Blog · 9 October 2026 · 4 minute read

Clip, reproject, buffer: three QGIS tools worth knowing

QGIS is free, open source GIS software, and it has hundreds of tools. Three of them do a lot of the everyday work. Here is what each one does and when you need it.

1. Clip

Clip cuts a layer down to a boundary. You give it the data and a shape, and it keeps only what falls inside the shape.

You need it because open data rarely arrives the size you want. Elevation tiles can cover square kilometres when your job is 50 metres of road. If the site sits across two tiles, you merge the tiles first and then clip the result to the site. What you hand over is small, fast to open and has nothing in it that the project does not need.

In QGIS: Vector menu, Geoprocessing Tools, Clip. For rasters such as elevation models: Raster menu, Extraction, Clip Raster by Extent or by Mask Layer.

2. Reproject

Every spatial file is stored in a coordinate reference system, or CRS. Some systems use latitude and longitude, which are measured in degrees. Others are map grids measured in metres. Around Sydney the usual grid is MGA Zone 56 on the GDA2020 datum.

Reprojecting converts a layer from one system to another. You need it for two reasons. Layers in different systems may not line up. And you cannot sensibly measure a distance or an area in degrees, so anything that involves metres should be done on a grid that is in metres.

One thing catches people out. Changing the CRS in a layer's properties only changes the label. It does not move the data. To convert it properly, use the Reproject Layer tool, which writes a new layer in the system you choose.

In QGIS: Processing Toolbox, search for Reproject Layer. GDA2020 MGA Zone 56 is listed as EPSG:7856.

3. Buffer

A buffer is the zone within a set distance of something. Picture a line for a new pipeline, and say you need everything within 1 kilometre of it at any point. The buffer tool draws that zone for you: 1 kilometre either side of the line, finishing in a semicircle at each end, because the end point also reaches 1 kilometre in every direction.

Try it. Move the slider and watch which trees fall inside the zone.

The buffer distance uses the units of the layer's CRS. If the layer is in degrees, a distance of 1000 means 1000 degrees, which is nonsense. Reproject to a grid in metres first, then buffer.

In QGIS: Vector menu, Geoprocessing Tools, Buffer.

Run all three in one go

Merge, reproject, clip and buffer usually happen in the same order, job after job. QGIS has a Model Designer that lets you chain tools together. You connect the output of one step to the input of the next, save it, and from then on the whole chain runs from a single window.

It saves time, and it also means every site gets processed the same way, which makes the results easier to check.

In QGIS: Processing menu, Model Designer.