Posted on December 30, 2022 (Updated on July 19, 2025)

How to find intersection polygons in single layer on QGIS version 3.4.2?

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Okay, here’s a revised version of the article, aiming for a more human and conversational tone, varying sentence structure, and injecting some personality:

Unlocking Spatial Insights: Finding Intersection Polygons in QGIS 3.4.2 (The Way a Real Person Would Explain It)

Ever needed to figure out where things overlap on a map? Like, really overlap? That’s where finding intersection polygons comes in, and QGIS – that awesome, free Geographic Information System – makes it surprisingly doable. We’re going to walk through how to do this in QGIS version 3.4.2. Now, I know what you might be thinking: “3.4.2? Isn’t that, like, ancient?” Well, it’s a Long Term Release (LTR), meaning it’s super stable. Plus, the basic process is the same even if you’re rocking a newer version. Think of it as learning to ride a bike – once you get the hang of it on an old Schwinn, you can handle a fancy carbon-fiber road bike too.

Why Bother with Intersections, Anyway?

So, why would you even want to find these overlapping areas? Turns out, it’s incredibly useful. Imagine you’re a city planner trying to figure out where new developments might run into zoning headaches. Or maybe you’re an environmental scientist mapping endangered species habitats and need to see where they bump up against protected areas. I even used it once to figure out which of my client’s properties were subject to overlapping floodplain regulations – saved them a ton of potential heartache! The possibilities are endless. Spotting these overlaps helps you make smarter decisions, plain and simple.

Let’s Get Our Hands Dirty: Finding Those Intersections

Alright, enough talk. Let’s get down to business. Here’s how you actually find those intersections in QGIS:

  • Bring in the Data: First things first, you need to load your polygon layer into QGIS. Think of it like importing your photos into Photoshop. You can drag and drop the shapefile right onto the map, or use the “Add Vector Layer” button. Either way works.

  • Find the Magic Tool: Next, head up to the “Vector” menu at the top of the screen. Click on “Geoprocessing Tools,” then “Intersection.” This opens up the Intersection dialog box – our main control panel for this operation.

  • Tell QGIS What to Intersect: This is where it gets slightly tricky, but stick with me. Since we’re looking for overlaps within the same layer, you’re going to select the same layer for both the “Input layer” and the “Overlay layer.” Yep, you read that right.

    • Input layer: Choose your polygon layer from the dropdown.
    • Overlay layer: Choose the same polygon layer again.
    • Input/Overlay fields: (Optional) If you want to bring over some information from the original layer, pick a field or two here.
    • Output layer: Give your new intersection layer a name and tell QGIS where to save it. Something like “intersections.shp” works great.

  • Hit “Run” and Watch the Magic Happen: Click that “Run” button and let QGIS do its thing. It might take a few seconds (or longer, depending on how big your data is), but eventually, it’ll spit out a brand-new layer showing you exactly where those polygons overlap.

  • Check Out Your Results: Once it’s done, the new layer pops up in your QGIS map view. Take a look! These are the polygons that represent the intersections. Open the attribute table to see any information that came along for the ride from the original layer.

    • A Few Things to Watch Out For (Because Nothing’s Ever Perfect)

    • Topology Troubles: Make sure your original data is clean! Overlaps, gaps, and weird self-intersections within your original polygons can cause headaches. Run the QGIS topology checker before you start intersecting to catch these gremlins.
    • CRS Chaos: Coordinate Reference Systems (CRS) – basically, how QGIS knows where things are on the planet – must match. If they don’t, reproject your layers to the same CRS before intersecting. Trust me on this one.
    • Performance Problems: Got a massive dataset? Intersections can take a while. Spatial indexing can help speed things up.
    • Sliver Alert: Sometimes, the intersection process creates tiny, skinny polygons called “slivers.” They’re usually just artifacts of the process. The “Eliminate Sliver Polygons” tool can help you get rid of them.

    Taking It to the Next Level

    The basic intersection tool is awesome, but QGIS has even more tricks up its sleeve:

    • “Difference” for the Opposite Effect: Want to see what doesn’t overlap? Use the “Difference” tool.
    • Focus Your Efforts with Selections: Select specific polygons before intersecting to zoom in on just those areas.
    • Become a QGIS Python Wizard: For serious customization, dive into the QGIS Python API. You can automate everything!

    Wrapping It Up

    Finding intersection polygons in QGIS 3.4.2 is a skill that’ll pay off big time. Follow these steps, keep an eye out for potential pitfalls, and you’ll be well on your way to unlocking all sorts of cool insights from your geospatial data. And hey, don’t be afraid to experiment! That’s how you really learn.

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