating slope

Read more

• Contours and slopes