DEM Analysis: Slope, Aspect & Hillshade
A Digital Elevation Model (DEM) contains raw elevation values, but its real analytical power comes from **derivative products** — Slope, Aspect, and Hillshade — which reveal terrain characteristics essential for planning, hydrology, and hazard assessment. ---
📋 Prerequisites
- QGIS Desktop 3.22+ installed on your computer.
- Basic understanding of GIS data concepts.
Step-by-Step Instructions
Understand Slope
**Slope** measures the steepness or gradient of the terrain at each pixel, usually expressed in degrees (0–90°) or percentage. **Applications:** Landslide susceptibility, road/infrastructure planning, agricultural suitability ### How to calculate in QGIS: 1. **Raster → Analysis → Slope** 2. Select your input DEM 3. Set Z-factor (usually 1, unless your DEM's vertical and horizontal units differ) 4. Click **Run** ---
Understand Aspect
**Aspect** indicates the compass direction a slope faces (0–360°, where 0/360 = North, 90 = East, 180 = South, 270 = West). **Applications:** Solar radiation studies, agriculture (south-facing slopes get more sun in the Northern Hemisphere), snowmelt patterns ### How to calculate in QGIS: 1. **Raster → Analysis → Aspect** 2. Select your input DEM 3. Click **Run** ---
Understand Hillshade
**Hillshade** simulates how sunlight illuminates the terrain, creating a 3D shaded-relief effect that makes elevation features visually intuitive. ### How to calculate in QGIS: 1. **Raster → Analysis → Hillshade** 2. Select your input DEM 3. Set **Azimuth** (sun direction, default 315° = NW) and **Altitude** (sun angle, default 45°) 4. Click **Run** **Tip:** Hillshade is often layered underneath other data (like NDVI or LULC) at reduced opacity to add visual depth to a map. ---
Combine These for Terrain Analysis
A common workflow: 1. Generate Slope to identify steep, landslide-prone zones 2. Generate Aspect to understand sun exposure for agriculture planning 3. Generate Hillshade as a visual backdrop for your final map 4. Use **Reclassify** (Raster → Raster Calculator or Reclassify by Table) to categorize slope into classes like "Flat," "Moderate," "Steep" ---
Interpreting Results
| Slope Value | Classification | |---|---| | 0°–5° | Flat/gentle — suitable for most development | | 5°–15° | Moderate — needs care for construction | | 15°–30° | Steep — erosion/landslide risk | | >30° | Very steep — generally unsuitable for development | ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Slope = steepness of terrain (degrees or %)
- Aspect = direction a slope faces (compass degrees)
- Hillshade = simulated sunlight shading for visualization
- All three are derived directly from a DEM using QGIS's Raster → Analysis tools
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## Practice Questions
1. What does an aspect value of 180° indicate?
2. Why might a slope map be useful for landslide risk assessment?
3. What do the Azimuth and Altitude parameters control in a Hillshade calculation?
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*Next Tutorial: Watershed Delineation using DEM*