Which tool or plugin for Theissen / Voronoi polygons in QGIS?

Thiessen/Voronoi Polygons in QGIS: Picking the Right Tool for the Job

Thiessen polygons, or Voronoi diagrams as some call them, are a seriously handy tool in spatial analysis. Think of them as a way to divide up a map into zones based on what’s closest to a specific point. Each zone, or polygon, grabs all the area nearer to its central point than to any other. Pretty neat, huh? This makes them super useful for all sorts of things, from figuring out market areas for businesses to understanding how water flows in a watershed. And QGIS? Well, it gives you a few different ways to create these polygons, each with its own little quirks. Let’s dive in!

The Built-In “Voronoi Polygons” Algorithm: Quick and Easy

QGIS actually has a “Voronoi Polygons” tool right there in the Processing Toolbox, under Vector Geometry. It’s probably the simplest way to whip up a Voronoi diagram from a point layer. You just point it at your data, maybe add a buffer, and boom – polygons!

Here’s the lowdown:

What you get is a brand-new polygon layer, with each polygon showing the area closest to its original point. Simple as that!

GRASS GIS: When You Need a Little More Muscle

Now, QGIS plays nice with GRASS GIS, which is another open-source powerhouse for geospatial stuff. GRASS has this module called v.voronoi, and it gives you another way to make these polygons. I’ve found it’s particularly useful when you need more control over the area you’re working with. You can really dial in the processing extent, using a layer’s boundaries or drawing your own area. Plus, you can even choose to get lines instead of polygons, which can be handy in some situations.

How to get GRASS working for you:

SAGA GIS: The Speed Demon?

Then there’s SAGA GIS. Like GRASS, it integrates with QGIS. SAGA also has a “Thiessen Polygons” algorithm accessible via the Processing Toolbox. Some folks swear that SAGA crunches through large datasets noticeably faster than the other options. I haven’t run extensive benchmarks myself, but it’s worth a shot if you’re dealing with a massive point cloud.

Pro Tips and Tricks

• Buffering is your friend: Seriously, don’t skip the buffer, especially if you’re going to be doing analysis with these polygons. Those clipped edges can mess with your results.
• Clip it good: If your buffered polygons are going wild and extending way beyond where you need them, clip them back to your area of interest using the “Clip” tool. It’s like giving them a haircut.
• Think about what you’re doing with them: Voronoi polygons are more than just pretty shapes. Use them to figure out service areas, spot patterns in your data, or even estimate rainfall over an area. Get creative!
• Lines to polygons (sort of): Voronoi diagrams are usually made from points, but you can use lines. The trick is to turn those lines into points first.
• Dissolve to simplify: If you have an ID attribute from a line layer, you can dissolve the resulting polygons using that attribute. Alternatively, you can perform a spatial join with the line layer.
• Splitting polygons: You can use Voronoi polygons to split polygons into equal parts.

Final Thoughts

QGIS gives you a solid toolkit for creating Thiessen/Voronoi polygons, no matter what you’re trying to do. Whether you stick with the built-in tool, tap into the power of GRASS, or give SAGA a whirl, understanding the strengths of each approach will help you get the job done right. So, go forth, experiment, and unlock the hidden insights in your spatial data!