Digitization: Creating Vector Layers from Satellite Imagery
**Digitization** is the process of manually tracing features (roads, buildings, land parcels, water bodies) from a raster image — like a satellite photo or scanned map — to create clean, usable vector data. ---
📋 Prerequisites
- QGIS Desktop 3.22+ installed on your computer.
- Basic understanding of GIS data concepts.
Step-by-Step Instructions
Load Your Base Imagery
1. **Layer → Add Layer → Add Raster Layer** 2. Load a georeferenced satellite image or basemap (e.g., via the QuickMapServices plugin for Google/Bing satellite imagery) 3. Zoom in to your area of interest for clear visibility ---
Create a New Vector Layer
1. **Layer → Create Layer → New Shapefile Layer** (or GeoPackage layer) 2. Choose the geometry type matching what you're digitizing: - Buildings → Polygon - Roads → Line - Points of interest → Point 3. Set the CRS to match your project 4. Add relevant attribute fields (e.g., "Name", "Type", "Area") ---
Start Editing
1. Select your new layer in the Layers Panel 2. Click the **Toggle Editing** pencil icon 3. Select the appropriate **Add Feature** tool from the Digitizing Toolbar ---
Trace the Features
- **For polygons (e.g., a building):** Click each corner of the building outline, then right-click to close the shape - **For lines (e.g., a road):** Click along the centerline of the road, right-click to finish - **For points:** Simply click once at the location After drawing, an attribute form pops up — fill in the relevant details and click **OK**. ---
Use Snapping for Accuracy
To avoid gaps or overlaps between adjacent features: 1. Go to **Project → Snapping Options** 2. Enable snapping and set a tolerance (e.g., 10 pixels) 3. This makes QGIS "snap" new vertices to existing ones automatically ---
Save Your Edits
1. Click **Save Edits** (disk icon) periodically to avoid losing work 2. When finished, click **Toggle Editing** again to exit edit mode ---
Clean Up Your Data
- Use **Vector → Geometry Tools → Check Validity** to find broken geometries - Use **Vector → Geometry Tools → Fix Geometries** to auto-repair common issues - Remove duplicate or overlapping features if needed ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Digitization = manually tracing features from imagery into vector layers
- Match geometry type to the real-world feature (point/line/polygon)
- Always enable snapping to keep adjacent features aligned
- Validate and fix geometries after digitizing
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## Practice Questions
1. Why is snapping important during digitization?
2. What geometry type would you choose for digitizing a lake?
3. Name one tool used to check and fix invalid geometries in QGIS.
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*Next Tutorial: Spatial Analysis – Buffer, Overlay, and Clip*