On The Fly Projection

Over the last few weeks I’ve seen a couple wonky things in QGIS when I scroll in super far, and have had a few friends call me with error messages related to coordinate reference systems (learn more about CRSs here). These issues are common when different layers in your QGIS project are using different CRSs.

Sometimes you might not notice that the CRSs in your project don’t all match, since under most circumstances they look like they line up. This is because QGIS (and other common programs like ArcMap) have on the fly projection enabled as default. On the fly projection re-projects the layers that don’t match the project CRS in real time so that everything can be displayed in the project CRS. You can turn this feature off and on by going to Settings->Options->CRS (see picture to left).

Most of the time on the fly projection works so well you don’t even notice that it’s running. One exception is when you zoom in really quickly, causing the layers to kinda drift apart and not line up. If you want to learn more, the documentation on the QGIS website for projections is a good place to start (here).

If you run a geoprocessing algorithm on two layers with different CRSs, you’ll get an error message. You can also run into issues when your project CRS isn’t the CRS of the layers you’re working with. In my next blog post, I’ll address how to fix both of these issues.