Dealing with coordinate reference systems

Whenever we load a data source, QGIS looks for usable CRS information, for example, in the shapefile's .prj file. If QGIS cannot find any usable information, by default, it will ask you to specify the CRS manually. This behavior can be changed by going to Settings | Options | CRS to always use either the project CRS or a default CRS.

The QGIS Coordinate Reference System Selector offers a filter that makes finding a CRS easier. It can filter by name or ID (for example, the EPSG code). Just start typing and watch how the list of potential CRS gets shorter. There are actually two separate lists; the upper one contains the CRS that we recently used, while the lower list is much longer and contains all the available CRS. For the elevp.csv file, we select NAD27 / Alaska Albers. With the correct CRS, the elevp layer will be displayed as shown in this screenshot:

If we want to check a layer's CRS, we can find this information in the layer properties' General section, which can be accessed by going to Layer | Properties or by double-clicking on the layer name in the layer list. If you think that QGIS has picked the wrong CRS or if you have made a mistake in specifying the CRS, you can correct the CRS settings using Specify CRS. Note that this does not change the underlying data or reproject it. We'll talk about reprojecting vectors and raster files in Chapter 3, Data Creation and Editing.

In QGIS, we can create a map out of multiple layers even if each dataset is stored with a different CRS. QGIS handles the necessary reprojections automatically by enabling a mechanism called on the fly reprojection, which can be accessed by going to Project | Project Properties, as shown in the following screenshot. Alternatively, you can click on the CRS status button (with the globe symbol and the EPSG code right next to it) in the bottom-right corner of the QGIS window to open this dialog:

All layers are reprojected to the project CRS on the fly, which means that QGIS calculates these reprojections dynamically and only for the purpose of rendering the map. This also means that it can slow down your machine if you are working with big datasets that have to be reprojected. The underlying data is not changed and spatial analyses are not affected. For example, the following image shows Alaska in its default NAD27 / Alaska Albers projection (on the left-hand side), a reprojection on the fly to WGS84 EPSG:4326 (in the middle), and Web Mercator EPSG:3857 (on the right-hand side). Even though the map representation changes considerably, the analysis results for each version are identical since the on the fly reprojection feature does not change the data.

In some cases, you might have to specify a CRS that is not available in the QGIS CRS database. You can add CRS definitions by going to Settings | Custom CRS. Click on the Add new CRS button to create a new entry, type in a name for the new CRS, and paste the proj4 definition string in the Parameters input field. This definition string is used by the Proj4 projection engine to determine the correct coordinate transformation. Just close the dialog by clicking on OK when you are done.

Note

If you are looking for a specific projection proj4 definition, http://spatialreference.org is a good source for this kind of information.