on January 7, 2023

QGIS Print Composer Legend: Polygon symbol not closed

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QGIS Print Composer Legend: Why Are My Polygons Open? (And How to Fix It!)

QGIS is fantastic, right? This open-source Geographic Information System packs a serious punch when it comes to making maps, especially with its Print Composer (or Print Layout, if you prefer). But let’s be honest, we’ve all been there: you’re putting the finishing touches on your masterpiece, and then BAM! Your legend looks…off. Specifically, those nice, neat polygon symbols? They’ve got gaps in their outlines. Argh!

It’s like forgetting to close the fridge door – annoying and makes everything look a bit sloppy. So, what’s going on, and more importantly, how do we fix this frustrating little problem?

The Case of the Missing Polygon Closure

Basically, this “polygon symbol not closed” thing happens when you’ve beefed up the outline (or stroke) width of your polygons. While the polygons themselves look perfect on the map, the legend decides to get creative and show breaks in the outline. It’s a visual quirk, a hiccup in how QGIS draws the legend patch – that little sample that’s supposed to represent your symbol. Think of it as a tiny rendering disagreement.

Decoding the Mystery: Causes and Killer Solutions

Okay, so why does this happen, and what can we do about it? Turns out, there are a few usual suspects, and the best solution depends on the culprit:

  • Just a Glitch in the Matrix (a.k.a. Rendering Artifact): Sometimes, it’s just a random rendering hiccup in the Print Composer. QGIS gets a bit confused.

    • The Quick Fix: Try nudging the outline width ever so slightly. Seriously, even going from 0.5 to 0.51 can sometimes kick QGIS back into shape and force it to render correctly. It’s like giving it a gentle nudge to remember what it’s supposed to do!

  • The Default Patch Problem: QGIS, by default, uses a simple rectangle as the legend patch for polygons. Now, when you’ve got a super thick outline, trying to cram that into a tiny rectangle can cause issues. It’s like trying to fit an elephant into a Mini Cooper – things are bound to get a little distorted.

    • Solution 1: Get Crafty with Custom Shapes: If you’re running QGIS version 3.14 or later (and you really should be!), you’ve got some serious power at your fingertips. You can actually define the shape of your legend patch using something called Well-Known Text (WKT). Sounds complicated, but it’s just a way of telling QGIS what shape you want. Want a circle? No problem! There are tons of online resources with WKT strings for common shapes. Just copy, paste, and voila!
    • Solution 2: Centroid Fill Magic: Here’s a clever trick: style your polygons using ‘centroid fill’. This basically puts a marker symbol right in the middle of your polygon. Then, tweak the marker size to get it looking just right. The upside? Your legend will show that symbol perfectly. The downside? You might have to fiddle with the marker size in both the map and the legend to get everything balanced. It’s a bit of trial and error, but can be worth it!

  • Geometry Gone Wild: Sometimes, the complexity of your layer’s geometry can throw a wrench in the works. Multipolygons or overly intricate shapes can confuse the default legend patch.

    • The Simplification Solution: Try simplifying the layer’s geometry, especially for the legend. QGIS has some handy geometry processing tools that can help you with this. Think of it as giving your data a little haircut to make it more presentable.

  • The Inverted Polygon Predicament: Are you using the “Inverted polygon” style? If so, and you’re filtering by map content, the legend might just decide to misbehave. I’ve seen this happen more than once!

    • Let’s Get Practical: Customizing Legend Patches Step-by-Step

    Ready to roll up your sleeves and get your hands dirty? Here’s how to customize those legend patches:

  • Fire Up the Print Composer: Head to Project > New Print Layout in the main QGIS window.
  • Summon the Legend: Grab the “Add Legend” tool and drag a box on your layout to create your legend.
  • Take Control: Disable Auto-Update: In the “Item Properties” panel for the legend, uncheck “Auto update.” This is key, as it lets you tweak things manually.
  • Edit the Specific Item: Double-click the layer name in the “Legend Items” list that’s giving you grief. This opens the “Legend Item Properties” window.
  • Shape-Shifting Time!
    • Click on “Shape.” You’ll see the current patch shape (that pesky rectangle) and the options to change it.
    • Choose a pre-defined shape from the dropdown, or unleash your inner artist and paste in a WKT string.
    • Adjust the “Width” and “Height” to get the size just right.
  • Apply and Admire: Hit the “Back” button, close the window, and bask in the glory of your customized polygon symbol!
  • Don’t Forget the Style Manager: Legend patches are like secret agents, managed from the QGIS Style Manager. This is where you can create and manage shapes for points, lines, and polygons. Think of it as your legend patch headquarters.

    • Pro Tips for Legend Perfection

    • Keep it Real: Preserve Aspect Ratio: When you’re customizing shapes, make sure “Preserve Aspect Ratio” is checked. Otherwise, things can get stretched and wonky.
    • Symbol Layer Shenanigans: Experiment with different symbol layer types within your polygon’s style. You might be surprised at what you discover!
    • The Duplicate Layer Dance: Here’s a sneaky trick I’ve used: duplicate the problem layer, make the duplicate’s symbol smaller, and then hide it under the original layer. Use that layer in the legend. It’s a bit of a workaround, but hey, whatever works, right?
    • Refresh, Refresh, Refresh: After tweaking the layer’s symbology in the main QGIS window, hit “Update All” in the legend’s “Item Properties” to see those changes reflected in the Print Composer. Don’t forget this step!

    So, there you have it! By understanding why those polygon outlines go AWOL and using these tricks, you can create legends that are not only accurate but also look fantastic. Go forth and make beautiful maps!

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