From a 3D Hill to a 2D Map
Slices of a potato stacked one over another look like a hill. From above they look like circles within circles.
Contour lines are used to show 3-D landforms like hills and mountains as 2-D pictures on paper.
Key idea 1 : The potato hill — Cut a large potato in half and take the flat-based half; measure its height in millimetres. Draw one circle near the top (smaller) and another near the base and slice at these. Push a stick through, keep slices in order, trace the lowest slice’s edge on paper, then remove and trace each next slice.
Key idea 2 : The resulting map — You finally get three circles within circles. Give the outermost circle the value ‘0’ and write the measured height at the centre of the innermost. Slice thickness helps value the lines between. We have turned a 3-D object into a 2-D picture.
Key idea 3 : Real mountains — In reality a mountain cannot be sliced and laid on paper. Mathematical and survey methods are used; these are learnt when Geography is studied as a special subject later. Maps made by them are contour maps showing height, slope and landforms.
- Easy : Value of the outermost circle? — 0.
- Medium : Value of the innermost circle? — Height of the hill.
- Exam-type : Hill seen from above? — Circles within circles.
In the figure: 3-D to 2-D.
Slice, trace edge = line.
Contour lines are not drawn on the land; they are imaginary lines on the map.
Memory clue: Outside 0 → inside height.
Spot the difference
| Step | Action | Result |
|---|---|---|
| 1 | Cut potato | Slices |
| 2 | Trace edge | Circles |
| 3 | Give values | Contour map |
Practice questions
- 1What did we achieve with the potato activity?
- 2What do two-dimensional and three-dimensional mean?
- 3How do we give values to the circles?
- 4Why 0 for the outer circle?
- 5Why can we not slice a real mountain?
- 6How does the activity relate to a map?
- 7How does the potato hill look from above?
- 8By which methods are contour maps made?
Show answers (try them yourself first)
- 1A 2-D picture of a 3-D object.
- 22-D = length-width; 3-D = length-width-height.
- 3From slice thickness and height.
- 4The base is the starting level.
- 5Mathematical and survey methods are needed.
- 6Contour maps follow the same principle.
- 7Circles within circles.
- 8Mathematical and survey methods.
Contour Lines and Their Properties
Joining all points at one level on a field gives a line. On a map such lines are contour lines.
Lines on a map joining places of the same altitude are contour lines. They generally do not cross each other.
Key idea 1 : What are contour lines? — A line joining all places of the same altitude is a contour line. Each line carries its height in metres. As the height is equal along a line, lines of different heights generally do not cross. The outermost line has the lowest value and inner lines higher.
Key idea 2 : Interval — The height difference between successive lines is decided beforehand. To draw contours from mean sea level up to your place, the interval is kept at most 50 m. This tells how many lines are needed.
Key idea 3 : Model and map — Landforms seen on a terrain model can be identified from the lines on its map. The model shows shape, while the map gives precise information about height and direction. A map is as seen from above; the observer is as if above the landform.
- Easy : Contours join? — Places of equal altitude.
- Medium : Do two contours cross? — Generally not.
- Exam-type : Maximum interval? — 50 m (in the village exercise).
In the figure: Height rises inward.
One line = one height.
Contours are not rivers or roads; they are imaginary equal-height lines.
Memory clue: Contour = same height.
Spot the difference
| Point | Contour line | Note |
|---|---|---|
| Meaning | Equal height | On a map |
| Outer line | Low value | Base |
| Inner line | High value | Top |
Practice questions
- 1What is a contour line?
- 2Why do contour lines not cross each other?
- 3How do the values change from outside to inside?
- 4How is the interval between two lines decided?
- 5How would you decide how many lines are needed for your village’s height?
- 6State the difference between a model and a map.
- 7Where is the observer when viewing a contour map?
- 8In which unit are contour values given?
Show answers (try them yourself first)
- 1A line joining places of equal altitude.
- 2Because every line has a different height.
- 3Lower outside, higher inside.
- 4By a predecided height difference.
- 5Divide the total height by the 50 m interval.
- 6Model shows shape; map shows height-direction.
- 7Above the landform, looking down.
- 8Metres.
Reading Slope and Landforms
While climbing a hill the slope is sometimes steep and sometimes gentle. The spacing of lines on a map shows exactly this.
If contour lines are close, the slope is steep; if the gap is more, the slope is gentle. The shape of the lines helps identify the landform.
Key idea 1 : Judging slope — If the gap between two lines is small the height changes a lot over a short distance, so the slope is steep. If the gap is large the slope is gentle. So spacing tells the slope.
Key idea 2 : Direction of slope and landforms — Seeing the direction in which line values rise tells where the high ground lies and the direction of slope. Lines also show in which direction ranges extend and where flat land lies. Closed circles within circles rising inward indicate a hill.
Key idea 3 : Choosing a route — To climb a hill by an easy, safe path choose the side where lines are widely spaced; the slope is gentle there. A river flows from high to low ground, so its direction can be inferred from heights.
- Easy : Close lines mean? — Steep slope.
- Medium : Widely spaced lines mean? — Gentle slope.
- Exam-type : Easy climbing side? — Where lines are far apart.
In the figure: Spacing shows slope.
Close = steep; far = gentle.
A side with many lines is not always easier; it may be steep.
Memory clue: Dense lines = steep climb.
Spot the difference
| Lines | Spacing | Slope |
|---|---|---|
| Close | Small | Steep |
| Far apart | Large | Gentle |
Practice questions
- 1What is the slope if contour lines are close?
- 2What is the slope if the gap is large?
- 3How does a map tell the slope direction?
- 4Which landform do circles within circles show?
- 5How would you choose a safe climbing route?
- 6How would you decide a river’s direction from a map?
- 7How would you identify where flat land is?
- 8Explain the link between spacing and change of height.
Show answers (try them yourself first)
- 1Steep.
- 2Gentle.
- 3By where the values increase.
- 4A hill.
- 5Choose the side with widely spaced lines.
- 6From high ground to low ground.
- 7Where lines are few or widely spaced.
- 8Small gap = big change = steep; large gap = gentle.
Uses of Contour Maps
Trekkers look at a contour map to understand the route and climb before going.
Contour maps show altitude, relief, slope, direction of slope and drainage, so mountaineers, trekkers, soldiers and planners find them useful.
Key idea 1 : Useful to whom — In studying landforms we must consider altitude, relief, slope, direction of slope and drainage. Contour maps made by specific methods are used for this. They are very useful to mountaineers, trekkers, soldiers and defence officers.
Key idea 2 : Use in planning — Contour maps help in regional planning. A farmer can learn which way water flows, which part is high or low and from where irrigation is possible. They also help decide the right places for dams, roads and settlements.
Key idea 3 : Local example — The textbook shows the Karha river basin near Saswad. In the north is the Mula-Mutha basin, in the south the Katraj-Diveghat range (spreading west to east) and beyond it part of the Karha basin. From the model and map one can find the direction of Purandar fort, the flow of the Karha and where the range’s height decreases.
- Easy : Users? — Trekkers, soldiers, farmers.
- Medium : For what information? — Height, slope, drainage.
- Exam-type : Karha river is near which town? — Saswad.
In the figure: Uses of contour maps.
As per the textbook the Karha river basin is near Saswad; Purandar fort and the Katraj-Diveghat range are shown in its map. (as per the textbook)
Height-slope-water = what the map tells.
A contour map is not only for mountains; it helps in farming and planning.
Memory clue: Height + slope + water = planning.
Spot the difference
| User | Use | Benefit |
|---|---|---|
| Mountaineer | Choose route | Safety |
| Soldier | Understand terrain | Defence planning |
| Farmer | Water-flow direction | Irrigation |
Practice questions
- 1To whom are contour maps useful?
- 2What information does a contour map give?
- 3How does a contour map help a farmer?
- 4What is their use in planning?
- 5Why do mountaineers need these maps?
- 6Which landforms appear in the Karha basin map?
- 7In which direction does the Katraj-Diveghat range extend?
- 8What steps would you follow to prepare a contour map of your village?
Show answers (try them yourself first)
- 1Mountaineers, trekkers, soldiers, planners, farmers.
- 2Altitude, relief, slope, slope direction, drainage.
- 3It shows water flow and high-low areas.
- 4Planning dams, roads, settlements.
- 5To pick a safe climbing path.
- 6Ranges, river basin, fort.
- 7West to east.
- 8Measure height, decide interval, draw lines and give values.