Site Context & Constraints Mapping

A good site analysis is not one map crammed with everything; it is a set of clean, single-subject layers you can read one at a time and then combine. Think of the site's world as separated into themes: access (roads, transit, walking distance to a station), amenities (schools, clinics, markets within reach), hazards (flood lines, steep slopes, high-tension lines, contaminated ground), heritage (protected monuments and the statutory buffers around them), and ecology (waterbodies, tree cover, drainage paths, wetlands).

Each theme is a layer, and each layer answers a different question about the plot. Loaded together, they turn a bare boundary into a briefing. The discipline is to keep them separate first - a flood layer that has been mixed into a road layer can never be re-weighted or switched off - and only then to overlay them. That separation is the whole reason we do this in a GIS rather than on a single traced sheet.

The move that made GIS famous in design came from Ian McHarg's Design with Nature (1969): draw each constraint on a transparent sheet, stack the sheets, and the darkest areas - where constraints pile up - are the places to avoid. GIS does this arithmetically and instantly. You mark each constraint as a zone (a flood polygon, a heritage-buffer ring, a steep-slope patch), then combine them so the plot is divided into no-build ground and what remains: the buildable envelope.

In practice you build constraint zones with two everyday operations you have met earlier in the course: the buffer (a distance band - say, a 100 m no-build ring around a protected monument, or a 30 m margin from the top of a stream bank) and the overlay (combining layers so you can select the ground that satisfies every rule at once). The output is not a pretty picture; it is a defensible statement: this is where a building may legally and sensibly go.

Some constraints are absolute - you do not build inside a flood line or a heritage buffer, full stop. Those are exclusions: the ground is simply removed. But many site factors are matters of degree. A gentle slope is better than a steep one but not forbidden; closer to transit is better than far but not mandatory. For these you move from hard exclusion to weighted suitability: score each factor from good to poor, decide how much each one matters (its weight), and combine the weighted scores into a single suitability surface.

The honesty test here is the weights. If access matters twice as much as ecology on this brief, say so, write the numbers down, and be ready to defend them - because someone will change them and watch the answer move. A structured way to set defensible weights is the Analytic Hierarchy Process (Saaty, 1977), where you compare factors two at a time. The technique matters less than the principle: the map is only as objective as the weights you fed it, so make them explicit.

You can assemble most of a site briefing for an Indian plot from free data. OpenStreetMap gives you roads, footpaths, and many amenities as clean vector features (use the QuickOSM plugin in QGIS). ISRO Bhuvan offers thematic layers and imagery, and Survey of India Open Series Maps provide 1:50,000 topographic context. Flood and drainage lines you often derive yourself from a terrain model (the next lesson) or trace from imagery.

Be honest about the gaps. OSM completeness varies street by street; a missing amenity is not an absent one. Heritage buffers and precise flood lines are frequently statutory and must come from the local authority, not a crowd map - use open data to find and reason, then verify the legally binding lines with the master plan or the competent department. And keep everything in one projected, metre-based CRS (for much of India, a UTM zone such as EPSG:32643) so your buffers are true distances and your areas are real square metres.