Selecting only polygons that contain another polygon with Select by Location in QGIS

Unlocking Spatial Secrets: How to Find Polygons Hiding Other Polygons in QGIS

Ever feel like your GIS data is holding back on you? Like there’s a secret layer of information just waiting to be uncovered? Well, you’re probably right! QGIS, that amazing open-source GIS powerhouse, has a trick up its sleeve called “Select by Location” that can help you find exactly that. And today, we’re going to crack the code on how to use it to find polygons that are playing host to other polygons. Think of it like finding Russian nesting dolls, but with maps!

So, what’s this “spatial relationship” business all about? Basically, it’s how GIS figures out if things are near each other, on top of each other, or, in our case, inside each other. “Select by Location” is the tool that lets you ask QGIS questions like, “Hey, which of these areas contain a school?” or, “Which counties have a river running through them?” It’s powerful stuff.

Now, the key to our polygon-finding mission is understanding the word “contains.” It’s not just about two shapes bumping into each other. When polygon A “contains” polygon B, it means polygon B is completely chilling inside polygon A, like a little shape within a bigger shape. Forget “intersects” – that’s just a polite hello. And “within” is the opposite game – we want the big guy.

Alright, let’s get our hands dirty. Here’s how you can become a “Select by Location” master:

Okay, so why should you care? Here are a few real-world scenarios where this trick can save the day:

• City Planning: Imagine you need to know which city council districts (the big polygons) have parks (the smaller polygons) within their boundaries. Boom! “Select by Location” to the rescue.
• Saving the Planet: Let’s say you’re tracking wetlands and want to know which watersheds contain them. This tool makes it a breeze.
• Real Estate Gold: Trying to find properties that have protected natural areas smack-dab in the middle? This is your shortcut.
• Managing Resources: Need to know which management zones include critical animal habitats? You guessed it – “Select by Location.”

A few things to keep in mind:

• Overlapping Mayhem: If your “containee” polygons are a bit messy and overlap, any “container” polygon that has any part of those overlapping polygons inside will get selected.
• Big Data Blues: Got a massive dataset? “Select by Location” can take a while. Spatial indexes are your friend here – they help QGIS speed things up.
• Other Options: “Select by Location” is great, but sometimes a spatial join might be a better fit, especially if you need to copy information from the “containee” layer to the “container” layer.

In conclusion, mastering “Select by Location” and the “contains” predicate is like unlocking a secret level in QGIS. It lets you ask powerful questions about your data and get answers that would otherwise take hours to find. So go forth, explore your geospatial data, and uncover those hidden polygon relationships! You might be surprised at what you discover.