Lesson 0.2Lesson 0.2 · Ground Rules — Thinking Spatially
Vector, Raster, Features & Layers
The two ways the world becomes data
A road is a line. A satellite photo is a grid. Both are “the map.”
Zoom into any GIS project and you will find two completely different kinds of thing pretending to be the same map. Some things are crisp, editable shapes with data behind them — a plot you can click to read its owner. Other things are photographs made of tiny coloured squares — a satellite image, an elevation surface. These are the two data models of GIS: vector and raster. Almost everything you will ever load is one or the other, and choosing wrong makes easy work hard.
Vector has edges. Raster has resolution. Ask which one your question needs.
Vector: the world as shapes with data
The vector model stores reality as three kinds of geometry:
• Points — a single location: a tree, a lamp post, a bore-well, a bus stop. • Lines (polylines) — a length: a road centreline, a drain, a pipe, a contour. • Polygons — an enclosed area: a plot, a building footprint, a ward, a lake.
What makes a vector shape a GIS feature rather than a mere drawing is the row of data attached to it. Every feature sits in an attribute table: one row per feature, one column per fact. A polygon of a plot carries its area, owner, land use and FSI. Click the shape, read the row; sort the row, find the shape. Vector is the model for anything with sharp edges and things you want to count, label or query one-by-one.
One feature = one shape + one row of data. Lose the row and you’ve just got a doodle.
Raster: the world as a grid of values
The raster model throws a fine grid over the world and stores one value in every cell (pixel). A satellite image is a raster where each cell holds a colour (really, brightness in several bands). An elevation model is a raster where each cell holds a height in metres. A temperature surface, a slope map, a rainfall map — all rasters.
Raster's defining number is its resolution — the ground size of one cell. Sentinel-2 imagery is 10 m; Landsat is 30 m; the ISRO CartoDEM elevation model is ~30 m. A 10 m cell means the smallest thing the data can “see” is roughly 10 m across; you cannot recover a footpath from 30 m data no matter how far you zoom. Raster is the model for continuous phenomena — things that have a value everywhere, not just at sharp edges: height, heat, greenness, wetness.
Vector or raster? Match the model to the phenomenon
The rule of thumb: discrete, countable, edged things → vector; continuous, everywhere things → raster.
A plot boundary is discrete — vector. Elevation is continuous — raster. But the same phenomenon can flip models depending on the question. Population is really a set of people (points), yet we often map it as a raster “density surface,” or as vector wards each carrying a count. Roads are vector lines for routing, but “distance to the nearest road” is naturally a raster surface. A large part of GIS craft is converting between the two — rasterising vectors, or vectorising rasters — to suit the analysis. You will do both in later modules.
Layers stack both models together
A layer is one dataset of one model shown as one sheet in the stack. A project mixes them freely: a raster satellite base at the bottom, vector roads and plots above, a raster slope surface half-transparent on top. Each layer has its own attribute table (vector) or value range (raster), its own style, and its own coordinate system that the GIS reconciles on the fly.
The discipline is to keep one kind of thing per layer — all the plots in one layer, all the trees in another — so each layer has a clean, single-subject attribute table you can query. Sloppy layering (roads and rivers in one file) is the commonest beginner mistake and it poisons every analysis downstream.
One subject per layer. A layer that holds “everything” can be queried for nothing.
Sentinel-2 (Copernicus)
Free 10 m optical satellite raster
The default free imagery raster for the built environment; 10 m cells. dataspace.copernicus.eu
CartoDEM (NRSC)
India's ~30 m elevation raster
Free at 30 m posting via Bhoonidhi; a raster where each cell is a height in metres.
OpenStreetMap (OSM)
Global vector features (points/lines/polygons)
The reference example of vector data: every road, building and POI is a feature with attributes.
Workshop — hold both models in one project
You will load a vector layer and a raster layer for the same area, read a feature's attribute table, and read a raster cell's value — feeling the difference in your hands.
QGIS 3.44 or ArcGIS Pro; a free USGS EarthExplorer or ISRO Bhoonidhi account for the DEM.
Goal: one project with (a) OSM building polygons and (b) an elevation raster Data: OSM buildings (free) + SRTM or CartoDEM 30 m DEM (free) Time: ~40 minutes
- 1In QGIS: use QuickOSM to download
buildingpolygons for a small area. In ArcGIS Pro: add OSM buildings from the Living Atlas or a downloaded extract. - 2Open the vector layer's attribute table (QGIS: right-click → Open Attribute Table; ArcGIS Pro: right-click → Attribute Table). One row per building — that is vector.
- 3Download a DEM raster: SRTM 30 m from USGS EarthExplorer, or CartoDEM 30 m from ISRO Bhoonidhi. Add it to the project.
- 4Use the Identify / value tool on the raster (QGIS: Identify Features on the raster; ArcGIS Pro: Explore → click). A single cell reports one number — an elevation. That is raster.
- 5Style the DEM with a colour ramp and set it 50% transparent over the buildings. You are now looking at both models, stacked — the everyday reality of a GIS project.
You’ll walk away with
A single project holding a vector layer (with a table) and a raster layer (with per-cell values), and a clear, felt sense of when each model applies.
Three altitudes on the same idea
Read the band that fits you — or all three.
You will live mostly in vector, and lean on raster for terrain and imagery. Plot, building, tree, setback line — vector. Contours you derive from a raster DEM; the satellite backdrop for a context plan — raster. Know that a 30 m DEM is too coarse for fine site grading but perfect for reading the plot's overall slope and drainage.
Statutory plans are vector; the evidence base is often raster. Land-use zones, ward boundaries and road networks are polygons and lines you edit and total. But land-use change, built-up spread and green cover come from raster satellite analysis (Module 6). A planner must be fluent moving counts between the two — e.g. summarising a raster land-cover map by vector ward.
Street and block analysis is vector; experiential surfaces are raster. Block polygons, street centrelines and frontages are vector. But sunlight hours, visibility and “distance to a park” are continuous — raster surfaces you drape over the plan. Choosing the right model keeps a walkability study honest instead of blocky.
“Vector is for accurate work and raster is just low-quality background imagery.”
Do it yourself
Sort the world into the two models.
- 1List five things on a site and tag each vector or raster: bus stop, ground temperature, plot line, tree, rainfall.
- 2For a vector plot layer, write three columns its attribute table should carry.
- 3Explain in one sentence why a 30 m DEM cannot show a 2 m-wide drain.
- 4Name one phenomenon you could store either way, and say what question would decide it.
The grammar, in one line
Peer-reviewed journals & authoritative standards
- 01Chang, K.-T. — Introduction to Geographic Information Systems, 9th ed. — McGraw-Hill Education, 2019.
- 02Burrough, P.A., McDonnell, R.A. & Lloyd, C.D. — Principles of Geographical Information Systems, 3rd ed. — Oxford University Press, 2015.
- 03Copernicus Data Space Ecosystem (Sentinel imagery) — European Space Agency, ongoing.
Every feature and every cell claims to sit at a spot on Earth — but Earth is round and our screens are flat. How we squash the globe onto a plane, and why two “correct” datasets can still refuse to line up, is the coordinate-systems lesson next.
The author
Amogh N P
Architect, interior designer, and creative polymath. Studio Matrx began in his notebooks — his vision of design made honest, useful, and open to everyone. Its Academy is written and taught in his memory, and free, forever.
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