Collecting geometries and selecting attribute-value using QGIS

QGIS: Wrangling Geometries and Pinpointing Data Like a Pro

QGIS. It’s a powerhouse, right? This open-source Geographic Information System lets you do just about anything with geospatial data – create, tweak, visualize, analyze, you name it. Two things you’ll find yourself doing a lot are collecting geometries and selecting features based on what they’re like. Let’s dive in!

Collecting Geometries: Making Sense of Scattered Pieces

Ever have a map where something should be one thing, but it’s broken up into a bunch of little pieces? Think of a country with a bunch of islands, or maybe a park split by a road. That’s where collecting geometries comes in. It’s basically like gluing those pieces back together into a single, multi-part feature.

Why bother?

Well, for starters, it cleans up your data. Fewer features mean easier management and faster analysis. Plus, it just looks better on a map. Imagine trying to analyze the total area of that island nation if you had to deal with each tiny islet separately! Collecting geometries lets you treat the whole thing as one unit, making analysis way more efficient.

How do we do it in QGIS? There are a couple of ways to skin this cat:

Real-World Example:

I once had a project where I was mapping communes in France. Some of these communes had exclaves – little detached bits of land. To calculate the total population of each commune, I needed to merge those exclaves. The Aggregate tool was perfect! I grouped by the commune ID, used first_value to keep the ID, and sum to add up the population from all the parts. Easy peasy.

Selecting Attribute Values: Finding Needles in a Haystack

Okay, now let’s talk about selecting features based on their attributes. This is like having a super-powered search function for your map. Want to find all the cities with a population over a million? Or maybe all the parks that have a playground? This is how you do it.

Why is this so useful?

Think about it: you can quickly explore your data, focus your analysis, extract specific subsets, and even create custom map styles. It’s all about finding the right features for the job.

QGIS gives you a couple of ways to select by attribute:

Expression Essentials:

Expressions are like little formulas. They use field names (in double quotes, like “population”), values (text in single quotes, like ‘City Name’), and operators (like =, >, <). You can even combine conditions with AND, OR, and NOT.

Expression Examples:

• Big cities only: “population” > 1000000
• Names starting with “N”: “name” LIKE ‘N%’
• Green energy power plants: “primary_fuel” IN (‘Biomass’, ‘Geothermal’, ‘Hydro’, ‘Solar’, ‘Wind’, ‘Storage’, ‘Wave and Tidal’)
• Power plants of a certain size: “capacity” >= 50000 AND “capacity” <= 100000

Pro Tip:

The “Select by Expression” tool has options to add to, remove from, or select within your current selection. Super handy for refining your results!

Saving Your Selection:

Once you’ve got your selection just right, you can save it as a new layer. Just right-click the layer in the Layers panel, go to Export > Save Selected Features As, and choose your format.

Mastering these techniques – collecting geometries and selecting attribute values – is like unlocking a secret level in QGIS. You’ll be able to wrangle your spatial data with confidence, turning it into insights and stunning visualizations. So go forth and map!