How can I get my Qgis area calculations in metres rather than degrees? My shapefiles are projected using WGS 1984 UTM fuseau 35 Sud

Getting Accurate Area Calculations in Meters with QGIS: A Human’s Guide

Ever wrestled with QGIS trying to get area calculations in meters, only to be stumped by degrees? Yeah, it’s a common head-scratcher, especially when you’re dealing with shapefiles projected in WGS 1984 UTM Zone 35S. It’s like trying to measure your living room for new carpet using a protractor – not exactly intuitive! But don’t worry, I’ve been there, and I’m here to walk you through it. The good news is, getting those accurate meter measurements isn’t as daunting as it seems.

The core issue? We’re dealing with two different ways of describing the Earth. Think of it this way: WGS 1984 is like using latitude and longitude to pinpoint a location – great for global navigation, but not so hot for local measurements. It uses degrees, which are angular measurements on a sphere. Now, try calculating the area of your backyard using angles! Projected coordinate systems, like UTM, are the game-changer. They flatten the Earth (well, a portion of it) onto a plane, allowing us to use good old meters for measuring distances and areas. UTM cleverly divides the world into zones, each with its own projection to minimize distortion.

So, how do we make QGIS play nice and give us those meter-based areas? The secret sauce is making sure your shapefile and your QGIS project are both set to the right coordinate system. Luckily, if you’re already using WGS 1984 UTM Zone 35S, you’re already partway there! UTM Zone 35S is specifically designed for accurate measurements in meters within its zone. Let’s dive into the nitty-gritty:

You can also use $area in other places, like labels, to display areas directly on your map. Pretty neat, huh?

A Few Things to Keep in Mind:

• Garbage In, Garbage Out: Your area calculations are only as good as your original data. If your shapefile was created with inaccurate methods, your areas will be off, too.
• Zone Hopping: UTM zones are great, but they’re not magic. If your study area crosses into another UTM zone, things get trickier. Consider using a different projection or splitting your data.
• “On-the-Fly” Isn’t Always Best: While convenient, “on-the-fly” transformations can sometimes slow things down. If you’re doing a lot of analysis, it’s often better to permanently reproject your data to the project CRS. You can do this by exporting the layer and choosing the correct CRS.
• Units, Units, Units: Always, always double-check your units! The field calculator will use the project CRS units. If you need hectares or something else, use the field calculator to convert (e.g., “$area”/10000 for square meters to hectares).

So there you have it! Calculating areas in meters in QGIS with WGS 1984 UTM Zone 35S shapefiles doesn’t have to be a headache. With these steps, you’ll be measuring like a pro in no time. Now go forth and map!