This section of the free chapter preview of Mastering QGIS looks at working with coordinate systems in QGIS. Mastering QGIS is designed to bring users who want to go beyond the basics of QGIS to an advanced level of QGIS expertise.
When working with spatial data, it is important that a coordinate reference system (CRS) be assigned to the data and QGIS project. To view the CRS for the QGIS Project, click Project | Project Properties and choose the CRS tab.
It is recommended that all data added to a QGIS project be projected into the same CRS as the QGIS project. However, if this is not possible, or convenient, QGIS can project layers ‘on the fly’ to the project’s CRS.
If you want to quickly search for a CRS, you can enter the EPSG code to quickly filter through the CRS list. An EPSG code refers to a specific CRS stored in the EPSG Geodetic Parameter Dataset online registry that contains numerous global, regional, or local CRS. An example of a commonly used EPSG code is 4326 which refers to WGS 84. The EPSG online registry is available at http://www.epsg-registry.org/.
To enable ‘on the fly’ projection, do the following:
- Click Project | Project Properties.
- Choose the CRS tab, and Enable ‘on the fly’ CRS transformation.
- Set the CRS we wish to apply to the project and have all layers not set to the project’s CRS become transformed ‘on the fly’.
To view the CRS for a layer:
- Open the layer’s properties by either clicking Layer | Properties, or by right-clicking on the layer in the Layers panel.
- Choose Properties from the context menu and then choose General tab.
- If the layer’s CRS is not set, or is incorrect, click Specify to open the CRS Selector window and select the correct CRS.
To project a layer to a different CRS:
- Right-click on the layer in the Layers panel, then choose Save As from the context menu.
- In the Save vector layer as dialog, set the file format, file name, set CRS to Selected CRS and click Change to set the target CRS, then save the file.
To create a new CRS, or modify an existing CRS:
- Click Settings | Custom CRS to open the Custom Coordinate Reference System Definition
- Click Add new CRS button to add a new entry to the CRS list above.
- With the new CRS selected, we can set the name and parameters of the CRS. The CRS properties are set using the 4 format. To modify an existing CRS, click Copy existing CRS and select the CRS to copy parameters from, otherwise, enter the parameters manually.
Proj.4 is another Open Source Geospatial Foundation (http://osgeo.org) project used by QGIS, like OGR and GDAL. This project is for managing coordinate systems and projections. For a detailed user manual for the Proj.4 format used to specify the CRS Parameters in QGIS, download it from ftp://ftp.remotesensing.org/proj/OF90-284.pdf.