Overlay analysis

Also called: Map overlay · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 12, Ch 4 "Spatial Information Technology"

Meaning

Overlay analysis (also called map overlay) is a GIS operation that places two or more thematic layers (maps that each show only one theme, such as soil, land use or roads) of the same area on top of each other. The result is a new layer that shows where the themes meet or match.

• It is called the hallmark (defining feature) of GIS because few other systems can do it.
• It answers two key planning questions: "What is found together in one place?" and "What has changed here over time?"

Explanation

How it works

• Starting point: the GIS and its two kinds of data
• GIS (Geographic Information System): a computer system that links data about places on Earth and shows it as a map [6].
• Spatial data tells you where a thing is: its location and shape (a point, a line or an area).

• Attribute data tells you what the thing is, such as land-use type or population.

• Process

• First, define the problem. Example: "Where has the city grown?" or "Where can a landfill go?"
• Collect one thematic layer for each factor, all covering the same area.
• Stack the layers so that matching places line up exactly.

• The GIS combines them into one new layer.

• Result: a map of where conditions overlap, or where land has changed. Planners can act on it.

• Britannica's description: overlay is GIS's ability to put new information on top of existing data and show it in colour on screen. It supports decisions in geology, ecology, land use, demographics (population studies) and transport [6].

From sieve mapping to digital overlay

• Sieve mapping was 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. These were the areas that met every condition.

• GIS overlay is the digital form of sieve mapping.

• It is faster.
• It is exact.
• It can be repeated whenever new data comes in.

Two main uses

• Change detection (comparing the same area at two dates)
• Layer 1: land use in an earlier year.
• Layer 2: land use in a later year.
• Overlay: a new layer showing which land changed use, such as farmland that became built-up area.

• This is how land transformation and urban sprawl (a city spreading outward in an unplanned way onto nearby farmland and open land) are mapped.

• Land-suitability analysis (finding 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 that groundwater and rivers stay clean.
• Layer B: settlements, removed along with a safe distance, for health and to keep smell away.
• Layer C: land use or slope.
• Overlay: only land that passes every condition is left. These are the suitable sites.
• Here buffers are drawn first, then overlaid. The two operations work together.

Place among NCERT's spatial analysis operations

• 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.

• Input: two or more thematic layers of the same area. Output: a new combined layer.

• Condition needed: all layers must cover the same area, so that places line up correctly.
• Better base data gives better results. Finer base maps make overlays more exact.

Where it occurs

• Aligarh, western Uttar Pradesh (NCERT case, Fig. 4.8–4.10)
• Aligarh lies in the Ganga–Yamuna Doab (the land between the Ganga and Yamuna rivers).
• Its urban land use in 1974 was overlaid on its urban land use in 2001.

• The result was a map of land transformation and urban sprawl.

• PM GatiShakti National Master Plan (NMP), all of India

• Launched on 13 October 2021. It is a GIS platform that stacks data layers of infrastructure, geographic features and demography (population data) for joint infrastructure planning [3][4].
• Developed by BISAG-N (Bhaskaracharya National Institute for Space Applications and Geo-informatics) [3].
• It has brought on board 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 the network-planning divisions of the infrastructure ministries, examines project proposals so that planning stays unified [3].

• Bhuvan (NRSC/ISRO): India's web GIS portal. It offers CartoDEM v3 (a height model made from Cartosat images) for India and nearby countries: Bangladesh, Bhutan, Nepal, Pakistan and Afghanistan. This base layer can be used in overlays [2].

• Software used: ArcView / ArcGIS (commercial, by Esri), GeoMedia (commercial) and QGIS (free and open-source).
• Policy support: the National Geospatial Policy, 2022 (notified on 28 December 2022) aims for 5–10 cm mapping of urban and rural areas, 50–100 cm mapping of forests and wastelands, and a high-accuracy national DEM by 2030. These finer base layers will make overlays more exact [5].

Don't confuse with

• Buffer analysis: draws a zone of a set distance around a point, line or area to find what lies inside it. It is also called proximity analysis. Overlay combines layers. A buffer measures nearness to one feature. NCERT's buffer case is the 2–10 km rings around hospitals in western UP. NCERT's overlay case is Aligarh, 1974 vs 2001.
• Network analysis: studies movement along connected lines, such as the shortest route, the fastest route or a service area. It works best on vector data. Overlay asks "what is where, together?" Network analysis asks "how do I get there?"
• Sieve mapping: this is the manual form of the same idea (tracings stacked on a light table). Overlay analysis is its digital form in GIS.
• Digital Terrain Model (DTM / DEM): a 3-D model of land height. It is a separate operation on NCERT's list, though a DEM can be used as one layer inside an overlay.

Prelims Hooks

• Overlay analysis is the digital form of sieve mapping, the manual method of stacking tracings on a light table.
• NCERT's overlay example: Aligarh urban land use, 1974 vs 2001, which shows urban sprawl. This is called change detection.
• NCERT's four spatial analysis operations are overlay, buffer, network analysis and DTM/DEM. Trap: "classification" and "geo-referencing" are not on this list.
• Overlay needs two or more thematic layers of the same area, and its output is a new layer. In landfill siting, buffers are drawn first and then overlaid.
• The PM GatiShakti NMP (launched 13 Oct 2021) is a national-scale layer-stacking GIS. It was developed by BISAG-N and has 1,614+ data layers [3][4].
• QGIS is free and open-source. ArcGIS and GeoMedia are commercial.

Mains Points

• Managing urban sprawl (GS-I)
• Overlaying land-use maps from two dates (Aligarh, 1974 vs 2001) shows farmland lost to built-up area.
• This helps with master plans, green belts and protecting farmland.

• The same method can be repeated for any fast-growing Indian city using Bhuvan and IRS images.

• Infrastructure planning (GS-III)

• PM GatiShakti stacks 1,614+ layers from 44 ministries and 36 states/UTs [3].
• Overlay keeps new projects away from forests and settlements. Buffers mark clearance zones. Network analysis plans multimodal routes (routes that link road, rail and other modes).

• This reduces delays and duplicated work [3].

• Environment and disaster planning (GS-III)

• Overlaying layers of water bodies, settlements and slope picks safe landfill sites, protecting groundwater and public health.
• The National Geospatial Policy 2022 targets of 5–10 cm mapping and a national DEM by 2030 will give more exact layers for such overlays [5].

Related concepts

• Spatial analysis
• Sieve mapping
• Buffer analysis
• Network analysis

Read more

• Spatial analysis: overlay, buffer and network