Spatial analysis: overlay, buffer and network

Detail

What spatial analysis is

• GIS (Geographic Information System): a computer system that links data about places on Earth and shows it as a map [6].
• Spatial analysis is the use of two kinds of data together to answer real questions about places:
• Spatial data: where a thing is (its location and shape, e.g. a hospital as a point, a road as a line, a district as an area).

• Attribute data: what the thing is (its details, e.g. number of beds, road type, population).

• Spatial analysis is what sets a GIS apart from other information systems. An ordinary database stores records. A GIS can also ask "where", "how near" and "what overlaps what".

• The sequence:
• First, define the problem (e.g. "Which villages have no hospital within 10 km?").
• Then, run the analysis on spatial and attribute data together.

• Result: answers and trend models that decision-makers can use.

• NCERT lists four operations: 1. Overlay analysis 2. Buffer analysis 3. Network analysis 4. Digital Terrain Model (DTM / DEM): a digital 3-D model of land height. It is covered in detail under topographical maps.

• India's base height data: CartoDEM v3 (a DEM made from Cartosat satellite images) is available on Bhuvan for India and nearby countries: Bangladesh, Bhutan, Nepal, Pakistan and Afghanistan [2].

Overlay analysis: the hallmark of GIS

• Overlay analysis: placing two or more thematic layers of the same area on top of each other to make a new layer (NCERT Fig. 4.7).
• Thematic layer: a map of one theme only, e.g. soil, land use, roads, water bodies.

• The new layer shows where the themes meet or match.

• It is called the hallmark (defining feature) of GIS because few other systems can do it. Britannica describes overlay as GIS's ability to put new information on top of existing data and show it in colour on screen. It is used for decisions in geology, ecology, land use, demographics and transport [6].

• Sieve mapping: the older, manual form of overlay.
• Each theme was traced on a transparent sheet.
• The sheets were stacked on a light table (a glass table lit from below).
• Light passed only through the unshaded parts, which showed the areas that met every condition.
• GIS overlay is the digital form of sieve mapping. It is faster, exact, and can be repeated with new data.

NCERT case: Aligarh urban sprawl (Uttar Pradesh)

• Location: Aligarh city, western Uttar Pradesh, in the Ganga–Yamuna Doab (the land between the Ganga and Yamuna rivers).
• Step by step (Fig. 4.8–4.10):
• Layer 1: urban land use of Aligarh in 1974.
• Layer 2: urban land use of Aligarh in 2001.
• Overlay the two → a new layer showing which land changed use (e.g. farmland to built-up area).

• Result: a map of land transformation and urban sprawl.

• Urban sprawl: a city spreading outward in an unplanned way onto nearby farmland and open land.

• This is called change detection: comparing the same area at two dates.

Land-suitability analysis

• Overlay also finds the best site for a project. Example: siting a landfill (a garbage dumping site).
• Layer A: water bodies, removed along with a safe distance around them, so groundwater and rivers stay clean.
• Layer B: settlements, removed along with a safe distance, for health and smell.
• Layer C: e.g. land use or slope.

• Overlay → only the land that passes every condition is left. These are the suitable sites.

• Note: here buffers and overlay work together. Buffers are drawn first, then overlaid.

National-scale overlay: PM GatiShakti

• PM GatiShakti National Master Plan (NMP), launched on 13 October 2021, is a GIS platform that stacks data layers of infrastructure, geographic features and demography (population data) for joint infrastructure planning [3][4].
• It was developed by BISAG-N (Bhaskaracharya National Institute for Space Applications and Geo-informatics) [3].
• It has onboarded 44 central ministries and 36 states/UTs, with 1,614+ data layers (PIB, June 2025) [3].
• A Network Planning Group (NPG), made up of the heads of network-planning divisions of the infrastructure ministries, examines project proposals so that planning stays unified and optimised [3].

Buffer analysis: proximity analysis

• Buffer analysis: drawing a zone of a set distance around a feature to find who or what lies inside it.
• It is also called proximity analysis (analysis of nearness).
• Buffers can be drawn around all three vector feature types:

Feature type	Example	Buffer shape	Use
Point	Hospital, well, factory	Circle or ring	Population served or exposed
Line	Road, river, railway	Corridor (strip) along the line	River-bank protection zone, road-widening impact
Area (polygon)	Lake, forest, city	Belt around the edge	Eco-sensitive zone around a park

NCERT case: hospitals in western Uttar Pradesh

• Districts (north to south): Saharanpur → Muzaffarnagar → Meerut → Ghaziabad → Gautam Buddh Nagar → Aligarh.
• Step by step (Fig. 4.12, 4.13):
• Mark hospitals as points.
• Draw rings of 2 to 10 km around each hospital.
• Overlay the rings on settlements and population.
• Result: places outside all rings are underserved (they lack nearby health care). New hospitals can be planned there.

Other buffer examples

• Exposure zone: households within 1 km of a chemical unit are marked as being at risk from leaks or pollution. This is useful for disaster planning.
• Municipal GIS on Bhuvan: NRSC's Bhuvan has a buffer analysis tool that finds amenities (schools, hospitals, ATMs) within a buffer distance the user chooses. It was shown with the Ludhiana Municipal Corporation (Punjab) GIS. Bhuvan also offers proximity analysis to find nearby places [2].

Network analysis

• Network analysis: analysis of movement along connected lines, i.e. a network of links (roads, tracks) and nodes (junctions, stations).
• What it finds:
• Shortest route: the least distance between two places.
• Fastest route: the least travel time, allowing for speed limits and traffic.

• Service area: all places reachable within a set time or distance, e.g. an ambulance's 15-minute zone.

• Networks studied: roads, railways, pipelines (oil, gas, water) and rivers (river flow and inland waterways).

• Why vector data suit it best:
• Vector data store features as points, lines and polygons, so a road is a connected line with a direction and a length.

• Raster data store the world as a grid of cells, which breaks a line into separate squares and loses the links between them.

• Buffer vs network service area: a buffer measures straight-line ("as the crow flies") distance. A network service area measures real distance along roads. A village 5 km away across a river with no bridge falls inside a 5 km buffer but may be outside the network service area.

• In India, network planning for roads, railways, gas pipelines and waterways is now done jointly on the GatiShakti GIS platform [3].

Comparing the three operations

Operation	Basic question	Input	Output	NCERT / Indian example
Overlay	What is where, together? What changed?	2+ thematic layers of the same area	New combined layer	Aligarh land use, 1974 vs 2001
Buffer	Who is near this?	A point, line or area plus a distance	Zone or ring	2–10 km rings around hospitals, western UP
Network	How do I get there? How far can I reach?	Connected line network (vector)	Route or service area	Road, rail and pipeline routing

Software named in NCERT

• ArcView / ArcGIS (commercial, by Esri).
• GeoMedia (commercial).
• QGIS (free and open-source).
• Indian platform: Bhuvan (NRSC/ISRO), a web portal with online buffer and proximity tools [2].

Policy setting: National Geospatial Policy, 2022

• Notified on 28 December 2022. It replaces the National Map Policy, 2005 [5].
• Vision up to 2035. It opens up and widens access to geospatial data so that governments, businesses and academia can use it [5].
• Targets by 2030:
• High-resolution topographical survey and mapping: 5–10 cm for urban and rural areas, 50–100 cm for forests and wastelands.

• A high-accuracy DEM for the whole country [5].

• Why it matters for spatial analysis: finer base maps give more exact overlays and buffers. A national DEM supports terrain analysis.

Exam pairing

• Urban change detection → overlay.
• Population served or affected → buffer.
• Best route or reach → network.

Prelims Hooks

• Overlay analysis is the digital form of sieve mapping (tracings stacked on a light table).
• NCERT's overlay example: Aligarh urban land use, 1974 vs 2001, showing urban sprawl.
• NCERT's buffer example: 2–10 km rings around hospitals in Saharanpur, Muzaffarnagar, Meerut, Ghaziabad, Gautam Buddh Nagar and Aligarh, all in western Uttar Pradesh.
• Buffer analysis is also called proximity analysis. Buffers can be drawn around points, lines and areas.
• NCERT's four spatial analysis operations are overlay, buffer, network analysis and the digital terrain model. Trap: "classification" or "geo-referencing" is not on this list.
• Network analysis suits vector data, not raster.
• QGIS is the free, open-source software. ArcGIS and GeoMedia are commercial.
• The PM GatiShakti NMP was developed by BISAG-N, has 1,614+ data layers, and was launched on 13 Oct 2021 [3][4].
• The National Geospatial Policy 2022 replaced the National Map Policy 2005 and has a vision to 2035 [5].
• The Bhuvan Municipal GIS buffer tool was shown with Ludhiana (Punjab) [2].

Mains Points

• Health-care equity (GS-I/II): buffer analysis shows underserved areas. In the western UP hospital case, villages outside the 2–10 km rings show where health care is lacking. Network service areas make this more exact by measuring real travel along roads instead of straight-line distance.
• Managing urban sprawl (GS-I): overlaying land-use maps from different years (Aligarh 1974 vs 2001) shows farmland lost to built-up area. This helps with master plans, green belts and protecting farmland, and can be repeated for any fast-growing Indian city with Bhuvan and IRS images.
• Infrastructure planning (GS-III): PM GatiShakti stacks 1,614+ layers from 44 ministries and 36 states/UTs. It applies overlay (avoiding forests and settlements), buffer (clearance zones) and network analysis (multimodal routes) at national scale. This cuts delays and duplicated work [3].
• Disaster and environment (GS-III): buffers around chemical units (1 km exposure zones), rivers and coasts, and overlays for landfill siting, support NDMA-style risk planning. The National Geospatial Policy 2022 targets of 5–10 cm mapping and a national DEM by 2030 will make these analyses more exact [5].