What is a topographical map?
Topography means the shape and features of the land surface. A topographical map is a large-scale map that shows relief, rivers, forests, roads, railways, villages and towns of a small area in detail.
They are made after careful ground surveys, with aerial photos and satellite data. In India they are prepared by the Survey of India, Dehradun.
They are used for planning, defence, tourism, and studying how land and people are linked.
Survey of India sheet numbering
India's maps are part of a series. Each sheet is named with a number and letters.
- Million sheets: 4° latitude × 4° longitude, scale 1:1,000,000, numbered (e.g. 53).
- Each is divided into 16 sheets of 1° × 1°, scale 1:250,000, lettered A to P (e.g. 53F). These are called degree sheets.
- Each degree sheet is divided into 16 sheets of 15′ × 15′, scale 1:50,000, numbered 1 to 16 (e.g. 53F/10).
- Each 15′ sheet is divided into 4 sheets of 7′30″ × 7′30″, scale 1:25,000, named NW, NE, SW, SE (e.g. 53F/10/NE).
A sheet also shows its number, scale, contour interval, magnetic north, the year of survey and an index of nearby sheets in its margin.
Conventional signs, symbols and colours
Topo sheets use agreed signs so they can be read anywhere without long notes.
- Blue: water bodies – rivers, canals, tanks, wells.
- Green: forests, orchards, trees.
- Brown: contours, sand dunes, rocky outcrops.
- Black: railways, telephone lines, names, grid lines, boundaries.
- Red: roads, tracks, huts and houses, grid numbers.
- Yellow: cultivated land.
- White: uncultivated or unsurveyed land.
Symbols include a cross for a church, a small dome for a temple, a flag for a post office, and dotted or dashed lines for tracks.
Contours and contour interval
A contour is an imaginary line joining places of the same height above mean sea level. The height is written on the line.
The contour interval (CI) is the constant height gap between two neighbouring contours. It is usually 20 m on 1:50,000 sheets. A smaller interval shows relief in more detail.
Features of contours
- They never cross each other (except at an overhanging cliff) and never split.
- Close contours show a steep slope; wide gaps show a gentle slope.
- Contours merging into one line show a vertical cliff or waterfall.
- Each contour is a closed loop, even if the loop runs off the sheet.
Other ways to show height: spot heights (a dot with height), benchmarks (BM, marked on buildings), triangulation stations (a triangle with height), hachures and layer tints.
Types of slope
- Gentle slope: contours far apart.
- Steep slope: contours close together.
- Uniform slope: contours at equal spacing.
- Concave slope: gentle at the bottom, steep at the top. Contours are wide apart at low heights and crowded at high heights.
- Convex slope: steep at the bottom, gentle at the top. Contours are crowded at low heights and wide apart near the top.
Drawing a cross-section
- Draw a straight line AB across the contours on the map.
- Place the straight edge of a strip of paper along AB. Mark every point where a contour cuts it, and write its height.
- On graph paper draw a horizontal base line equal to AB. Choose a vertical scale (for example 1 cm = 20 m), keeping it larger than the horizontal scale so the shape is clear.
- Place the strip on the base line and raise a perpendicular from each mark to its height.
- Join the tops with a smooth curve. Shade below it. Write both scales.
The result is the profile: what the land looks like if you cut it and look from the side. The ratio of vertical to horizontal scale is the vertical exaggeration.
Identifying landforms
- Conical hill: near-circular contours, evenly spaced, with the highest value in the middle.
- Plateau: close contours on the sides and a wide empty flat top.
- V-shaped valley: V-shaped contours with the V pointing upstream (towards higher ground); a stream runs along the middle.
- U-shaped valley: like a flat-floored U; made by glaciers.
- Gorge: a deep narrow valley with very close contours on both sides.
- Spur: a tongue of high land into lower land; V-shaped contours pointing downhill.
- Cliff: contours merge into one line.
- Waterfall: a river crossing a point where contours merge.
- Col / saddle: a low dip between two hills; pass: a deep gap through a mountain range.
Reading settlements and human features
Settlement patterns seen on topo sheets:
- Compact (nucleated): houses close together, often where land is fertile and water is easy.
- Dispersed (scattered): houses far apart, as in hilly or forested land.
- Linear: houses in a line along a road, river, canal or ridge.
- Circular or star-shaped: around a tank, or along several roads leaving a centre.
Also read transport (roads, railways and where they avoid steep slopes), occupation (farming, forestry), and water sources (wells, tanks, canals).
Link them to relief: villages sit on flat land near water; roads follow valleys and passes.
Try it
Make a hill from clay or a potato. Stand it in a glass box, pour water in 1 cm at a time and draw the waterline with a marker each time. Look from above: those lines are contours. In the 3D (step 3), change the contour interval and count the lines.
Key formulas and definitions
- Contour = line of equal height above sea level
- Contour interval = height difference between two neighbouring contours
- Number of contours ≈ relief ÷ contour interval
- Vertical exaggeration = vertical scale ÷ horizontal scale
- Sheet series: 4°×4° (1:1M) → 1°×1° (1:250K) → 15′×15′ (1:50K) → 7½′×7½′ (1:25K)
Worked examples
1. Sheet 53F is divided into 15′ × 15′ sheets. How many, and what is the scale?
1° = 60′; 60 ÷ 15 = 4 per side, so 4 × 4 = 16 sheets (53F/1 to 53F/16), at 1:50,000.
2. A hill rises from 200 m to 500 m and the contour interval is 50 m. How many contours are drawn from 250 m to 500 m?
250, 300, 350, 400, 450, 500 → 6 contours.
3. On a map, the contours are crowded near the valley floor and wide apart near the summit. What slope is it?
Steep at the bottom and gentle at the top: a convex slope.
4. V-shaped contours point towards higher values. Is this a valley or a spur?
V pointing uphill (towards higher contours) is a valley.
5. Horizontal scale 1:50,000 and vertical scale 1 cm = 50 m. What is the vertical exaggeration?
Vertical: 1 cm = 5,000 cm → 1:5,000. VE = 50,000 ÷ 5,000 = 10 times.
6. Houses on a topo sheet form a long line along a river levee. Name the pattern and a reason.
Linear settlement: people build on the higher, drier levee along the river, close to water and the route.
Common mistakes
- Reading the V of a valley the wrong way. In a valley the V points uphill; in a spur it points downhill.
- Thinking contours can cross. They never cross, except at an overhanging cliff.
- Using the same vertical and horizontal scale in a cross-section, which makes the profile look flat.
- Mixing up colours: roads and huts are red, railways are black.