Spatial Analysis: Buffer, Overlay & Clip
Spatial analysis tools let you answer real-world location-based questions — "What's within 500m of this river?", "Which land parcels fall inside the flood zone?" This tutorial covers the three most-used tools: **Buffer, Overlay, and Clip**. ---
📋 Prerequisites
- QGIS Desktop 3.22+ installed on your computer.
- Basic understanding of GIS data concepts.
Step-by-Step Instructions
Buffer – Create a Zone Around a Feature
A **Buffer** creates a polygon representing a fixed distance around a point, line, or polygon. **Use case:** Find all buildings within 200m of a road. ### How to do it in QGIS: 1. **Vector → Geoprocessing Tools → Buffer** 2. Select your input layer (e.g., Roads) 3. Set the buffer **Distance** (e.g., 200 meters — make sure your CRS uses meters!) 4. Click **Run** ---
Overlay – Combine Two Layers
**Overlay** operations combine two vector layers based on their spatial relationship. | Overlay Type | Result | |---|---| | **Intersect** | Keeps only overlapping areas of both layers | | **Union** | Combines both layers, keeping all areas | | **Difference** | Keeps areas in Layer A that don't overlap Layer B | | **Symmetrical Difference** | Keeps areas that don't overlap in either layer | ### How to do it in QGIS: 1. **Vector → Geoprocessing Tools → Intersection** (or Union/Difference) 2. Choose your Input layer and Overlay layer 3. Click **Run** **Use case:** Find which agricultural land parcels fall within a flood-prone zone (Intersect the parcels layer with the flood zone layer). ---
Clip – Cut a Layer Using a Boundary
**Clip** extracts the portion of a layer that falls within a boundary layer — everything outside is removed. ### How to do it in QGIS: 1. **Vector → Geoprocessing Tools → Clip** 2. Input layer: the data you want to cut (e.g., Roads for the whole country) 3. Overlay layer: your boundary (e.g., a single district) 4. Click **Run** — the output contains only roads within that district ---
Buffer vs Clip vs Overlay — When to Use What
- **Need a zone/proximity area around something?** → Buffer - **Need to combine info from two layers?** → Overlay (Intersect/Union) - **Need to cut a large dataset down to your study area?** → Clip ---
A Common Real Workflow
1. **Clip** your national roads layer to your study district 2. **Buffer** the clipped roads by 100m (representing a "noise impact zone") 3. **Intersect** the buffer with a residential land-use layer to find affected houses ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Buffer = zone of fixed distance around a feature
- Overlay = combine two layers (Intersect, Union, Difference)
- Clip = cut a layer using another layer as a boundary
- Combine these tools in sequence for real-world spatial questions
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## Practice Questions
1. What's the difference between Clip and Intersect?
2. Which tool would you use to find a 500m safety zone around a factory?
3. Why is CRS important when creating a buffer with a distance in meters?
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*Next Tutorial: NDVI Calculation & Vegetation Health Mapping*