10. Trabalhando com Projeções

Um Sistema de Referência de Coordenadas, ou SRC, é um método de associar coordenadas numéricas com uma posição na superfície da Terra. O QGIS tem suporte para aproximadamente 7.000 SRCs padrão, cada um com diferentes casos de uso, prós e contras! Escolher um sistema de referência apropriado para seus projetos e dados QGIS pode ser uma tarefa complexa, mas felizmente o QGIS ajuda a guiá-lo através desta escolha, e torna o trabalho com diferentes CRSs tão transparente e preciso quanto possível.

10.1. Visão geral do Suporte a Projeções

QGIS has support for approximately 7,000 known CRSs. These standard CRSs are based on those defined by the European Petroleum Search Group (EPSG) and the Institut Geographique National de France (IGNF), and are made available in QGIS through the underlying “Proj” projection library. Commonly, these standard projections are identified through use of an authority:code combination, where the authority is an organisation name such as “EPSG” or “IGNF”, and the code is a unique number associated with a specific CRS. For instance, the common WGS 84 latitude/longitude CRS is known by the identifier EPSG:4326, and the web mapping standard CRS is EPSG:3857.

Custom, user-created CRSs are stored in a user CRS database. See section Sistema de Referência de Coordenadas personalizado for information on managing your custom coordinate reference systems.

10.2. Sistemas de referência de coordenadas da camada

In order to correctly project data into a specific target CRS, either your data must contain information about its coordinate reference system or you will need to manually assign the correct CRS to the layer. For PostGIS layers, QGIS uses the spatial reference identifier that was specified when that PostGIS layer was created. For data supported by OGR or GDAL, QGIS relies on the presence of a recognized means of specifying the CRS. For instance, for the Shapefile format this is a file containing an ESRI Well-Known Text (WKT) representation of the layer’s CRS. This projection file has the same base name as the .shp file and a .prj extension. For example, alaska.shp would have a corresponding projection file named alaska.prj.

Whenever a layer is loaded into QGIS, QGIS attempts to automatically determine the correct CRS for that layer. In some cases this is not possible, e.g. when a layer has been provided without retaining this information. You can configure QGIS behavior whenever it cannot automatically determine the correct CRS for a layer:

  1. Abrir :menuseleção:`Configurações-->` opções Opções… ► SRC`

    ../../../_images/options_crs.png

    Fig. 10.1 A guia SRC na Caixa de Diálogo de Opções QGIS

  2. Under the CRS for layers group, set the action to do when a new layer is created, or when a layer is loaded that has no CRS. One of:

    • radioButtonOn Leave as unknown CRS (take no action): there will be no prompt to select a CRS when a layer without CRS is loaded, defering CRS choice to a later time. Convenient when loading a lot of layers at once. Such layers will be identifiable in the Layers panel by the indicatorNoCRS icon next to them. They’ll also be un-referenced, with coordinates from the layer treated as purely numerical, non-earth values, i.e. the same behavior as all layers get when a project is set to have no CRS.

    • radioButtonOff Prompt for CRS: it will prompt you to manually select the CRS. Selecting the correct choice is crucial, as a wrong choice will place your layer in the wrong position on the Earth’s surface! Sometimes, accompanying metadata will describe the correct CRS for a layer, in other cases you will need to contact the original author of the data to determine the correct CRS to use.

    • radioButtonOff Usar SRC do projeto

    • radioButtonOff Use default layer CRS, as set in the Default CRS for layers combobox above.

Dica

Atribuir o mesmo SRC a várias camadas que não tenham SRC ou que tenham um SRC errado em uma única operação:

  1. Selecione as camadas no painel :guilabel:`Camadas’.

  2. Press Ctrl+Shift+C. You could also right-click over one of the selected layers or go to Layer ► Set CRS of layer(s)

  3. Encontre e selecione o CRS adequado para usar

  4. And press OK. You can confirm that it has been set correctly in the Source tab of the layers’ properties dialog.

Note that changing the CRS in this setting does not alter the underlying data source in any way, rather it just changes how QGIS interprets the raw coordinates from the layer in the current QGIS project.

10.3. Sistemas referência de coordenadas projetadas

Cada projeto no QGIS também tem um Sistema de Referência de Coordenadas associado. O projeto SRC determina como os dados são projetados de suas coordenadas brutas subjacentes para o mapa plano renderizado em sua tela de mapa QGIS.

QGIS supports “on the fly” CRS transformation for both raster and vector data. This means that regardless of the underlying CRS of particular map layers in your project, they will always be automatically transformed into the common CRS defined for your project. Behind the scenes, QGIS transparently reprojects all layers contained within your project into the project’s CRS, so that they will all be rendered in the correct position with respect to each other!

É importante fazer uma escolha apropriada de SRC para seus projetos QGIS. Escolher um SRC inadequado pode fazer com que seus mapas pareçam distorcidos e reflitam mal os tamanhos relativos do mundo real e as posições das feições. Normalmente, ao trabalhar em áreas geográficas menores, haverá vários SRCs padrão usados em um determinado país ou área administrativa. É importante pesquisar quais SRCs são escolhas apropriadas ou padrão para a área que você está mapeando e garantir que seu projeto QGIS siga esses padrões.

By default, QGIS starts each new project using a global default projection. This default CRS is EPSG:4326 (also known as “WGS 84”), and it is a global latitude/longitude based reference system. This default CRS can be changed via the CRS for New Projects setting in the CRS tab under Settings ► opções Options… (see Fig. 10.1). There is an option to automatically set the project’s CRS to match the CRS of the first layer loaded into a new project, or alternatively you can select a different default CRS to use for all newly created projects. This choice will be saved for use in subsequent QGIS sessions.

The project CRS can also be set through the CRS tab of the Project ► Properties… dialog. It will also be shown in the lower-right of the QGIS status bar.

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

Fig. 10.2 Caixa de Diálogo de Propriedades de Projeto

As opções disponíveis são:

  • desmarcado No CRS (or unknown/non-Earth projection): Checking this setting will disable ALL projection handling within the QGIS project, causing all layers and map coordinates to be treated as simple 2D Cartesian coordinates, with no relation to positions on the Earth’s surface. It can be used to guess a layer CRS (based on its raw coordinates or when using QGIS for non earth uses like role-playing game maps, building mapping or microscopic stuff. In this case:

    • Nenhuma reprojeção é feita durante a renderização das camadas: as feições são apenas desenhadas usando suas coordenadas brutas.

    • A elipsóide é bloqueada e forçada a “Nenhum/Planimétrico”.

    • As unidades de distância e área, e a exibição de coordenadas são bloqueadas e forçadas a “unidades desconhecidas”; todas as medições são feitas em unidades de mapa desconhecidas, e nenhuma conversão é possível.

  • or an existing coordinate reference system that can be geographic, projected or user-defined. A preview of the CRS extent on earth is displayed to help you select the appropriate one. Layers added to the project are translated on-the-fly to this CRS in order to overlay them regardless their original CRS. Use of units and ellipsoid setting are available and make sense and you can perform calculations accordingly.

Whenever you select a new CRS for your QGIS project, the measurement units will automatically be changed in the General tab of the Project properties dialog (Project ► Properties…) to match the selected CRS. For instance, some CRSs define their coordinates in feet instead of meters, so setting your QGIS project to one of these CRSs will also set your project to measure using feet by default.

Dica

Configurando a projeção SRC a partir de uma camada

Você pode atribuir um SRC ao projeto usando um SRC de camada:

  1. In the Layers panel, right-click on the layer you want to pick the CRS

  2. Selecione Configurar SRC do projeto a partir da Camada.

The project’s CRS is redefined using the layer’s CRS. Map canvas extent, coordinates display are updated accordingly and all the layers in the project are on-the-fly translated to the new project CRS.

10.4. Seletor do Sistemas de Referência de Coordenadas

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.

  • ** Sistemas de referência de coordenadas recentemente utilizados**: Se você tiver certos SRCs que você usa freqüentemente em seu trabalho diário de SIG, estes serão exibidos nesta lista. Clique em um destes itens para selecionar o SRC associado.

  • Sistemas de referência coordenados do mundo: Esta é uma lista de todos os SRCs suportados pelo QGIS, incluindo os sistemas de referência de coordenadas geográficas, projetadas e personalizadas. Para definir um SRC, selecione-o da lista expandindo o nó apropriado e selecionando o SRC. O SRC ativo é pré-selecionado.

  • PROJ text: This is the CRS string used by the PROJ projection engine. This text is read-only and provided for informational purposes.

The CRS selector also shows a rough preview of the geographic area for which a selected CRS is valid for use. Many CRSs are designed only for use in small geographic areas, and you should not use these outside of the area they were designed for. The preview map shades an approximate area of use whenever a CRS is selected from the list. In addition, this preview map also shows an indicator of the current main canvas map extent.

10.5. Sistema de Referência 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 projeçãoPersonalizada 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.

Defining a custom CRS in QGIS requires a good understanding of the PROJ 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 https://pubs.usgs.gov/of/1990/of90-284/ofr90-284.pdf).

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

A janela Definição de Sistema de Referência de Coordenadas personalizado exige apenas dois parâmetros para definir um SRC ao usuário:

  1. Um nome descritivo

  2. Os parâmetros cartográficos em formato PROJ ou WKT

Para criar um novo SRC:

  1. Click the Adicionar simbologia Add new CRS button

  2. Insira um nome descritivo

  3. Select the format: it can be Proj String or WKT

  4. Adicionar ao SRC Parâmetros.

    Nota

    Prefer storing the CRS definition in WKT format

    Although both Proj String and WKT formats are supported, it’s highly recommended to store projection definitions in the WKT format. Therefore, if the available definition is in the proj format, select that format, enter the parameters and then switch to WKT format. QGIS will convert the definition to the WKT format that you can later save.

  5. Click Validate to test whether the CRS definition is an acceptable projection definition.

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

Fig. 10.3 Custom CRS Dialog

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.

10.5.1. Integrate an NTv2-transformation in QGIS

To integrate an NTv2 transformation file in QGIS you need one more step:

  1. Place the NTv2 file (.gsb) in the CRS/Proj folder that QGIS uses (e.g. C:\OSGeo4W64\share\proj for windows users)

  2. Add nadgrids (+nadgrids=nameofthefile.gsb) to the Proj definition in the Parameters field of the Custom Coordinate Reference System Definition (Settings ► Custom Projections…).

    ../../../_images/nadgrids_example.png

    Fig. 10.4 Configurar uma transformação NTv2

10.6. Transformações de Datum

In QGIS, ‘on-the-fly’ CRS transformation is enabled by default, meaning that whenever you use layers with different coordinate systems QGIS transparently reprojects them to the project CRS. For some CRS, there are a number of possible transforms available to reproject to the project’s CRS!

By default, QGIS will attempt to use the most accurate transformation available. However, in some cases this may not be possible, e.g. whenever additional support files are required to use a transformation. Whenever a more accurate transformation is available, but is not currently usable, QGIS will show an informative warning message advising you of the more accurate transformation and how to enable it on your system. Usually, this requires download of an external package of transformation support files, and extracting these to the proj folder under your QGIS user profile folder.

Se desejar, o QGIS também pode lhe solicitar sempre que várias transformações possíveis possam ser feitas entre dois CRSs, e permitir que você faça uma seleção informada de qual é a transformação mais apropriada para usar para seus dados.

This customization is done in the Settings ► opções Options ► Transformations tab menu under the Default datum transformations group:

  • using caixa de seleção Ask for datum transformation if several are available: when more than one appropriate datum transformation exist for a source/destination CRS combination, a dialog will automatically be opened prompting users to choose which of these datum transformations to use for the project. If the Make default checkbox is ticked when selecting a transformation from this dialog, then the choice is remembered and automatically applied to any newly created QGIS projects.

  • ou definir uma lista de transformações de dados apropriadas para usar como padrão ao carregar uma camada em um projeto ou ao reprojetar uma camada.

    Use the Adicionar simbologia button to open the Select Datum Transformations dialog. Then:

    1. Choose the Source CRS of the layer, using the drop-down menu or the definir projeção Select CRS widget.

    2. Fornecer o Destino SRC da mesma forma.

    3. Uma lista das transformações disponíveis da origem ao destino será mostrada na tabela. Clicando em uma linha serão mostrados detalhes sobre as configurações aplicadas e a correspondente precisão e área de uso da transformação.

      ../../../_images/datumTransformation.png

      Fig. 10.5 Selecionando uma transformação padrão de datum preferida

      In some cases a transformation may not be available for use on your system. In this case, the transformation will still be shown (greyed) in this list but can not be picked until you install the required package of transformation support. Usually, a button is provided to download and install the corresponding grid, which is then stored under the proj folder in the active user profile directory.

    4. Encontre a transformação de sua preferência e selecione

    5. Set whether you caixa de seleção Allow fallback transforms if preferred operation fails

    6. Clique OK.

      A row is added to the table under Default Datum Transformations with information about the Source CRS, the Destination CRS, the Operation applied for the transformation and whether Allow fallback Transforms is enabled.

    From now, QGIS automatically uses the selected datum transformations for further transformation between these two CRSs until you remove it (removerSimbologia) from the list or change the entry (toggleEditing) in the list.

Datum transformations set in the Settings ► opções Options ► Transformations tab will be inherited by all new QGIS projects created on the system. Additionally, a particular project may have its own specific set of transformations specified via the CRS tab of the Project properties dialog (Project ► Properties…). These settings apply to the current project only.