Slope, Aspect, and Hillshade

Derive terrain analysis products from Digital Elevation Models in QGIS

A comprehensive guide to extracting and visualizing slope, aspect, curvature, and hillshade from DEM data for professional terrain analysis and mapping

Tutorial: DEM Derivatives - Slope, Aspect & Hillshade

Master terrain analysis by deriving professional-grade slope, aspect, curvature, and hillshade products from Digital Elevation Models.

75 minutes

Intermediate

GIS Raster Analysis QGIS

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Learning Objectives

By the end of this tutorial, you will be able to:

• Objective 1 {#obj-1}: Calculate slope and aspect from a DEM → Go to Step 1
• Objective 2 {#obj-2}: Create hillshade visualizations for professional mapping → Go to Step 2
• Objective 3 {#obj-3}: Derive curvature and plan/profile curvature → Go to Step 3
• Objective 4 {#obj-4}: Combine derivatives for multivariate terrain analysis → Go to Step 4
• Objective 5 {#obj-5}: Style and symbolize DEM derivatives for publication → Go to Step 5

These skills are essential for geomorphological analysis, landslide hazard assessment, hydrological modeling, and terrain-based land use planning.

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📍 Learning Path: Objectives → Steps

Objective	Related Step	Time	Key Outcome
🎯 Calculate slope & aspect	Step 1: Slope & Aspect	15 min	Slope and aspect rasters
🎯 Create hillshade	Step 2: Hillshade	15 min	Shaded relief visualization
🎯 Derive curvature	Step 3: Curvature	15 min	Curvature and morphology
🎯 Combine derivatives	Step 4: Multivariate Analysis	15 min	Integrated analysis
🎯 Style for publication	Step 5: Styling & Symbolization	15 min	Professional cartography

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Workflow Summary

This tutorial follows these key steps:

┌─────────────────────────────────────────────────────┐
│ STEP 1: Slope & Aspect Calculation                  │
│ What: Derive basic terrain parameters               │
│ Time: ~15 min                                       │
└──────────────────────┬──────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────┐
│ STEP 2: Hillshade Creation                          │
│ What: Generate shaded relief for visualization      │
│ Time: ~15 min                                       │
└──────────────────────┬──────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────┐
│ STEP 3: Curvature Derivatives                       │
│ What: Calculate terrain curvature products          │
│ Time: ~15 min                                       │
└──────────────────────┬──────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────┐
│ STEP 4: Multivariate Analysis                       │
│ What: Combine derivatives for integrated analysis   │
│ Time: ~15 min                                       │
└──────────────────────┬──────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────┐
│ STEP 5: Styling & Symbolization                     │
│ What: Create publication-ready maps                 │
│ Time: ~15 min                                       │
└──────────────────────┬──────────────────────────────┘
                       ↓
┌─────────────────────────────────────────────────────┐
│ RESULT: Complete DEM analysis package               │
└─────────────────────────────────────────────────────┘

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Prerequisites & Requirements

Before you start, make sure you have:

• QGIS 3.x or higher installed with GDAL/OGR tools
• Processed DEM from the “Working with Terrain Data” tutorial (or any clipped DEM)
• Study area shapefile for reference (optional but recommended)
• Basic understanding of raster data concepts
• ~200 MB free disk space for processing and outputs

Knowledge assumed: - Familiarity with QGIS interface and raster layers - Understanding of coordinate reference systems - Basic map projection concepts

⚠️ Important: DEM derivatives are highly sensitive to data quality. Use high-quality DEMs (LiDAR preferred, SRTM acceptable) for accurate results.

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Data & Resources

What you need:

Resource	Type	Purpose
Clipped DEM	GeoTIFF raster	Input for all derivatives
Study Area Shapefile	Vector boundary	For clipping outputs (optional)
QGIS GDAL Tools	Built-in	Processing algorithms

Optional reference materials: - QGIS Raster Analysis Documentation: https://docs.qgis.org/latest/en/docs/user_manual/processing_algs/qgis/rasteranalysis.html - Terrain Analysis Basics: https://en.wikipedia.org/wiki/Digital_elevation_model - Slope/Aspect Standards: https://pubs.usgs.gov/pp/p1395/report.pdf

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Step-by-Step Instructions

Step 1: Slope & Aspect

Achieves: ✅ Learning Objective 1: Calculate slope and aspect

What you’ll do: Derive slope and aspect rasters from your DEM, which are fundamental terrain parameters used in countless analyses.

1.1 — Understanding Slope and Aspect

Slope measures terrain steepness (in degrees or percent). Aspect measures terrain orientation (0-360°, where 0° = North).

These are calculated using the elevation gradients in the raster: - Slope = rate of maximum elevation change - Aspect = direction of maximum elevation change

1.2 — Calculate Slope

QGIS Raster Menu

Raster 
  → Analysis 
  → Slope

Configuration:

Parameter	Setting	Notes
Input layer	dem_clipped.tif	Your DEM raster
Z factor	1.0	Default; increase for exaggeration
Output layer	slope_degrees.tif	Output filename

Important: Choose degrees for accessibility (0-90°) rather than percent for professional analysis.

Click Run. QGIS calculates slope for every cell based on neighboring elevation values.

1.3 — Calculate Aspect

QGIS Raster Menu

Raster 
  → Analysis 
  → Aspect

Configuration:

Parameter	Setting
Input layer	dem_clipped.tif
Output layer	aspect.tif

Aspect interpretation: - 0° / 360° = North - 90° = East - 180° = South - 270° = West

1.4 — Interpret and Classify Results

Slope classification for terrain analysis:

Slope Range	Terrain Type	Characteristics
0-2°	Flat/plains	Prone to flooding, poor drainage
2-5°	Gentle slopes	Suitable for agriculture
5-15°	Moderate slopes	Good for development with care
15-30°	Steep slopes	Erosion risk, landslide hazard
30°+	Very steep	Mountainous, unstable

Pro Tip: Slope values >30° are typically unsuitable for building and prone to landslides, but excellent for identifying geological hazards.

← Back to Learning Objectives

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Step 2: Hillshade

Achieves: ✅ Learning Objective 2: Create hillshade visualizations

What you’ll do: Generate a shaded relief visualization that makes terrain features visually apparent and professional-looking.

2.1 — Understanding Hillshade

Hillshade simulates lighting effects on terrain, making subtle variations visible. It’s created by calculating slope and aspect, then combining them with a light source direction (typically from the northwest at 45° angle).

2.2 — Generate Hillshade

QGIS Raster Menu

Raster 
  → Analysis 
  → Hillshade

Configuration:

Parameter	Setting	Explanation
Input layer	dem_clipped.tif	Your DEM
Z factor	1.5	Exaggeration (1-3 typical)
Azimuth of light	315	Direction from top-left (0-360°)
Vertical angle	45	Height angle of light source (0-90°)
Output layer	hillshade.tif	Output filename

Light source explained: - Azimuth 315° = light from northwest (standard for Northern Hemisphere) - Vertical angle 45° = light from above at 45° (default, good visibility)

Click Run. QGIS creates grayscale hillshade (0-255 values).

2.3 — Enhanced Hillshade (Optional)

For better visual impact, combine hillshade with original DEM or slope data:

1. Load hillshade layer
2. Set Blend Mode to “Multiply” or “Overlay”
3. Adjust Opacity to 60-80%
4. Stack with slope or DEM underneath

This creates dramatic terrain visualization!

2.4 — Professional Hillshade Tips

• Exaggerate Z-factor (2-3) for small areas with subtle terrain
• Use Z=1 for large areas to avoid visual distortion
• Flip azimuth to 135° for Southern Hemisphere (opposite lighting)
• Stack multiple hillshades with different azimuths for better detail

← Back to Learning Objectives

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Step 3: Curvature

Achieves: ✅ Learning Objective 3: Derive curvature and plan/profile curvature

What you’ll do: Calculate terrain curvature to identify topographic features like ridges, valleys, and saddle points.

3.1 — Understanding Curvature

Curvature measures how much terrain bends (concave/convex):

• Positive values = convex slopes (ridges)
• Negative values = concave slopes (valleys)
• Zero values = straight slopes

Plan Curvature = horizontal curvature (side-to-side)
Profile Curvature = vertical curvature (up-down)

3.2 — Calculate Plan Curvature

QGIS Processing Toolbox

Search for “Plan Curvature” in the Processing Toolbox.

Configuration:

Parameter	Setting
Input DEM	dem_clipped.tif
Output layer	plan_curvature.tif

Plan Curvature interpretation: - Ridge areas show positive values (bright colors) - Valley areas show negative values (dark colors) - Used for identifying water accumulation patterns

3.3 — Calculate Profile Curvature

Search for “Profile Curvature” in the Processing Toolbox.

Configuration:

Parameter	Setting
Input DEM	dem_clipped.tif
Output layer	profile_curvature.tif

Profile Curvature interpretation: - Positive = slope becomes steeper downhill - Negative = slope becomes gentler downhill - Used for erosion and water flow modeling

3.4 — Terrain Classification Using Curvature

Combine curvature products to classify landforms:

Ridge areas:        Plan > 0, Profile > 0
Valley areas:       Plan < 0, Profile < 0
Slopes:             Plan ≈ 0, Profile varies
Saddle points:      Plan > 0, Profile < 0

← Back to Learning Objectives

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Step 4: Multivariate Analysis

Achieves: ✅ Learning Objective 4: Combine derivatives for multivariate analysis

What you’ll do: Integrate multiple DEM derivatives to create comprehensive terrain analysis products.

4.1 — Terrain Position Index (TPI)

TPI classifies terrain based on elevation relative to surrounding areas:

QGIS Processing Toolbox

Search for “Terrain Position Index” or use Raster Calculator:

TPI = DEM - FOCUS_OPERATION(DEM, "mean", radius=100)

TPI interpretation: - TPI > 0 = ridges and peaks - TPI ≈ 0 = slopes and flats - TPI < 0 = valleys and depressions

4.2 — Topographic Ruggedness Index (TRI)

TRI measures roughness of terrain (standard deviation of slope):

QGIS Raster Calculator

TRI = SQRT(slope_x² + slope_y²)

TRI applications: - Identify rough/mountainous terrain - Assess terrain accessibility - Hazard zone identification

4.3 — Combined Terrain Classification

Create a multivariate classification using multiple derivatives:

QGIS Raster Calculator

Terrain_Class = IF(slope < 5, 1,           # Flat
                IF(slope < 15, 2,          # Gentle
                IF(slope < 30, 3,          # Steep
                4)))                        # Very Steep

Then refine by aspect for exposure: - North-facing = cooler, shadier - South-facing = warmer, sunnier - E/W = transitional

4.4 — Landslide Hazard Assessment (Example Application)

Combine derivatives for hazard mapping:

Hazard = HIGH if (slope > 30° AND 
                  plan_curvature < 0 AND 
                  profile_curvature > 0)

This identifies valleys with steepening slopes = high hazard zones.

← Back to Learning Objectives

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Step 5: Styling & Symbolization

Achieves: ✅ Learning Objective 5: Style DEM derivatives for publication

What you’ll do: Apply professional styling to DEM derivatives for cartographic publication.

5.1 — Color Ramps for Slope

Best practices for slope visualization:

QGIS Layer Properties

Symbology 
  → Raster 
  → Render type: Singleband pseudocolor

Recommended color schemes:

Slope Range	Color	Meaning
0-5°	Green	Safe, buildable
5-15°	Yellow	Caution
15-30°	Orange	Risk
30°+	Red	Hazard

QGIS provides presets: “Viridis”, “Plasma”, “RdYlGn_r” (good for slope)

5.2 — Aspect Circular Color Ramp

Aspect is circular data (0-360° = same as 360-0°), so use circular color ramps:

QGIS Aspect Styling

Symbology 
  → Raster 
  → Color ramp: "Spectral" (circular)

Standard aspect colors: - North (0°) = Purple - East (90°) = Green - South (180°) = Yellow - West (270°) = Red

5.3 — Hillshade as Background Layer

Layer stack for professional maps:

Top:    Vector overlays (shapefiles, contours)
        ↓
Mid:    Slope or curvature (with transparency)
        ↓
Bottom: Hillshade (grayscale background)

Configuration: 1. Set hillshade Blend Mode to “Multiply” 2. Set derivative Opacity to 60-75% 3. Enable vector borders for clarity

5.4 — Add Legend and Metadata

QGIS Composer/Print Layout

Add Legend 
  → Include all derivative layers
  → Add title, scale, north arrow
  → Add metadata box explaining:
     - Data source (SRTM, LiDAR, etc.)
     - Processing method
     - Color scheme meaning
     - Derived date

Professional cartography requirements: - ✅ Clear legend with units (degrees, percent) - ✅ Scale bar visible - ✅ North arrow oriented correctly - ✅ Data source and processing date noted - ✅ CRS/Projection clearly labeled

5.5 — Export for Publication

QGIS Export Options

For printed maps:

File 
  → Export as Image 
  → PNG at 300 DPI 
  → Or PDF for vector export

For web/presentations:

File 
  → Export as Image 
  → PNG at 150 DPI 
  → Or export QML style for web GIS

Pro Tips: - Export as GeoTIFF to maintain georeference - Use PDF for reports (vector text, smaller files) - Test on color-blind palette (use “Cividis” colormap)

← Back to Learning Objectives

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Result & Expected Outcome

What you should have now:

✅ Slope raster (degrees, 0-90°)
✅ Aspect raster (direction, 0-360°)
✅ Hillshade layer (grayscale relief)
✅ Curvature products (plan & profile)
✅ Publication-ready maps with proper styling

Your complete DEM derivatives package:

dem_derivatives/
├── slope/
│   ├── slope_degrees.tif
│   ├── slope_classified.tif
│   └── slope_percent.tif
├── aspect/
│   ├── aspect.tif
│   ├── aspect_classified.tif
│   └── shaded_relief.tif
├── curvature/
│   ├── plan_curvature.tif
│   ├── profile_curvature.tif
│   ├── terrain_position_index.tif
│   └── terrain_ruggedness_index.tif
├── integrated_analysis/
│   ├── terrain_classification.tif
│   ├── landslide_hazard.tif
│   └── sunlight_exposure.tif
└── maps/
    ├── slope_map.pdf
    ├── aspect_map.pdf
    ├── hillshade_composite.pdf
    └── terrain_analysis_summary.pdf

Applications of your derivatives: - 🏗️ Site suitability for construction - 💧 Hydrological flow modeling - 🌳 Vegetation and landcover analysis - ⚠️ Hazard assessment (landslides, avalanches) - 🗺️ Terrain-based mapping and planning - 🔬 Geomorphological research

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Exercises & Challenges

Challenge 1: Intermediate — Classify Slope for Land Use Planning

🟡 Create Land Use Suitability Map Intermediate

Objective: Create a slope-based land use classification suitable for urban planning.

Your task:

1. Calculate slope from your DEM
2. Create 5 classes: Flat (<5°), Buildable (5-15°), Limited (15-30°), Restricted (30-45°), Prohibited (>45°)
3. Assign colors: Green (best) → Red (worst)
4. Overlay with existing land use shapefile
5. Identify conflicts and suitable areas for development

Hint: Use Raster Calculator with nested IF statements for classification.

Solution (click to reveal):

Tip

# In QGIS Raster Calculator:
IF(slope < 5, 1,
   IF(slope < 15, 2,
      IF(slope < 30, 3,
         IF(slope < 45, 4, 5))))

Then apply color ramp: Green (1) → Yellow (2) → Orange (3) → Red (4-5)

Interpretation: Green areas are safest for building. Red areas require special engineering or are unbuildable.

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Challenge 2: Intermediate — Identify Ridges and Valleys

🟡 Geomorphological Feature Mapping Intermediate

Objective: Extract and map ridge lines, valley lines, and saddle points.

Your task:

1. Calculate plan curvature from your DEM
2. Ridges: Identify areas where plan_curvature > 0.5 AND slope > 10°
3. Valleys: Identify areas where plan_curvature < -0.5
4. Saddle points: Identify areas where aspect changes abruptly
5. Vectorize and map these features
6. Verify against hillshade for visual accuracy

Resources: - QGIS Raster to Vector conversion: https://docs.qgis.org/latest/en/docs/user_manual/processing_algs/qgis/rasteranalysis.html - Terrain morphometry guide: https://en.wikipedia.org/wiki/Terrain_morphology

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Tips, Tricks & Warnings

💡 Pro Tips

• Tip 1: Z-factor is crucial — Adjust Z-factor (slope multiplier) to match your terrain type. Use higher values (2-3) for subtle terrain, lower values (0.5-1) for dramatic terrain.

• Tip 2: Check for data artifacts — SRTM has artifacts over water bodies. Use -9999 as Nodata to exclude these from calculations.

• Tip 3: Combine derivatives strategically — Slope + Aspect alone misses terrain complexity. Always include curvature for better landform classification.

• Tip 4: Color choice matters — Use perceptually uniform colormaps (Viridis, Cividis) for scientific visualization. Avoid “rainbow” ramps—they distort perception of data.

• Tip 5: Test your hillshade azimuth — Northern Hemisphere uses 315° (NW light). Southern Hemisphere uses 135° (SE light). Test both for visual preference.

• Tip 6: Stack layers intelligently — Hillshade + slope (60% opacity) + contours = excellent terrain visualization for analysis and presentation.

⚠️ Common Pitfalls

Watch out for these mistakes:

1. Using wrong Z-factor: Z-factor too high (>5) creates unrealistic exaggeration. Z-factor too low (<0.5) makes subtle terrain invisible.

   ❌ Z=10 for a 30m DEM = 10x exaggeration = unrealistic
   ✅ Z=1.5 for a 30m DEM = subtle, realistic

2. Ignoring DEM quality issues: Artifacts in input DEM = artifacts in all derivatives. Always inspect DEM for holes, spikes, or water artifacts first.

3. Not checking nodata values: Nodata (-9999 or 0) propagates through calculations and corrupts derivatives. Always set nodata properly.

4. Misinterpreting aspect: Aspect values are 0-360° (circular). Standard statistics (mean, median) don’t work! Use circular statistics instead.

5. Forgetting CRS for slope calculation: Slope accuracy depends on CRS. Use projected coordinates (UTM, not geographic lat/lon) for accurate degree measurements.

🔴 Critical Warnings

These mistakes can invalidate your analysis:

⚠️ Never use geographic coordinates (lat/lon) for slope/aspect! Results will be completely wrong:

❌ WRONG: EPSG:4326 (WGS 84 lat/lon)
✅ CORRECT: EPSG:32632 (WGS 84 / UTM zone 32N)

Always reproject to projected CRS before calculating derivatives.

⚠️ Check DEM vertical units match horizontal! Mismatched units cause wrong slope values:

Example: 30m resolution DEM but elevation in feet
Result: Slope values 3x too large!

Verify: DEM resolution and units should be consistent (e.g., both in meters).

⚠️ Document your Z-factor choice! Always note the Z-factor used in metadata. Different Z-factors make results non-comparable between studies.

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Related Tutorials & Next Steps

After completing this tutorial, you might be interested in:

Level Up Your Skills

1. Advanced Terrain Analysis: Hydrological Modeling — Calculate flow accumulation and watersheds
   • Estimated time: 60 minutes
   • Difficulty: Advanced
2. Landslide Hazard Assessment Using Terrain — Combine DEM derivatives with geological data
   • Estimated time: 90 minutes
   • Difficulty: Advanced
3. Vegetation Analysis from Terrain — Link terrain properties to landcover classification
   • Estimated time: 75 minutes
   • Difficulty: Intermediate

Related Topics

• Solar Radiation Modeling — Calculate sunlight exposure using slope and aspect
• Geomorphological Mapping — Use terrain derivatives for landform classification
• Terrain-Based Landcover Prediction — Predict vegetation from terrain derivatives
• Alpine Hazard Assessment — Identify avalanche and rockfall zones using slope

Practice Projects

• Project 1: Create a land use suitability map for your region
• Project 2: Perform geomorphological analysis of a mountain range
• Project 3: Develop a terrain-based natural hazard assessment

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References & Further Reading

Official Documentation

• QGIS Raster Analysis Manual: https://docs.qgis.org/latest/en/docs/user_manual/processing_algs/qgis/rasteranalysis.html
  • Comprehensive guides for Slope, Aspect, Hillshade, Curvature
• GDAL Raster Functions: https://gdal.org/python/osgeo_gdal_gdalconst.html
  • Technical documentation for underlying algorithms

Academic References

1. “Terrain Analysis: Principles and Applications” — John C. Gallant & John P. Wilson (2000)
   • Standard reference for terrain derivatives
   • Explains mathematical foundations
2. “Digital Elevation Model Technologies and Applications” — David F. Maune (2007)
   • Practical applications of DEM derivatives
   • Quality assessment and error analysis
3. “Geomorphometry: Concepts, Software, Applications” — Hengl & Reuter (2009)
   • Comprehensive guide to terrain analysis
   • Includes QGIS workflows

Online Resources

• OpenTopography Project: https://www.opentopodata.org/
  • Free global elevation data and analysis tools
• USGS DEM Standards: https://pubs.usgs.gov/pp/p1395/report.pdf
  • Technical specifications and quality standards
• R Package ‘raster’: https://CRAN.R-project.org/package=raster
  • Alternative to QGIS for batch processing DEM derivatives

Tools & Software

• QGIS — Free, open-source GIS (what we use here)
• SAGA GIS — Specialized for terrain analysis, free
• WhiteboxTools — Modern terrain analysis library, free
• ArcGIS Spatial Analyst — Commercial but powerful

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Questions or Issues?

If you encounter problems:

1. Verify your DEM: Check for nodata values, data spikes, or negative elevations
2. Check CRS: Confirm you’re using projected coordinates, not lat/lon
3. Test on subset: Verify workflow on small area before processing entire DEM
4. Review QGIS documentation: https://docs.qgis.org/
5. Ask GIS Stack Exchange: https://gis.stackexchange.com/ with [qgis] tag

Common issues:

Issue	Solution
Slope values unrealistic (>90°)	Check that DEM CRS is projected (UTM, not lat/lon)
Hillshade shows artifacts	Smooth DEM with Gaussian filter before calculating
Aspect values wrap at 0°/360°	Use circular statistics, not standard mean
Curvature shows noise	Increase search radius or smooth DEM first

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Tutorial Version: 1.0
Last Updated: February 5, 2025
Difficulty: Intermediate
Estimated Duration: 75 minutes
Status: ✅ Complete and tested in QGIS 3.28.0

Prerequisites: Completion of “Working with Terrain Data” tutorial recommended but not required (any processed DEM works).