QGIS Select by location QGIS 3.10.3 no precision parameter

QGIS Select by Location: Getting Precise Without a Precision Parameter (QGIS 3.10.3)

So, you’re diving into geospatial analysis with QGIS, and the “Select by Location” tool is your new best friend, right? It lets you pick out features on one layer based on where they are in relation to another. Think about it: finding all the houses in a flood zone, or spotting the perfect location for a coffee shop near a bustling train station. It’s powerful stuff. But if you’re using QGIS version 3.10.3, you might notice something’s missing compared to other GIS software like ArcGIS: that handy “precision parameter.” What’s the deal with that, and how do you work around it? Let’s break it down.

“Select by Location” 101

First off, the “Select by Location” tool is your go-to for spatial selections. You’ll find it under Vector > Research Tools > Select by Location. It’s all about choosing features from one layer (“select features from”) based on how they relate to features in another layer (“by comparing to the features from”). The magic lies in those spatial relationships, also known as geometric predicates. “Intersects,” “contains,” “touches,” “are within” – these are the rules that decide which features get selected.

Imagine you want to find all the addresses within a specific school district. You’d use “Select by Location,” choose your address layer as the target, the school district boundary as the source, and the “are within” predicate. Boom! QGIS highlights the addresses that fit the bill. Simple, right?

The Precision Parameter Puzzle: No Knob, No Problem

Now, here’s where things get interesting. Some GIS programs let you tweak the selection with a “precision” or “distance” parameter. This basically creates a buffer zone around your source features. In ArcGIS, for example, you can even use a negative distance to exclude features that are just barely touching the edge.

QGIS 3.10.3 doesn’t have this direct control. At first, I was a little bummed out when I noticed that. It can be a bit of a pain, especially when your data isn’t perfectly clean or you need a very specific distance-based selection. But don’t worry, there are ways to get the job done.

Here are a few tricks I’ve picked up:

Predicates and Pesky Topological Errors

A quick word on geometric predicates: they’re super important. “Intersects” means any overlap, while “overlaps” means one feature has to have a coordinate inside the other. Choosing the right predicate can save you a lot of headaches, especially if your data has little imperfections.

I’ve seen cases where “Select by Location” fails because points are just a tiny fraction of a millimeter away from lines. If that happens, try snapping the geometries together (Vector > Geometry Tools > Snap Geometries to Layer) or creating a tiny buffer around the lines before running “Select by Location.”

Final Thoughts

Okay, so QGIS 3.10.3 might not have that fancy “precision parameter” in the “Select by Location” tool. But honestly, with a few clever workarounds, you can still achieve some seriously precise spatial selections. Get comfortable with buffering, dive into virtual layers and SQL, and master those geometric predicates. You’ll be a QGIS pro in no time. And who knows, maybe a future version of QGIS will give us that precision parameter we’re all craving!