Solar, Slope & Microclimate Analysis

A digital elevation model (DEM) is a raster in which every cell holds one number - the ground height at that spot. For India the everyday free choices are CartoDEM (from ISRO's Cartosat-1, ~30 m, via Bhuvan) and NASA SRTM (~30 m, near-global). The cell size matters: a 30 m DEM sees the shape of a hill but not a garden step, so it is superb for site context and useless for a plinth level. Know your resolution before you trust a number.

One quiet but critical distinction: a DEM ideally describes the bare earth (a digital terrain model, DTM), whereas a surface model (DSM) includes buildings and trees. Global 30 m products sit somewhere between the two. For slope and drainage you want bare earth; for shadows cast by buildings you need a surface with the buildings in it - which is why the last lesson of this module turns to LiDAR.

Slope is how steeply the ground falls, computed for each cell from the height difference across its neighbours (rise over run) and reported in degrees or percent. It drives everything from drainage and cut-and-fill cost to whether a road can climb the plot and where you must not build at all. A simple rule of thumb that survives contact with reality: under ~10% is comfortable, 10-25% needs terracing and care, and above that costs escalate fast.

Aspect is the compass direction a slope faces - the answer to 'which way does this ground look?' It is computed from the same neighbourhood of cells. Aspect governs sun and shelter: in India (northern hemisphere) south-facing slopes receive the most sun and are warm and dry, while north-facing slopes stay cool, shaded and damp. Slope and aspect together are the first, cheapest environmental read on any sloping site - and both come free from the DEM.

Aspect tells you the general solar disposition; a solar/shadow analysis tells you the specifics - how much sun a spot actually receives once the sun's yearly path and the surrounding obstructions are accounted for. GIS tools model the sun's position through the day and year and cast the resulting shadows across the terrain (and, with a surface model, across buildings), producing hours-of-sun surfaces and overshadowing maps.

The case that usually governs is the low winter sun, which throws the longest shadows and decides whether a courtyard or a neighbour's window gets any light in December. QGIS offers hillshade and solar/shadow tools (some via the UMEP plugin and the SAGA/GRASS providers); ArcGIS Pro has Hillshade, Areas Solar Radiation and shadow tools built in. Even a plain hillshade - a shaded-relief image made by lighting the DEM from a chosen sun angle - is a fast, intuitive way to see how light rakes across a site.

Terrain also shapes air. Ridges accelerate and expose; valleys and lee slopes shelter but can trap cold air and pollution; a south-west opening funnels the pre-monsoon wind. You cannot get a full computational-fluid-dynamics wind model out of a DEM, and you should not pretend to - but you can reason well from slope, aspect, exposure and landform position, combined with local wind-rose data, to place openings, courtyards and planting sensibly.

This is where GIS terrain analysis meets passive design: orient the long facade to catch prevailing breeze and winter sun while shading the summer west; put water and planting where the drainage and cool north slope already are; keep habitable rooms off the frost-pocket valley floor. The DEM will not design the building, but it hands you the environmental grain of the site - and building with that grain is most of passive comfort in the Indian climate.