Map interpretation

Detail

1. What map interpretation means

• Map interpretation means reading a topographical map to find out how its features are linked. It asks why a feature is where it is, not only what is there.

• A topographical map is a large-scale map that shows both natural features (relief, rivers, forests) and man-made features (roads, villages, canals) of a small area.

• It looks for cause-and-effect links between features:

• Forests on steep slopes: steep land is hard to plough and its soil is thin and quickly washed away. So it stays under trees.
• Crops on plains: flat land has deep soil and is easy to plough and water with canals. So it is farmed.

• Settlements along roads and rivers: rivers give water and fertile land. Roads give links to markets. So houses line up along them.

• Where the maps come from in India:

• The Survey of India (SOI) makes and maintains the topographic map data for the whole country. It was set up in 1767, which makes it India's oldest scientific department [2][5].
• Under the National Map Policy 2005, SOI maps come in two series [2]:

Feature	Defence Series Maps (DSMs)	Open Series Maps (OSMs)
Purpose	Defence and national security	Development work by the public, planners and students
Datum (the base model of the Earth's shape)	Everest or WGS-84	WGS-84
Projection	Polyconic or UTM	UTM (Universal Transverse Mercator)
Content	Full content, with heights and contours, at full accuracy	Different sheet numbers; "Unrestricted" once the Ministry of Defence gives a one-time clearance
Access	Classified; the Ministry of Defence sets the rules for use	Public, as paper and digital copies

• On 11 April 2017, the Nakshe portal was launched for SOI's 250th anniversary. It lets users download 1:50,000 OSMs as PDF files free of cost, after Aadhaar-based login [5].

2. Heads of interpretation

A head is a topic under which you study the map, one by one.

2.1 Marginal information

• Marginal information is the information printed in the margins (borders) of the sheet. It includes the sheet (index) number, the scale, the contour interval, the latitudes and longitudes, the north direction, the districts or states covered, the legend (key to the conventional signs) and the survey year.
• Always read it first. It tells you where the area is, how big it is, and how old the information is.

2.2 Relief

Relief means how the land rises and falls (its heights and slopes). On the map, contours show it. A contour is a line joining points of equal height above mean sea level.

• Hills and their slope types:

Slope type	How the contours look	What it means
Gentle	Widely spaced	Land rises slowly; can be farmed
Steep	Closely spaced	Land rises fast; forests or bare rock
Uniform (even)	Evenly spaced	Same steepness all along
Concave	Close at the top, wide at the bottom	Steep above, gentle below
Convex	Wide at the top, close at the bottom	Gentle above, steep below
Terraced	Close and wide bands one after another	Step-like slope

• Plateaus: high land that is flat-topped and steep-sided. On a map you check whether it is broad or narrow, flat or undulating (gently rolling), and whether it is dissected (cut into pieces by many river valleys).

• Example: the Chota Nagpur plateau (Jharkhand) is a dissected plateau.

• Plains: low, flat land. Contours are few and wide apart. Interpret them by origin:

Plain type	How it forms	Example
Alluvial	Rivers lay down silt	Ganga plain (Uttar Pradesh, Bihar)
Glacial	Glaciers and their meltwater lay down material	Outwash plains in the Himalaya; north European and North American plains
Karst	Water dissolves limestone, leaving sinkholes and caves	Limestone areas of Meghalaya; the Karst region of Slovenia
Coastal	Sea and rivers lay down sediment along the shore	Coromandel coast (Tamil Nadu)
Marshy	Badly drained, waterlogged low land	Sundarbans (West Bengal); Rann of Kachchh (Gujarat)

• Mountains:
• Peaks are the highest points. They are shown by spot heights or triangulation stations.
• Passes are low gaps across a ridge. On the map, contours dip between two high points.
• Examples: Zoji La (Ladakh), Shipki La (Himachal Pradesh), Nathu La (Sikkim).

2.3 Drainage

Drainage means the system of rivers and streams that carry water away from an area.

• Find the main river and its tributaries (smaller streams that join it). Note the direction of flow; it tells you which way the land slopes.
• Valley types: read them from the contours.
• V-shaped valley: cut by a fast river in hills. The contours form a V that points upstream.
• U-shaped valley: carved by a glacier.

• Gorge or canyon: a deep, narrow valley with very steep sides.

• Drainage patterns (the shape the river network makes on the map):

Pattern	Shape	Cause	Example
Dendritic	Like the branches of a tree	Rock of the same hardness everywhere	Rivers of the Indo-Gangetic plain
Radial	Streams flow outward from a centre	A dome or high point	Amarkantak (Madhya Pradesh); Narmada, Son and Mahanadi rise here
Ring (annular)	Streams curve around a dome	Rings of hard and soft rock around a dome	Eroded domes
Trellis	Main streams parallel; tributaries join at right angles	Folded rocks, with hard and soft bands side by side	Old folded mountains, e.g. Appalachians (USA)
Internal (centripetal)	Streams flow inward into a basin or lake	A closed, low basin	Loktak Lake (Manipur)

2.4 Land use

Land use means what people use each piece of land for.

• Forests are shown in green. They are marked by legal status:

Category	Meaning
Reserved Forest (RF)	Strictest protection. Most rights such as grazing and cutting are banned unless allowed.
Protected Forest (PF)	Some rights are allowed unless banned.
Unclassed (unclassified) Forest	Forest land not put into either group above.

• Together these three make up the Recorded Forest Area (RFA) [4].

• The mix differs a lot by state (ISFR 2019) [4]:

  • Karnataka: RFA 38,284 sq km. Reserved 28,690, Protected 3,931, Unclassed 5,663 sq km.
  • Bihar: RFA 6,877 sq km. Most of it is Protected (6,183 sq km); Reserved is only 693 sq km.
  • Punjab: RFA 3,084 sq km. Unclassed (1,903 sq km) is the largest group.

• Agriculture (shown in yellow), orchards, wasteland and industry (factories, mines, quarries).

• Services: schools, hospitals, post offices and airports. These show how developed a place is.
• Today: NRSC (National Remote Sensing Centre) has made satellite-based Land Use/Land Cover (LULC) maps at 1:50,000 scale for 2005-06, 2011-12 and 2015-16. The third cycle (2015-16) used Resourcesat-2 LISS-III images from three seasons. It has 54 classes, merged into 24 classes on the Bhuvan portal [3].

2.5 Transport and communication

• Roads (national highways, state highways, metalled and unmetalled roads), railways (broad, metre or narrow gauge), cart tracks, footpaths, bridges, post and telegraph offices.
• Look at how routes follow the relief:
• Roads and railways keep to valleys, plains and passes.
• On steep slopes, roads bend in hairpin bends to reduce the gradient.

2.6 Settlement

• Settlement means the places where people live. Read their type and pattern:
• Compact (nucleated): houses clustered together, common on fertile plains.
• Scattered (dispersed): houses spread apart, common in hills and forests.
• Linear: houses in a line along a road, river or canal.

• Circular and star-shaped patterns are also seen.

• Also note where settlements are placed: on high ground above flood level, near water, at road junctions.

2.7 Occupation

You work out occupation (how people earn a living) from land use. The map does not state it directly.

What the map shows	Likely main occupation
Villages among farmland and canals	Agriculture
Tribal settlements in forested hills	Lumbering and shifting cultivation (clear a forest patch, farm it for a few years, then move on)
Coastal villages, creeks, harbours	Fishing
Towns with schools, hospitals, offices, rail stations	Services and trade
Mines, quarries, factories	Mining and industry

3. Procedure of map interpretation

1. Locate the sheet. - Use the index number (the sheet's reference number in SOI's numbering grid) to place the sheet on the map of India. In the older numbering, 53 is a 1:1,000,000 sheet, 53F is a 1:250,000 sheet and 53F/1 is a 1:50,000 sheet. - The location tells you the physiographic region (for example the Himalaya, the Great Plains or the Peninsular plateau). From that you know what landforms to expect. - Note the scale and the contour interval (the height gap between two contours that sit next to each other). - OSMs carry a different sheet-numbering scheme from DSMs [2].

2. Trace each head separately. Copy major landforms, drainage, land use, and settlement with transport onto separate tracing sheets.

3. Describe each pattern in turn, for example "dendritic drainage, flowing north-east".
4. Superimpose pairs of tracings (lay one over another) to explain the links: - Contours over land use: forests sit on the closely spaced contours (steep slopes) and crops on the widely spaced ones. This shows how slope controls land use. - Drainage over settlement: villages line up along rivers. - Relief over transport: roads and railways follow valleys and passes.

4. Updating the map, and the link to GIS

• Why update: a sheet shows the land as it was on its survey date. Forests, towns and rivers change after that.
• How: compare the sheet with aerial photographs and satellite images of the same scale, then mark the changes.
• Hand overlay → GIS overlay:
• The tracing-sheet method is the ancestor of GIS overlay. A GIS (Geographic Information System) is computer software that stores each theme as a separate digital layer and stacks the layers for analysis.
• Bhuvan (ISRO's geoportal) publishes its theme layers as OGC-standard web services (WMS and WMTS). These can be laid over other layers in any GIS software. This is the digital version of laying tracing sheets over each other [3].
• NRSC's LULC layers are made in GIS vector format on WGS-84 datum [3].
• See remote-sensing-gis.

Prelims Hooks

• Survey of India was set up in 1767. It is India's oldest scientific department [5].
• National Map Policy 2005 created two series: DSMs (Everest/WGS-84 datum, Polyconic/UTM projection, classified) and OSMs (UTM projection, WGS-84 datum, unrestricted after one-time MoD clearance) [2].
• The Nakshe portal (2017) lets users download 1:50,000 OSMs free [5].
• Drainage pattern pairs: radial → Amarkantak; internal (centripetal) → Loktak Lake; trellis → folded rocks; dendritic → same-hardness rock and plains.
• Closely spaced contours mean a steep slope. If they are close at the top and wide at the bottom, the slope is concave.
• V-shaped contours that point upstream mark a river valley.
• Recorded Forest Area = Reserved + Protected + Unclassed forests [4].
• Trap: occupation is never printed on a toposheet. It is worked out from land use.
• NRSC's national LULC map is at 1:50,000 scale. Its third cycle is 2015-16, using Resourcesat-2 LISS-III data [3].

Mains Points

• Relief controls land use (GS-I):
• Steep slopes stay forested or are used for shifting cultivation. Plains are farmed and densely settled.

• Laying contour tracings over land-use tracings shows this clearly. It helps in planning watershed work and soil conservation.

• Settlement and transport follow terrain (GS-I):

• Linear settlements grow along rivers and roads. Routes in hills run through valleys and passes.

• This explains why the Himalaya and the Peninsular plateau have fewer, scattered villages than the Ganga plain.

• Access to maps vs security (GS-III):

• The split into DSMs and OSMs (2005) lets planners and citizens use accurate maps while defence data stays protected [2].

• Free OSM downloads (2017) support local planning [5].

• From hand overlay to GIS (GS-III):

• Satellite LULC layers updated each cycle (2005-06, 2011-12, 2015-16) and shared through Bhuvan make overlay analysis possible at national scale [3].
• Uses include tracking forest loss, urban growth and wasteland change.