Working with Projections

QGIS allows users to define a global and project-wide CRS (Coordinate Reference System) for layers without a pre-defined CRS. It also allows the user to define custom coordinate reference systems and supports on-the-fly (OTF) projection of vector and raster layers. All these features allow the user to display layers with different CRS and have them overlay properly.

Overview of Projection Support

QGIS has support for approximately 2,700 known CRS. Definitions for each of these CRS are stored in a SQLite database that is installed with QGIS. Normally you do not need to manipulate the database directly. In fact, doing so may cause projection support to fail. Custom CRS are stored in a user database. See Section Custom Coordinate Reference System for information on managing your custom coordinate reference systems.

The CRS available in QGIS are based on those defined by the European Petroleum Search Group (EPSG) and the Institut Geographique National de France (IGNF) and are largely abstracted from the spatial reference tables used in GDAL. EPSG identifiers are present in the database and can be used to specify a CRS in QGIS.

In order to use OTF projection, your data must contain information about its coordinate reference system or you have to define a global, layer or project-wide CRS. For PostGIS layers QGIS uses the spatial reference identifier that was specified when the layer was created. For data supported by OGR, QGIS relies on the presence of a recognized means of specifying the CRS. In the case of shapefiles, this means a file containing the Well Known Text (WKT) specification of the CRS. This projection file has the same base name as the shapefile and a prj extension. For example, a shapefile named alaska.shp would have a corresponding projection file named alaska.prj.

Whenever you select a new CRS, the used layer units will automatically be changed in the General menu of the mActionOptions Project Properties dialog under the Project (Gnome, OSX) or Settings (KDE, Windows) menu.

Global Projection Specification

QGIS starts each new project using the global default projection. The global default CRS is EPSG:4326 - WGS 84 (proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs) and comes predefined in QGIS. This default can be changed using the [Select...] button in the first section, used to defining the Default Coordinate Reference System to use when starting new projects, as shown in figure_projection_1. This choice will be saved for use in subsequent QGIS sessions.

Figure Projection 1:

../../../_images/crsdialog.png

CRS tab in the QGIS Options Dialog nix

When you use layers that do not have a CRS, you need to define how QGIS responds to these layers. This can be done globally or project-wide in the CRS menu under Settings ‣ mActionOptions Options.

The options shown in figure_projection_1 are:

  • radiobuttonoff Prompt for CRS
  • radiobuttonoff Use project CRS
  • radiobuttonon Use default CRS displayed below

If you want to define the coordinate reference system for a certain layer without CRS information, you can also do that in the General menu of the raster (see General Menu) and vector (see General Menu) properties dialog. If your layer already has a CRS defined, it will be displayed as shown in Vector Layer Properties Dialog .

Tip

CRS in the Map Legend

Right-clicking on a layer in the Map Legend (Section Map Legend) provides two CRS short cuts. Set layer CRS takes you directly to the Coordinate Reference System Selector dialog (see figure_projection_2). Set project CRS from Layer redefines the project CRS using the layer’s CRS

Define On The Fly (OTF) Reprojection

QGIS supports OTF reprojection for both raster and vector data. However, OTF is not activated by default. To use OTF projection, you must activate the checkbox Enable on the fly CRS transformation checkbox in the CRS menu of the mActionProjectProperties Project Properties dialog.

There are three ways to achieve this end:

  1. Select mActionOptions Project Properties from the Project (Gnome, OSX) or Settings (KDE, Windows) menu.
  2. Click on the geographic CRS status icon in the lower right-hand corner of the statusbar.
  3. Turn OTF on by default, by selecting the CRS tab of the Options dialog and selecting checkbox Enable ‘on the fly’ reprojection by default.

If you have already loaded a layer, and want to enable OTF projection, the best practice is to open the Coordinate Reference System menu of the Project Properties dialog, select a CRS, and activate the checkbox Enable on the fly CRS transformation checkbox. The geographic CRS status icon will no longer be greyed-out and all layers will be OTF projected to the CRS shown next to the icon.

Figure Projection 2:

../../../_images/projectionDialog.png

Project Properties Dialog nix

The Coordinate Reference System menu of the Project Properties dialog contains five important components as shown in Figure_projection_2 and described below.

  1. Enable on the fly CRS transformation - this checkbox is used to enable or disable OTF projection. When off, each layer is drawn using the coordinates as read from the data source and the components described below are inactive. When on, the coordinates in each layer are projected to the coordinate reference system defined for the map canvas.
  2. Coordinate Reference System - this is a list of all CRS supported by QGIS, including Geographic, Projected and Custom coordinate reference systems. To define a CRS, select it from the list by expanding the appropriate node and selecting the CRS. The active CRS is preselected.
  3. Proj4 text - this is the CRS string used by the Proj4 projection engine. This text is read-only and provided for informational purposes.
  4. Filter - if you know the EPSG code, the identifier or the name for a Coordinate Reference System, you can use the search feature to find it. Enter the EPSG code, the identifier or the name.
  5. Recently used CRS - if you have certain CRS that you frequently use in your everyday GIS work, these will be displayed in the table at the bottom of the Projection Dialog. Click on one of these buttons to select the associated CRS.

Tip

Project Properties Dialog

If you open the Project Properties dialog from the Project (Gnome, OSX) or Settings (KDE, Windows) menu, you must click on the CRS menu to view the Coordinate Reference System settings.

The geographic CRS status icon will also automatically bring the CRS menu to the front.

Custom Coordinate Reference System

If QGIS does not provide the coordinate reference system you need, you can define a custom CRS. To define a CRS, select mActionCustomProjection Custom CRS from the Settings menu. Custom CRS are stored in your QGIS user database. In addition to your custom CRS, this database also contains your spatial bookmarks and other custom data.

Figure Projection 3:

../../../_images/customProjectionDialog.png

Custom CRS Dialog nix

Defining a custom CRS in QGIS requires a good understanding of the Proj.4 projection library. To begin, refer to the Cartographic Projection Procedures for the UNIX Environment - A User’s Manual by Gerald I. Evenden, U.S. Geological Survey Open-File Report 90-284, 1990 (available at ftp://ftp.remotesensing.org/proj/OF90-284.pdf ).

This manual describes the use of the proj.4 and related command line utilities. The cartographic parameters used with proj.4 are described in the user manual and are the same as those used by QGIS.

The Custom Coordinate Reference System Definition dialog requires only two parameters to define a user CRS:

  1. a descriptive name and
  2. the cartographic parameters in PROJ.4 format.

To create a new CRS, click the mIconNew New button and enter a descriptive name and the CRS parameters. After that you can save your CRS by clicking the mActionFileSave Save button.

Note that the Parameters must begin with a +proj=-block, to represent the new coordinate reference system.

You can test your CRS parameters to see if they give sane results by clicking on the [Calculate] button inside the Test block and pasting your CRS parameters into the Parameters field. Then enter known WGS 84 latitude and longitude values in North and East fields respectively. Click on [Calculate] and compare the results with the known values in your coordinate reference system.