Geographical Techniques I: Sources of Data, Types of Maps, Map-Making Techniques, Data Representation, GIS Data Models and Functions, DEM, Georeferencing and GIS Applications
2. Techniques of map-making and data representation
| Technique | How it works | Suitable data and cautions |
|---|---|---|
| Choropleth | Areal units such as districts are shaded in graded tones by class of value | Only standardised values — densities, percentages, rates — never raw totals; assumes uniformity within each unit; the result depends on the class intervals (equal interval, quantile, natural breaks) and on the units chosen (the modifiable areal unit problem) |
| Isarithmic (isopleth) | Lines join points of equal value, interpolated between observations | Continuous phenomena: contours (height), isotherms, isobars, isohyets (rainfall), isobaths (depth), isohalines (salinity), isochrones (travel time) |
| Dasymetric | Refines a choropleth by using ancillary data (land use, settlement) to redistribute values into zones of more uniform density, ignoring administrative boundaries | Population density, removing uninhabited forest, water and desert; formalised by J.K. Wright (1936) |
| Chorochromatic | Distinct colours or tints show the extent of different categories | Qualitative (nominal) data only — land use, soil types, geology, languages; no quantity implied |
| Flow map | Lines or arrows whose width is proportional to the volume moving along a route | Traffic, trade, migration; Charles Minard's map of Napoleon's Russian campaign (1869) is the classic |
Other standard techniques are dot maps (each dot a fixed quantity, good for showing density and clustering) and proportional symbols — circles whose radius is proportional to the square root of the value, so that their areas are proportional to it (spheres use the cube root). For data representation, a pie diagram divides a circle into sectors whose angles are proportional to the parts: angle = value ÷ total × 360°. If 45 of a village's 120 workers are in agriculture, their sector is 45 ÷ 120 × 360° = 135°. Bar diagrams are simple, multiple (bars side by side for comparison) or divided (compound) bars showing components; line graphs show change over time; and climatic data are plotted as climographs and Griffith Taylor's hythergraph (temperature against rainfall month by month).
3. The GIS database: raster, vector and attribute data; functions of GIS; the DEM
A geographic information system (GIS) is a system of hardware, software, data, people and methods for capturing, storing, querying, analysing and displaying geographically referenced data; Roger Tomlinson's Canada Geographic Information System of the 1960s is usually counted the first. Spatial data are stored in one of two models. The vector model represents discrete features as points, lines and polygons defined by coordinates, and can store topology — connectivity, adjacency and containment — which makes it precise, compact and good for networks and boundaries (formats such as the shapefile, GeoJSON and KML). The raster model divides space into a grid of cells (pixels), each holding a value; its accuracy depends on cell size (resolution), it is ideal for continuous surfaces and satellite images, and it makes overlay simple, but files are large (formats such as GeoTIFF; compressed by run-length encoding or quadtrees). Attribute data are held in tables linked to features by an identifier, usually in a relational database, and may be nominal, ordinal, interval or ratio.
| GIS function | Operations |
|---|---|
| Conversion | Digitising paper maps (on a tablet or on screen) and scanning; vector-to-raster conversion (rasterisation) and raster-to-vector conversion (vectorisation); format and projection conversion |
| Editing | Correcting digitising errors — overshoots, undershoots (dangles) and sliver polygons; building topology; edge-matching adjacent sheets; rubber-sheeting to fit one layer to another |
| Analysis | Spatial and attribute queries; buffering; overlay (union, intersect, clip); network analysis (shortest path, service areas); map algebra on rasters (local, focal, zonal); interpolation (inverse distance weighting, kriging); terrain analysis |
A digital elevation model (DEM) is a representation of the land surface's height, usually as a raster of elevation values; a digital surface model includes buildings and trees, while a digital terrain model is the bare ground. The vector alternative is a triangulated irregular network (TIN), which models terrain with irregular triangles (a Delaunay triangulation) — dense where relief is complex and sparse where it is flat. From a DEM a GIS derives slope, aspect, hillshade, viewsheds, flow direction and flow accumulation, and watershed and drainage-network delineation — the automated counterpart of the morphometric analysis in the third chapter of this unit. Widely used DEMs include the Shuttle Radar Topography Mission (SRTM, flown in 2000), ASTER GDEM and ISRO's CartoDEM from Cartosat-1.
4. Georeferencing: coordinate systems, projections and datum; GIS applications
Georeferencing assigns real-world coordinates to a scanned map or image by matching identifiable ground control points to their known coordinates and fitting a transformation (affine or polynomial); the fit is judged by its root mean square (RMS) error. Coordinates are either geographic — latitude and longitude in degrees on a curved surface — or projected — Cartesian x and y in metres on a plane. The most widely used projected system is Universal Transverse Mercator (UTM), which divides the world into 60 zones, each 6° of longitude wide, each with its own transverse Mercator projection; India spans zones 42 to 47. A datum defines the reference ellipsoid (the mathematical figure approximating the Earth) and its position relative to the Earth; the true, gravity-defined surface is the geoid. India long used the Everest spheroid (1830) in its Indian datum; GPS works in WGS 84, and mixing datums without transformation shifts positions by hundreds of metres.
| Projection property | What is preserved | Examples |
|---|---|---|
| Conformal (orthomorphic) | Shape and angles locally; area is distorted | Mercator (navigation), transverse Mercator, Lambert conformal conic |
| Equal-area (homolographic) | Area; shape is distorted | Albers equal-area conic, Mollweide, sinusoidal, Lambert azimuthal equal-area |
| Equidistant | Distances along certain lines or from one point | Equidistant conic, azimuthal equidistant |
| Azimuthal (zenithal) | True direction from the centre | Gnomonic, stereographic, orthographic |
GIS applications named by the document are two. Thematic cartography — automated production of choropleth, isarithmic, dot and flow maps from databases, with classification and symbolisation under the analyst's control. Spatial decision support systems (SDSS) combine a GIS with analytical models and multi-criteria evaluation — for example the analytic hierarchy process, weighting slope, land use, distance to roads and flood risk — to help planners choose sites for landfills, hospitals or industries and to compare scenarios. In India the ISRO geoportal Bhuvan and the PM Gati Shakti National Master Plan (2021), a GIS platform coordinating infrastructure across ministries, are large-scale examples.
Key takeaways
- Primary and secondary sources; spatial (location) and non-spatial (attribute) data; maps by scale (cadastral, topographical, atlas) and by content (general-reference, thematic); scale as statement, RF and graphical scale — 4 cm at 1:50,000 is 2 km.
- Choropleth needs standardised data; isarithmic lines join equal values (isohyets, isobars); dasymetric refines with ancillary data; chorochromatic shows categories; flow lines are scaled to volume; proportional circles use the square root; pie angle = value ÷ total × 360°.
- Vector (points, lines, polygons, topology) is precise and compact; raster (cells) suits continuous surfaces and overlay; attributes sit in linked tables; GIS functions are conversion, editing and analysis.
- DEMs (raster) and TINs (vector) model terrain; slope, aspect, viewshed and watersheds are derived from them; SRTM and CartoDEM are common sources.
- Georeferencing fits control points (judged by RMS error); UTM has 60 zones of 6°; conformal projections keep shape (Mercator), equal-area ones keep area (Albers); datums pair an ellipsoid with an origin (Everest 1830, WGS 84); SDSS and thematic cartography are the key applications.
Practice questions (12)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
On a topographical sheet with a representative fraction of 1:50,000, two villages are 4 cm apart. What is the ground distance between them in kilometres? Type a number.
Numerical answer — type the value.
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Answer: 2
4 cm × 50,000 = 200,000 cm = 2,000 m = 2 km. The RF uses the same unit on map and ground, so the conversion is only a matter of units.Which of the following is appropriate for mapping as a choropleth of districts?
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Answer: A — Population density (persons per km²)
Choropleths need standardised values such as densities or percentages; raw totals mislead because larger districts look higher simply by size. Village locations suit a dot map, and freight volumes a flow map.A population map that uses land-use data to remove uninhabited forest and water from each district and redistributes people into zones of more uniform density is a:
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Answer: A — Dasymetric map
Dasymetric mapping (formalised by J.K. Wright, 1936) refines a choropleth with ancillary information so that zones follow real variations in density rather than administrative boundaries.A map showing land use — forest, cropland, pasture and built-up areas — in distinct colours with no quantitative value is a:
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Answer: D — Chorochromatic map
Chorochromatic maps show qualitative (nominal) categories by colour. Choropleths grade quantitative values by tone, isarithmic maps join equal values, and proportional circles scale symbols to quantities.Lines on a map joining places receiving equal amounts of rainfall are called:
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Answer: C — Isohyets
Isohyets join equal rainfall; isobars join equal pressure, isohalines equal salinity and isochrones equal travel time. All are forms of isarithmic mapping.Of a village's 120 workers, 45 work in agriculture. In a pie diagram of the workforce, what angle in degrees should the agriculture sector have? Type a number.
Numerical answer — type the value.
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Answer: 135
Angle = value ÷ total × 360° = 45 ÷ 120 × 360° = 0.375 × 360° = 135°. The sectors of all components must add up to 360°.Which of the following are advantages of the raster data model over the vector model? Select all that apply.
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Answer: A — It is well suited to continuous surfaces such as elevation and satellite imagery; C — Overlay of layers is computationally simple
Grids overlay cell by cell and naturally hold continuous values and imagery. Precise boundaries and compact topology are strengths of the vector model — hence the mnemonic "raster is vaster, vector is correcter".In GIS, converting a raster land-cover grid into polygons with boundaries is called:
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Answer: B — Vectorisation
Raster-to-vector conversion is vectorisation; the reverse is rasterisation. Georeferencing assigns coordinates, and rubber-sheeting stretches one layer to fit another during editing.Which of the following map projections preserves area (is equal-area)?
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Answer: C — Albers conic
The Albers conic projection is equal-area and widely used for mid-latitude countries. Mercator and stereographic are conformal, and the gnomonic shows all great circles as straight lines; none of those three preserves area.In the Universal Transverse Mercator (UTM) system, each zone is:
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Answer: A — 6° of longitude wide, with 60 zones around the globe
UTM divides the Earth between 80°S and 84°N into 60 zones of 6° of longitude, each with its own transverse Mercator projection to keep distortion small. India falls in zones 42 to 47.Assertion (A): A triangulated irregular network (TIN) represents terrain with irregularly spaced points joined into triangles. Reason (R): A TIN is a raster data model in which every cell holds an elevation value. Choose the correct option.
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Answer: C — A is true, but R is false
A is true: a TIN places more points where relief is complex and joins them by Delaunay triangulation. R is false: the TIN is a vector model; the grid of elevation cells described in R is a raster DEM.Which of the following cannot be derived from a digital elevation model alone?
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Answer: B — Land-use classes
Slope, aspect and viewsheds are functions of elevation and can be computed from a DEM. Land use is information about cover and activity, which needs imagery or field data, not heights.