Interpretation of Topographical Maps

Maps are classified as follows:

1. On the basis of Scale —
   1. Large scale maps — Large scale maps show a small area in great details.These maps are further classified into two types:
      1. Cadastral maps
      2. Topographical maps
   2. Small scale maps — Small scale maps show a larger area in less detail and space. These maps are further classified into two types:
      1. Atlas maps
      2. Wall maps
2. On the basis of their function —
   1. Political — e.g. World political
   2. Physical — e.g. Relief, Climatic
   3. Thematic — e.g. Population

The use of colours allows topographical maps to convey a lot of information in a compact and easy-to-understand format. These colours are called Conventional Colours as they are accepted all over the world. For example, yellow shows cultivable land, green shows forest area while black shows all dry streams and surveyed trees.

The objectives of Survey of India map are:

1. To have all topographical control and conduct required surveys within the country.
2. Mapping and production of political and physical map and aeronautical charts.
3. Demarcation of external boundaries of the Republic of India
4. Research and development in the fields of cartography, printing, photography related to topographical surveys of India.

1. Broken Ground

2. Temple

3. Church

4. Contours showing one side steep, the other side gentle slope

5. Surveyed tree

1. Causeway — A raised road or platform across a stream is known as Causeway. A stream having a causeway is motorable during dry season. Hence too many causeways indicate scanty or seasonal rainfall.

2. Contours — The lines drawn in brown colour, joining places having same elevation above sea level are called contours.

3. Depression — A hollow created on the top of mound or dune in sandy area, formed by blowing away of sand by the wind is called depression.

4. Stony waste — The land covered with rock boulders and stones, where cultivation is not possible is called Stony waste. It is shown on a survey sheet as a white patch.

5. Fire line — A clearing made in the forest to prevent spread of fire is called fire line.

6. Surveyed tree — It serves as a prominent survey point, shown in black.

7. Brackish— Brackish written beside a perennial lined well indicates that the water of the well is salty, and not fit for irrigation or drinking.

8. Meander— A bend in the river flowing through a flat land or plain during its middle stage is called meander.

9. Broken ground— Uncultivable land, generally found along the banks of a stream of river in arid regions where the top soil is easily eroded due to flooding during rainy season is called broken ground.

10. Embankment— A bank or mound constructed along a tank or artificial lake to prevent flooding is called embankment.

11. Diggi— A tank that is used for the collection of water is called diggi.

12. Pylons— Tall metal structures that holds electric cables high above the ground are called pylons.

The conventional signs and symbols help in giving maximum information about a place or area. They allow topographical maps to convey a lot of information in a compact and easy-to-understand format. They are universally accepted. The use of conventional signs and symbols help us to quickly get an idea of the layout and characteristics of an area just by looking at a topographical map.
For example, permanent settlements are shown by red squares, hospitals are shown by plus sign of red colour, aerodrome , helipad, post office, police station, etc. are shown by their own unique symbols.

1. 1. Depression — A hollow created on the top of mound or dune in sandy area, formed by blowing away of sand by the wind is called depression.
   2. Stony waste — The land which is mostly covered with rock boulders and stones and where cultivation is not possible. It is shown as a white patch on the map.
   3. Sheet rock — A region which is mostly covered by a sheet of rock is called sheet rock. It is shown in white and is printed on the survey sheet.
   4. Rock-out-crop — Subsoil is exposed in such areas and cultivation is not possible. It is printed on the survey sheet.
   5. Surveyed tree — It serves as a prominent survey point, shown in black.
   6. Embankment — A bank or mound constructed along a tank or artificial lake to prevent flooding is called embankment. It is generally a cemented structure.
   7. Stone quarry — It is the underground quarrying for stones. Its presence indicates the occupation of the people in the region. It also shows the trade possibilities.
   8. Meander — A bend in the river flowing through a flat land or plain during its middle stage is called meander.
2. 1. A Ridge — A long, narrow highland with edge along the top of hills or mountains is called ridge. The contours of a ridge are elliptical or elongated in shape. It is called watershed or water divide as it divides two drainage systems.
   2. Spot height — Spot height is the height of a particular point measured and is shown on the survey map as .240. It means that the height of that particular point is 240 metres above sea level.
   3. Relative height — 3r. besides a river bank, tank with embankment or sand dune shows the relative height between top and bottom. Hence, it should be explained as the relative height of the embankment of the tank is 3 metres.
   4. Fire line — A clearing made in the forest to prevent spread of fire is called Fire line.
   5. Causeway — A raised road or platform across a stream is known as Causeway. A stream having a causeway is motorable during dry season. Hence too many causeways indicate scanty or seasonal rainfall.
   6. Brackish — Brackish written beside a perennial lined well indicates that the water of the well is salty, and not fit for irrigation or drinking.
   7. Overhead tank — It is a big water container made on elevated structure for storing water. The stored water is meant for public supply.

Seasonal stream

Contour

NW

Broken ground

2.25 km

A raised platform across a stream

A pack track

A tank which is used for collection of water

A clearing made in the forest to prevent spread of fire called fire line.

A prominent survey point shown as surveyed tree at the height of 158 m above sea level

A railway track and metalled road

It is Bench Mark, cut on a rock wall or a building showing the height of that mark above sea level is 224 m

Trellised

Trellised

Radial

A thin water channel

Spot height showing the height of a particular point above sea level is 381 m

Triangulated height - showing the height of a point surveyed by the triangulated method is 224m

1 sq km

Footpath, pack track and metalled road

4099

Compact settlement or Nucleated settlement

Scattered or dispersed settlements

Left bank as the arrow shows river is flowing from east towards SW. If we stand at the source in the east facing the river, Marol is on the left bank

(a) Line of Longitude, 37'30"

(b) Line of Latitude, 32'30"

Brick kiln

Forest area

A steep slope

A gentle slope

(a) Cultivable land

(b) Perennial lined wells

10 r means that the relative height of the embankment of the perennial artificial tank is 10m

NE to SW as the spot heights are receding from NE to SW eg., △ 166, 138 ...

Reason— Receding spot heights can be seen from NE to SW which means that the slope of the land is towards SW and hence the direction of the flow of river.

3 r means relative height of the dry tank is 3m

(a) An artificial channel for conveying water typically in the form of a bridge across a valley

(b) Broken ground

6733

Working on the canal construction site

Contours are lines drawn in brown colour, joining places having the same elevation above mean sea level. The contour lines on a map provide a useful insight into the topography of an area.

Hachures are short lines representing directions of a slope. For steep slopes they are drawn closer together than for the gentle slopes. They cannot be used for flat ground.

The term gradient refers to the slope of the concerned feature. It is determined by calculating the Horizontal Equivalent. Horizontal Equivalent is the distance between the two contour lines. If it is less, the slope is steep and if it is more, the slope is gentle.

Form lines are the contour lines drawn as broken lines which are used to indicate the approximate heights above sea level. They are used to indicate the elevations of the area which are not accessible for proper survey.

Spot heights are points which show the height of that point from the mean sea level. They are represented by a dot with a number beside it. If the spot height is shown as .220, it means that the height of that point is 220 m from the mean sea level.

Bench marks are the permanent survey points shown by a stone or a building. They show the height of that point above sea level through accurate survey. The Bench mark is represented on the map as 'BM' with a number beside it. For example, 'BM 230 m' shows a bench mark with 230 m height.

In a contour map, every fifth line in multiple of 20 m is accentuated and is called the Index Contour.
Index contours make the contour map distinct. They also indicate whether the height of the terrain is increasing or decreasing.

The network of horizontal and vertical lines, or the Eastings and Northings on a topo sheet is called the grid.

The Following point should be kept in mind while reading a grid reference:

1. Origin is the south-west corner of the map. The reading on the map is always taken with reference to this origin.
2. Eastings are always read to the East of the origin.
3. Northings are always read to the North of the origin.
4. While giving a grid reference, Eastings are always stated first, followed by the Northings.
5. The readings are always taken to the right of the Eastings and to the North of the Northings

A Hill flat top gentle slope

A v-shaped Valley

A Ridge

A hill with gentle and steep slope

A conical hill with steep slope

A conical hill