How does QGIS reproject data onscreen when the project CRS is geographic?

QGIS and Geographic Project CRS: How On-the-Fly Reprojection Works (The Human Touch)

QGIS, that trusty open-source GIS, is a real workhorse when it comes to visualizing, analyzing, and wrangling spatial data. One of its coolest tricks? Handling data from all sorts of different Coordinate Reference Systems (CRSs) without batting an eye. This is especially handy when your project CRS is set to a geographic coordinate system. So, how exactly does QGIS pull this off with its on-the-fly reprojection? Let’s dive in and take a look.

CRSs: A Quick and Painless Explanation

First things first, let’s demystify CRSs. Think of a CRS as a way to pin locations down on a map. Because the Earth isn’t flat (no matter what some folks might say!), projecting it onto a 2D surface is always going to involve some compromises. There are two main flavors of CRS:

• Geographic Coordinate Systems (GCS): These use latitude and longitude – those familiar gridlines you see on globes – to pinpoint locations. A classic example is WGS 84 (EPSG:4326). GCSs are like sticking pins directly into the Earth’s surface; they’re not projected.
• Projected Coordinate Systems (PCS): These systems take the Earth and flatten it out using some fancy math. PCSs use units like meters or feet, which makes measuring distances and areas much easier. UTM (Universal Transverse Mercator) and State Plane Coordinate Systems are common examples.

Why Your Project CRS Matters

In QGIS, the project CRS is like the lens through which you view all your spatial data. It dictates how the raw coordinates of your data are projected onto the map you see on your screen. Think of it as choosing your favorite map projection. You can pick any CRS that makes sense for your project area. You can find and set your project CRS in the CRS tab of the Project ► Properties… dialog. Plus, it’s always displayed in the lower-right corner of the QGIS window, so you always know what you’re looking at.

On-the-Fly Reprojection: The Magic Behind the Curtain

QGIS boasts “on-the-fly” (OTF) reprojection, or “on-the-fly” CRS transformation, which is really where the magic happens. This means QGIS can automatically translate data from its original CRS into the project CRS for display, no matter where the data came from. Starting with QGIS 3.0, on-the-fly reprojection is always on. It just works!

Here’s how it plays out: When you drag a new layer into QGIS, the software sniffs out its CRS. If that CRS doesn’t match the project CRS, QGIS quietly performs a behind-the-scenes transformation. It rejiggers the layer’s coordinates to match the project CRS, ensuring everything lines up perfectly on your map. The best part? It doesn’t mess with your original data! The raw data stays untouched in its original CRS.

Geographic Project CRS: What’s the Catch?

So, what happens when your project CRS is a geographic one (like WGS 84)? QGIS still does its on-the-fly reprojection thing, but with a twist.

A Few Things to Keep in Mind

• Distortion Alert: When your project CRS is geographic, you’re essentially “unprojecting” everything to latitude and longitude for display. This can lead to some funky visual distortions, particularly across large areas. Geographic coordinate systems aren’t known for preserving shape, size, or distance.
• Measurement Caveats: Trying to measure distances or areas in a geographic CRS? Not the best idea. Geographic CRSs aren’t designed for accurate measurements because of those distortions we just talked about. For precision, stick to a projected coordinate system that’s appropriate for your region.
• Processing Considerations: While on-the-fly reprojection is great for quick visualization, it’s often smarter to reproject your data before you start doing any serious spatial analysis. This makes sure your calculations are based on data that’s optimized for accuracy.
• Transformation Choices: Sometimes, there are multiple ways to transform between CRSs. QGIS usually picks the most accurate one by default.

Making the Change Permanent

Need to permanently change a layer’s CRS? Maybe for analysis or to share data with someone who needs it in a specific CRS? No problem. QGIS has you covered:

• Export Magic: Right-click the layer, go to “Export,” and then “Save Features As…”. You can then specify the desired CRS for the new file.
• Reproject Layer Tool: Dig into the Processing Toolbox and find the “Reproject layer” algorithm. This tool spits out a brand-new layer with the reprojected geometries.
• GDAL Warp (for Rasters): Got raster data? Use the “Warp (Reproject)” tool in the Processing Toolbox (it’s under Raster > Projections).

The Bottom Line

QGIS’s on-the-fly reprojection is a fantastic tool that lets you mix and match data from different CRSs without a headache. When your project CRS is geographic, QGIS automatically juggles everything into latitude and longitude for display. Just remember the potential for distortion, and consider reprojecting to a projected CRS when accuracy is paramount. It’s all about choosing the right tool for the job!