Buffers and Polygon Simplification: Two Everyday GIS Operations
How buffer distances and simplification tolerances work, when each is appropriate, and how to shrink a heavy polygon without wrecking its shape.
What a buffer really is
A buffer creates a new polygon containing every location within a given distance of an input feature. It answers proximity questions: which properties are within 100 m of the pipeline, what falls inside a 5 km catchment.
Buffer distance is measured in ground units, so a metre-based buffer needs metre-based reasoning — buffering raw degrees produces an ellipse that widens toward the poles.
Choosing a buffer distance
Let the question set the number, not the map. Regulatory setbacks, service radii and noise contours all come with defined distances; using a round number because it looks tidy is how analyses get challenged.
Negative buffers (erosion) are useful too — shrinking a parcel inward to model a build envelope.
Simplification: fewer vertices, same story
Coastlines and administrative boundaries digitised at survey precision can carry hundreds of thousands of vertices. On a web map at country zoom, most of those vertices land inside the same pixel.
Douglas–Peucker style simplification removes vertices that fall within a tolerance of the line between their neighbours. Larger tolerance means smaller files and blockier shapes.
How to pick a tolerance
Iterate visually rather than guessing:
- Start small and increase until you can see the shape change, then step back
- Match the tolerance to display scale, not to storage ambitions
- Keep the original file — simplification is lossy and irreversible
- Re-check areas and lengths afterwards if the numbers are going into a report