Extracting land cover data (in percent) from shapefile in QGIS?

Unlocking Land Cover Secrets: A QGIS How-To for Percentage Extraction

Ever wondered exactly how much of your town is covered in trees versus concrete? Or maybe you’re trying to track changes in farmland over time? Land cover data is the key, and QGIS, that awesome free GIS software, is the tool to unlock it. We’re talking serious insights for environmental monitoring, urban planning – you name it! This isn’t just some dry technical manual; I’m going to walk you through extracting land cover percentages from shapefiles in QGIS, step by step. Think of it as learning to read the landscape.

Know Your Terrain: Understanding the Data

First things first, let’s talk data. You’ll generally be juggling two main files: your land cover map and your area of interest. The land cover map, usually a shapefile, is like a patchwork quilt of different land types – forests, cities, fields, you get the picture. Each patch (or polygon, in GIS speak) is tagged with what it represents. Then you have your area of interest – the specific zone you’re analyzing. It could be anything from a whole watershed to just your neighborhood.

Let’s Get Practical: The Extraction Process

Alright, time to get our hands dirty! Here’s how to pull those land cover percentages out of QGIS:

1. Data Prep: Getting Everything in Order

Fire up QGIS and load both your shapefiles. Now, this is important: make sure they’re both speaking the same language, or rather, using the same Coordinate Reference System (CRS). Think of it like making sure your measurements are all in feet or all in meters. If they’re not, you’ll need to “reproject” one to match the other. Trust me, getting this right saves a world of headaches later. For larger areas, pick a projection that keeps area consistent – it’ll give you more accurate calculations.

2. Sizing Up the Patches: Calculating Polygon Areas

Next, we need to know the area of each land cover patch. This is where the Field Calculator comes in handy. Open the attribute table of your land cover shapefile – that’s where all the data about each patch is stored. Flick the editing switch to “on” (that little pencil icon), and then summon the Field Calculator (the calculator icon, naturally).

Create a new field, maybe call it “Area_km2,” to hold the area values. Tell QGIS it’s a decimal number. Now for the magic: in the expression box, type $area/1000000. This tells QGIS to calculate the area of each polygon in square kilometers. If you prefer square meters, just use $area. Hit OK, and boom! Each patch now has its area calculated. Remember to turn editing off and save your work!

3. Focus on Your Zone: Intersecting the Layers

Okay, now we need to see what land cover falls inside your area of interest. For that, we use the “Intersection” tool. Find it under Vector -> Geoprocessing Tools. Set your land cover shapefile as the “Input layer” and your area of interest as the “Overlay layer.” Give the output a name and run it. This creates a new shapefile with only the land cover portions that are within your zone.

4. Area, Round Two: Calculating Intersected Areas

Time to calculate areas again, but this time for the intersected polygons. Repeat the steps from Step 2 on your new intersected shapefile. This gives you the area of each land cover type within your area of interest.

5. Grouping Similar Patches: Dissolving by Land Cover

To get the total area for each land cover type, we need to “dissolve” the polygons. Think of it as melting all the forest patches into one big forest patch. Go to Vector -> Geoprocessing Tools -> Dissolve. Set the input to your intersected shapefile. In the “Dissolve field(s)” option, pick the attribute field that identifies the land cover type (like “Forest,” “Urban,” etc.). Run it, and QGIS will merge all adjacent patches of the same type.

6. The Big Picture: Total Area of Interest

If you haven’t already, calculate the total area of your area of interest shapefile. Use the Field Calculator, just like in Step 2.

7. The Grand Finale: Calculating Percentages

Here’s where it all comes together! Open the attribute table of your dissolved shapefile (from Step 5). Turn on editing, open the Field Calculator, and create a new field called “Percentage.”

In the expression box, type (Area_km2 / Total_Area) * 100. “Area_km2” is the area of each dissolved land cover polygon (from Step 4), and “Total_Area” is the total area of your area of interest (from Step 6). Hit OK, and QGIS will calculate the percentage of each land cover type. Save your changes, and you’re done!

Bonus Round: Alternative Routes

• Zonal Statistics: If you’re working with a raster (gridded) land cover map, the “Zonal Statistics” tool is your friend. It crunches the numbers for each zone, defined by your area of interest. It’s a great option, but you’ll need to convert your vector data to raster first.
• LecoS Plugin: For the serious landscape ecologists out there, the LecoS plugin is packed with tools, including land cover percentage calculations.
• “Raster layer zonal statistics”: This tool is available in the Processing Toolbox under Raster Analysis.

Pro Tips and Things to Keep in Mind

• Garbage In, Garbage Out: Your results are only as good as your data. High-quality land cover data is key.
• Projection Matters: Seriously, double-check your projections.
• Units, Units, Units: Keep your units consistent throughout the process.
• Overlapping Zones: If your area of interest has overlapping parts, the intersection might get weird. Dissolve it first!
• Simplify for Speed: Big files can be slow. Simplify the shapes if needed to speed things up.

So there you have it! Extracting land cover percentages in QGIS isn’t rocket science, and it opens up a world of possibilities for understanding and managing our landscapes. Now go forth and analyze!