Lesson 5.2Lesson 5.2 · Raster & Terrain Analysis
Slope, Aspect, Hillshade and Contours
Four terrain products the whole DEM was waiting to give you
The DEM you downloaded looks like a grey blur. Everything useful is hidden inside it.
A raw Digital Elevation Model - a grid where each cell holds a height - is almost unreadable to the eye. But four short operations unlock it: slope tells you how steep, aspect which way it faces, hillshade makes the relief leap off the screen, and contours turn it into the lines an architect actually draws with. These four derivatives are where terrain analysis begins, and all of them come free from one elevation grid. This lesson computes and, more importantly, reads each one.
Drape everything on a hillshade. It is the cheapest way to make a map look like it knows something.
Start with a clean DEM - and know its resolution
A DEM is a raster of ground elevation. In India your default is CartoDEM (30 m or 90 m) from Bhuvan; globally, SRTM (~30 m) and ASTER GDEM (~30 m) are the free standbys, and ALOS PALSAR offers ~12.5 m radiometrically terrain-corrected tiles for finer work. Higher still, OpenTopography distributes lidar-derived DEMs where they exist.
Resolution is destiny here. A 30 m cell means the smallest landform the DEM can represent is 30 m across - it will smooth out a bund, a nala or a plinth. For regional siting and drainage context, 30 m is fine; for site grading and cut-fill you need lidar or a surveyed surface. Two housekeeping notes before you derive anything: work in a metre-based projected CRS so slope comes out in real degrees, and be wary of a DEM whose horizontal and vertical units differ.
A 30 m DEM cannot see your kerb. Match the DEM to the question, not the other way round.
Slope: how steep, in degrees or per cent
Slope is the rate of elevation change at each cell. The software slides a 3x3 window across the DEM and, from the eight neighbours, computes the steepest tilt - the classic rise-over-run. It reports either degrees (0 flat, 90 vertical) or per cent (rise/run x 100). Know which your tool is giving you: 100 per cent slope is 45 degrees, a fact that surprises people.
Slope is the single most-used terrain derivative in the built environment. It sets what is buildable without heroic earthwork, where roads and ramps can go (accessibility ramps have legal maximum gradients), how water will run, and where erosion and landslip risk climb. Reclassify it (Module 5.1) into buildable / cut-and-fill / avoid bands and it becomes a decision surface.
A subtlety worth knowing: several algorithms exist for the same slope. The common Horn method (the default in most tools) weights all eight neighbours; simpler methods use fewer. On smooth terrain the differences are tiny, but on noisy DEMs they matter, so state which DEM and method produced your slope layer - the same honesty you owe every derived product.
Aspect: which way the land faces
Aspect is the compass direction of the steepest downhill slope at each cell - the way the ground faces. It is reported in degrees clockwise from north (0 to 360) and usually binned into the eight cardinal and intercardinal directions, with flat cells flagged separately.
For design in India, aspect is a solar and comfort tool. South-facing slopes receive the most sun across the year and warm early; north-facing slopes stay cooler and shadier. That drives passive-design decisions - where to open up for winter sun in the north, where to shade against the west in the Deccan and the plains. Aspect also shapes drainage direction and, combined with prevailing wind, exposure. It is a circular quantity, so never average aspect naively - 350 degrees and 10 degrees average to 0 (north), not 180 (south).
Aspect is a direction, not a number line. North wraps around; your average must too.
Hillshade: a fake sun that makes relief readable
Hillshade simulates how the terrain would be lit by the sun from a chosen direction and height, shading each cell by how much light it would catch. It computes nothing new about the ground - it is purely a way of seeing - but it is transformative, turning a flat grey DEM into a landscape the eye reads instantly.
The two controls are azimuth (the sun's compass direction, conventionally 315 degrees / northwest) and altitude (its height above the horizon, often 45 degrees). The northwest default is a cartographic convention that avoids an optical illusion where valleys look like ridges. Use hillshade as a base layer under everything else: drape a semi-transparent slope or land-use layer over a hillshade and your maps suddenly have depth and legibility.
Contours: turning the grid back into lines
Contours are lines joining points of equal elevation, generated from the DEM at a chosen interval (say every 1 m, 5 m or 10 m). They are the terrain representation architects and planners have drawn for a century, and they remain the cleanest way to communicate landform on a plan and to set finished floor and road levels.
Contour interval is a judgement: too coarse and you lose the site's shape; too fine and the plan is a black smear. Match the interval to the terrain and the scale. Remember that DEM-derived contours inherit the DEM's resolution and any noise, so 30 m-DEM contours will look wobbly and are unfit for grading - smooth them, or derive from a surveyed surface for construction drawings. Contours close on themselves, never cross, and bunch where it is steep - reading that spacing is a core site-analysis skill.
CartoDEM (via Bhuvan)
India national DEM from Cartosat-1 stereo, 30 m and 90 m
Free via Bhuvan; the default source for slope/aspect/hillshade/contours over Indian sites.
NASA SRTM
Global radar DEM, ~30 m (1 arc-second)
Public domain via USGS EarthExplorer; the global standby for terrain derivatives.
ALOS PALSAR (ASF)
L-band SAR with ~12.5 m radiometrically terrain-corrected DEM tiles
Free (JAXA data via Alaska Satellite Facility, registration); a finer grid than SRTM for hillier sites.
OpenTopography
High-resolution lidar point clouds and DEMs, plus global DEM access
Free (per-dataset licences, often CC-BY); where lidar exists it is the leap from context to grading-grade terrain.
Workshop - unlock a DEM into four layers
Take one CartoDEM (or SRTM) tile and produce all four derivatives, then compose a readable site-topography sheet.
QGIS 3.44 (GDAL built in) or ArcGIS Pro with Spatial Analyst; a CartoDEM/SRTM tile.
Given: one DEM tile covering your site and 1-2 km around it Goal: slope, aspect, hillshade and contour layers + one composed map Time: ~60 minutes
- 1Download the DEM (CartoDEM from Bhuvan, or SRTM from EarthExplorer) and reproject it to a metre-based CRS (UTM zone for your area). In QGIS: Raster > Projections > Warp; in ArcGIS Pro: Project Raster.
- 2Compute slope and aspect. In QGIS: Raster > Analysis > Slope, then Aspect (GDAL). In ArcGIS Pro: Spatial Analyst > Surface > Slope and Aspect.
- 3Compute hillshade with azimuth 315 and altitude 45. In QGIS: Raster > Analysis > Hillshade. In ArcGIS Pro: Spatial Analyst > Surface > Hillshade.
- 4Generate contours at a sensible interval (try 5 m, adjust to the terrain). In QGIS: Raster > Extraction > Contour (GDAL). In ArcGIS Pro: Spatial Analyst > Surface > Contour.
- 5Compose the sheet: put hillshade at the bottom, a semi-transparent slope on top, contours as lines above, and label. Note in the margin the DEM's resolution so no one mistakes it for survey data.
You’ll walk away with
A four-layer terrain sheet for your site plus a one-line honesty note on the DEM's resolution and its fitness for use.
Three altitudes on the same idea
Read the band that fits you — or all three.
These four derivatives ARE your site's topography brief. Slope decides the buildable footprint and the earthwork budget; aspect drives passive-solar orientation and shading; hillshade makes your context panels legible; contours set your levels and section cuts. Master them and you arrive at the site meeting already knowing how the ground behaves.
Terrain derivatives underpin developable-land and hazard assessments. Slope thresholds define which land a plan can zone for development versus conservation; aspect and drainage feed micro-climate and green-network decisions. For hill towns and coastal ghats, a slope-and-aspect layer is a statutory-grade constraint, not a nicety.
Slope is the quiet author of walkability. Comfortable walking and cycling live under gentle gradients; a slope surface shows where a street grid or a public stair will invite or repel movement. Aspect tells you which side of a square will be sunlit at 4 pm - the side people will actually sit on.
“Hillshade and contours give me an accurate model of my site's surface.”
Do it yourself
No software needed - read terrain like a section.
- 1A slope layer reads 100 per cent on a steep bank. Convert that to degrees. (Hint: rise equals run.)
- 2For a house in Pune wanting winter morning sun and afternoon shade, which two aspects are you hunting for and which are you avoiding?
- 3Why is 315 degrees the conventional hillshade azimuth, and what illusion does it prevent?
- 4You draw contours from a 30 m SRTM tile for a construction drawing. Give two reasons that is a mistake.
- 5On a contour plan, the lines suddenly bunch tightly together. What does the ground do there?
The one line to carry out
Peer-reviewed journals & authoritative standards
- 01Wilson, J.P. & Gallant, J.C. (eds) — Terrain Analysis: Principles and Applications — Wiley, 2000.
- 02Burrough, P.A., McDonnell, R.A. & Lloyd, C.D. — Principles of Geographical Information Systems, 3rd ed. — Oxford University Press, 2015.
- 03de Smith, M.J., Goodchild, M.F. & Longley, P.A. — Geospatial Analysis: A Comprehensive Guide, 7th ed. — Winchelsea Press, 2025.
- 04Chang, K.-T. — Introduction to Geographic Information Systems, 9th ed. — McGraw-Hill Education, 2019.
- 05Longley, P.A., Goodchild, M.F., Maguire, D.J. & Rhind, D.W. — Geographic Information Science and Systems, 4th ed. — Wiley, 2015.
Slope and aspect describe the surface; next we make the surface DO things - decide what an observer can see and where every drop of rain will flow.
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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