Trabalhando com Projecções¶

Visão geral do Suporte a Projecções¶

QGIS has support for approximately 2,700 known CRSs. Definitions for each 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 CRSs are stored in a user database. See section Sistema de Coordenadas personalizado for information on managing your custom coordinate reference systems.

The CRSs 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, either your data must contain information about its coordinate reference system or you will need 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 layer units will automatically be changed in the General tab of the Project Properties dialog under the Project (or Settings) menu.

Especificação Geral da Projecção¶

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 it comes predefined in QGIS. This default can be changed via the ^{Select CRS} button in the first section, which is used to define the default coordinate reference system for new projects, as shown in figure_projection_options. This choice will be saved for use in subsequent QGIS sessions.

CRS tab in the QGIS Options Dialog

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 tab under Settings ‣ Options.

The options shown in figure_projection_options are:

• Prompt for CRS
• Use project CRS
• 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 tab of the raster and vector properties dialog (see General Properties for rasters and General Properties for vectors). If your layer already has a CRS defined, it will be displayed as shown in General tab in vector layers properties dialog.

Define On The Fly (OTF) CRS Transformation¶

QGIS supports on the fly CRS transformation for both raster and vector data. However, OTF is not activated by default. When OTF is off, each layer is drawn using the coordinates as read from the data source. When OTF is on, each layer’s coordinates are transformed to the CRS of the project.

There are three ways to enable On The Fly CRS Transformation:

• Select Project Properties ‣ CRS from the Project ( or Settings) menu. You can then activate the Enable on the fly CRS transformation (OTF) checkbox in the CRS tab and select the CRS to use (see Coordinate Reference System Selector)
• Click on the ^{CRS status} icon in the lower right-hand corner of the status bar, leading you to the previous dialog.
• Turn OTF on by default in the CRS tab of the Settings ‣ Options dialog by selecting Enable ‘on the fly’ reprojection by default or Automatically enable ‘on the fly’ reprojection if layers have different CRS.

If you have already loaded a layer and you want to enable OTF reprojection, the best practice is to open the CRS tab of the Project Properties dialog, activate the Enable ‘on the fly’ CRS transformation checkbox, and select a CRS. The ^{CRS status} icon will no longer be greyed out, and all layers will be OTF projected to the CRS shown next to the icon.

Project Properties Dialog

Coordinate Reference System Selector¶

This dialog helps you assign a Coordinate Reference System to a project or a layer, provided a set of projection databases. Items in the dialog are:

• 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.
• Recently used coordinate reference systems: If you have certain CRSs that you frequently use in your everyday GIS work, these will be displayed in this list. Click on one of these items to select the associated CRS.
• Coordinate reference systems of the world: This is a list of all CRSs 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.
• PROJ.4 text: This is the CRS string used by the PROJ.4 projection engine. This text is read-only and provided for informational purposes.

Sistema de Coordenadas personalizado¶

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

Custom CRS Dialog

Defining a custom CRS in QGIS requires a good understanding of the PROJ.4 projection library. To begin, refer to “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.

A janela Definição de Sistema de Coordenadas personalizado exige apenas dois parâmetros para definir um SC do utilizador:

1. Um nome descritivo

2. Os parâmetros cartográficos no formato PROJ.4

To create a new CRS, click the ^{Add new CRS} button and enter a descriptive name and the CRS parameters.

Note que os Parâmetros devem começar com um bloco +proj=, para representar o novo sistema de coordenadas.

You can test your CRS parameters to see if they give sane results. To do this, 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.

Transformações de datum pré-definidas¶

OTF depends on being able to transform data into a ‘default CRS’, and QGIS uses WGS84. For some CRS there are a number of transforms available. QGIS allows you to define the transformation used otherwise QGIS uses a default transformation.

In the CRS tab under Settings ‣ Options you can:

• set QGIS to ask you when it needs define a transformation using Ask for datum transformation when no default is defined
• edit a list of user defaults for transformations.

QGIS asks which transformation to use by opening a dialogue box displaying PROJ.4 text describing the source and destination transforms. Further information may be found by hovering over a transform. User defaults can be saved by selecting Remember selection.