Last reviewed 16 Sept 2026 · 9 min read
Plans and maps
- A plan is a large-scale drawing of a small area (e.g. a building site) where the earth's curvature is irrelevant.
- A map is a small-scale representation of a larger area of the earth on a plane, showing natural and artificial features, usually with a projection and grid.
Classification of maps
By scale
| Category | Typical scales | Examples |
|---|---|---|
| Large-scale | 1 : 500 to about 1 : 10 000 | Building plans, estate plans, town plans, cadastral maps |
| Medium-scale | About 1 : 25 000 to 1 : 50 000 | Topographic maps (toposheets), district planning |
| Small-scale | 1 : 250 000 and smaller | State maps, atlases, wall maps |
By purpose/content
Topographic maps (relief, drainage, settlements, communications, vegetation), cadastral maps (property boundaries, land records), engineering maps (project sites, routes), contour maps, thematic maps (geology, soils, land use, population, rainfall), road and railway maps, navigation charts, city guide maps.
Choice of scale
The scale is chosen considering:
- Purpose of the map and the detail required.
- Size of the area and the paper/screen size.
- Plotting accuracy — the smallest distance that can be plotted and read reliably on paper is about 0.25 mm; ground features smaller than cannot be shown to scale.
- Time and cost of surveying (larger scales need more detail).
FormulaPlotting accuracy and scale
Survey of India topographic maps
- The Survey of India (SOI) publishes topographic maps of the country.
- Traditional sheet system: India was covered by 1 : 1 000 000 sheets of 4° × 4°; each divided into 16 sheets of 1° × 1° at 1 : 250 000 (e.g. sheet 53E); each degree sheet divided into 16 sheets of 15′ × 15′ at 1 : 50 000 (e.g. 53E/1 to 53E/16); each 1 : 50 000 sheet into 4 sheets of 7′30″ × 7′30″ at 1 : 25 000 (e.g. 53E/1/NW).
- Under the National Map Policy (2005), SOI publishes two series:
- Defence Series Maps (DSMs) — for defence and national security (restricted).
- Open Series Maps (OSMs) — for public and development use, on the WGS 84 datum and UTM projection, with sensitive details excluded.
- Digital topographic databases and online geoportals are increasingly used; India's geospatial guidelines and National Geospatial Policy (2022) liberalised access to geospatial data.
Map projections
A map projection is a systematic method of representing the curved surface of the earth (graticule of latitudes and longitudes) on a flat surface. No projection preserves all properties simultaneously.
Properties that may be preserved:
- Conformal (orthomorphic) — shapes of small areas and angles preserved.
- Equal-area (equivalent) — areas preserved.
- Equidistant — distances preserved along certain lines.
- Azimuthal — directions from a central point preserved.
| Projection | Developable surface | Features / use |
|---|---|---|
| Cylindrical equal-area | Cylinder tangent at equator | Areas true; shapes distorted towards poles |
| Mercator | Cylinder (conformal) | Conformal; rhumb lines (constant bearing) are straight lines — navigation charts; large area distortion at high latitudes |
| Transverse Mercator | Cylinder tangent along a central meridian | Conformal; accurate in a narrow north–south zone — topographic mapping |
| Universal Transverse Mercator (UTM) | Secant transverse Mercator in 6° longitude zones | Scale factor 0.9996 at the central meridian; coordinates in metres with false easting of 500 000 m; widely used for GNSS and topographic maps (India spans several UTM zones) |
| Conical projections (e.g. Lambert conformal conic) | Cone tangent or secant along standard parallels | Suitable for mid-latitude regions extending east–west |
| Polyconic projection | Series of cones along each parallel | Used for Survey of India's older topographic sheets; minimal distortion for small sheets |
| Azimuthal (zenithal) | Plane tangent at a point | Polar maps; directions from centre true |
Grids, graticules and references
- Graticule — network of parallels (latitudes) and meridians (longitudes).
- Grid — rectangular network of lines (eastings and northings) on a projected map, with numbered grid lines.
- Grid reference — location given by eastings first, then northings ("read right, then up"): e.g. a six-figure reference such as 435782 means easting 43.5 and northing 78.2 grid units within the sheet's labelling.
- Three norths: true north (geographic), grid north (direction of grid lines), magnetic north; the angles between them (grid convergence and magnetic declination) are shown in the map margin.