QGIS as OGC Data Client¶

Resumo do suporte WMS¶

QGIS currently can act as a WMS client that understands WMS 1.1, 1.1.1 and 1.3 servers. It has particularly been tested against publicly accessible servers such as DEMIS.

WMS servers act upon requests by the client (e.g. QGIS) for a raster map with a given extent, set of layers, symbolization style, and transparency. The WMS server then consults its local data sources, rasterizes the map, and sends it back to the client in a raster format. For QGIS this would typically be JPEG or PNG.

WMS is generically a REST (Representational State Transfer) service rather than a fully-blown Web Service. As such, you can actually take the URLs generated by QGIS and use them in a web browser to retrieve the same images that QGIS uses internally. This can be useful for troubleshooting, as there are several brands of WMS servers in the market and they all have their own interpretation of the WMS standard.

Camadas WMS podem ser adicionadas simplesmente, desde que você saiba a URL para acessar o servidor WMS, você tem uma conexão útil para esse servidor, e o servidor HTTP entende como mecanismo de transporte de dados.

Overview of WMTS Support¶

QGIS can also act as a WMTS client. WMTS is an OGC standard for distributing tile sets of geospatial data. This is a faster and a more efficient way of distributing data than WMS because with WMTS the tile sets are pre-generated and the client only requests the transmission of the tiles and not their production. A WMS request typically involves both the generation and transmission of the data. A well known example of a non-OGC standard for viewing tiled geospatial data is Google Maps.

In order to display the data at a variety of scales close to what the user might want, the WMTS tile sets are produced at several different scale levels and are made available for the GIS client to request them.

This diagram illustrates the concept of tile sets:

Figure WMTS 1:

Concept of WMTS tile sets

The two types of WMTS interfaces that QGIS supports are via Key-Value-Pairs (KVP) and RESTful. These two interfaces are different and you need to specify them to QGIS differently.

1) In order to access a WMTS KVP service, a QGIS user opens the WMS/WMTS interface and adds the following string to the URL of the WMTS tile service:

"?SERVICE=WMTS&REQUEST=GetCapabilities"

An example of this type of address is

http://opencache.statkart.no/gatekeeper/gk/gk.open_wmts?\
  service=WMTS&request=GetCapabilities

For testing the topo2 layer in this WMTS works nicely. Adding this string indicates that a WMTS web service is to be used instead of a WMS service

2) The RESTful WMTS service takes a different form, it is a straightforward URL, the format recommended by the OGC is:

{WMTSBaseURL}/1.0.0/WMTSCapabilities.xml

This format helps you to recognize that it is a RESTful address. A RESTful WMTS is accessed in QGIS by simply adding its address in the WMS setup in the URL field of the form. An example for an Austrian basemap of this type of address is http://maps.wien.gv.at/basemap/1.0.0/WMTSCapabilities.xml

Selecting WMS/WMTS Servers¶

A primeira vez que você usar o recurso de WMS, não há servidores definidos.

Comece clicando no : sup: botão Adicionar camada WMS dentro da barra de ferramentas, ou através do :menuselection: menu Camada –> Adicionar Camada WMS ....

O diálogo: guilabel: ‘Adicionar camada(s) de um servidor` para adicionar camadas do servidor WMS aparecerá. Você pode adicionar alguns servidores para jogar clicando no botão [Adicionar servidores padrões] Isto irá adicionar dois servidores de demonstração de WMS para você usar, os servidores WMS do DM Solutions Gmap e Lizardtech server. Para definir um servidor WMS novo no guia: guilabel: Camadas, selecione o botão [Novo]. Em seguida, insira os parâmetros para se conectar ao servidor WMS desejado, conforme listado na table_OGC_1:

Tabela OGC 1: Parâmtros de coneção WMS

If you need to set up a proxy-server to be able to receive WMS-services from the internet, you can add your proxy-server in the options. Choose menu Settings ‣ Options and click on the tab Network & Proxy. There you can add your proxy-settings and enable them by setting the Use proxy for web access. Make sure that you select the correct proxy type from the Proxy type dropdown menu.

Once the new WMS Server connection has been created, it will be preserved for future QGIS sessions.

Loading WMS/WMTS Layers¶

Once you have successfully filled in your parameters you can use the [Connect] button to retrieve the capabilities of the selected server. This includes the Image encoding, Layers, Layer Styles and Projections. Since this is a network operation, the speed of the response depends on the quality of your network connection to the WMS server. While downloading data from the WMS server, the download progress is visualized in the left bottom of the WMS dialog.

Sua tela deve agora visualizar como na figure_OGR_1, que mostra a resposta fornecida pelo servidor WMS DM Solutions GMap.

Figure OGR 1:

Dialog for adding a WMS server, showing its available layers

Codificação de imagem

A: guilabel: Codificação da Imagem a secção agora lista todos os formatos que são suportados pelo cliente e servidor. Escolha um de acordo com suas exigências de precisão de imagem.

Opções

The Options field provides a text field where you can add a Layer name for the WMS-layer. This name will be presented in the legend after loading the layer.

Below the layer name you can define Tile size, if you want to set tile sizes (eg. 256x256) to split up the WMS request into multiple requests.

A: guilabel: Limite de feição para GetFeatureInfo define quais feições do servidor serão consultadas.

Se você selecionar um WMS da lista um campo com a projeção padrão, fornecido pelo mapserver, aparecerá. Se o botão [Mudar ...] estiver ativo, você pode clicar nele e mudar a projeção padrão do WMS para outro SRC, fornecidos pelo servidor WMS.

Ordem de camada

A guia: guilabel: Ordem de camada lista as camadas selecionadas disponíveis a partir do servidor WMS atual conectado. Você pode notar que algumas camadas são expansíveis, isso significa que a camada pode ser exibida em uma escolha de estilos de imagem.

You can select several layers at once, but only one image style per layer. When several layers are selected, they will be combined at the WMS Server and transmitted to QGIS in one go.

Transparência

In this version of QGIS, the Global transparency setting from the Layer Properties is hard-coded to be always on, where available.

Sistema de coordenada de referência

A Coordinate Reference System (CRS) is the OGC terminology for a QGIS Projection.

Each WMS Layer can be presented in multiple CRSs, depending on the capability of the WMS server.

To choose a CRS, select [Change...] and a dialog similar to Figure Projection 3 in Trabalhando com Projeções will appear. The main difference with the WMS version of the screen is that only those CRSs supported by the WMS Server will be shown.

Server search¶

Within QGIS you can search for WMS-servers. Figure_OGC_2 shows the tab Server Search with the Add Layer(s) from a Server dialog.

Figure OGR 2:

Dialog for searching WMS servers after some keywords

As you can see it is possible to enter a search-string in the text field and hit the [Search] button. After a short while the search result will be populated into the list below the text field. Browse the result list and inspect your search results within the table. To visualize the results, select a table entry, press the [Add selected row to WMS-list] button and change back to the tab Layers. QGIS automatically has updated your server list and the selected search result is already enabled in the list of saved WMS-servers in the Layers tab. You only need to request the list of layers by clicking the [Connect] button. This option is quite handy when you want to search maps by specific keywords.

Basicamente, esta opção é um frontend para o API de http://geopole.org.

Tilesets¶

When using WMTS (Cached WMS) Services like

http://opencache.statkart.no/gatekeeper/gk/gk.open_wmts?\
  service=WMTS&request=GetCapabilities

you are able to browse through the tab Tilesets given by the server. Additional information like tile size, formats and supported CRS are listed in this table. In combination with this feature you can use the tile scale slider from the Settings ‣ Panels (KDE and Windows) or View ‣ Panels (Gnome and MacOSX) then choose Tile scale, which gives you the available scales from the tileserver with nice slider docked in.

Usando a ferramenta de identificação¶

Once you have added a WMS server, and if any layer from a WMS server is queryable, you can then use the ^Identify tool to select a pixel on the map canvas. A query is made to the WMS server for each selection made. The results of the query are returned in plain text. The formatting of this text is dependent on the particular WMS server used.

Format selection

If multiple output formats are supported by the server, a combo box with supported formats is automatically added to the identify results dialog and the selected format will is stored in project for the layer.

GML format support

The ^Identify tool supports WMS server response (GetFeatureInfo) in GML (it is called Feature in QGIS GUI in this context) format. If “Feature” format is supported by the server and selected, results of the Identify tool are vector features like from regular vector layer. When a single feature is selected in the tree, it is highlighted in the map and it can be copied to clipboard and pasted to another vector layer. See example setup of UMN Mapserver below to support GetFeatureInfo GML format.

# in layer METADATA add which fields should be included and define geometry (example):

"gml_include_items"   "all"
"ows_geometries"      "mygeom"
"ows_mygeom_type"     "polygon"

# Then there are two possibilities/formats available, see a) and b):

# a) basic (output is generated by Mapserver and does not contain XSD)
# in WEB METADATA define formats (example):
"wms_getfeatureinfo_formatlist" "application/vnd.ogc.gml,text/html"

# b) using OGR (output is generated by OGR, it is send as multipart and contains XSD)
# in MAP define OUTPUTFORMAT (example):
OUTPUTFORMAT
    NAME "OGRGML"
    MIMETYPE "ogr/gml"
    DRIVER "OGR/GML"
    FORMATOPTION "FORM=multipart"
END

# in WEB METADATA define formats (example):
"wms_getfeatureinfo_formatlist" "OGRGML,text/html"

Exibindo propriedades

Depois de ter adicionado um servidor WMS, você pode ver suas propriedades clicando sobre ela na legenda, e selecionando: menuselection: Propriedades.

Guia Metadata

The tab Metadata displays a wealth of information about the WMS server, generally collected from the Capabilities statement returned from that server. Many definitions can be cleaned by reading the WMS standards (see OPEN-GEOSPATIAL-CONSORTIUM Referências Bibliográficas e Web), but here are a few handy definitions:

• Propriedades do servidor

  • Versão WMS — A versão WMS suportada pelo servidor.

  • Image Formats — The list of MIME-types the server can respond with when drawing the map. QGIS supports whatever formats the underlying Qt libraries were built with, which is typically at least image/png and image/jpeg.
  • Identity Formats — The list of MIME-types the server can respond with when you use the Identify tool. Currently QGIS supports the text-plain type.

• Propriedades da camada

  • Selecionado — Seja ou não essa camada foi selecionada quando o servidor foi adicionado a este projeto.

  • Visible — Whether or not this layer is selected as visible in the legend. (Not yet used in this version of QGIS.)

  • Pode identificar — ou não essa camada retornará nenhum resultado quando a ferramenta Identificar é usada nela.

  • Can be Transparent — Whether or not this layer can be rendered with transparency. This version of QGIS will always use transparency if this is Yes and the image encoding supports transparency
  • Can Zoom In — Whether or not this layer can be zoomed in by the server. This version of QGIS assumes all WMS layers have this set to Yes. Deficient layers may be rendered strangely.
  • Cascade Count — WMS servers can act as a proxy to other WMS servers to get the raster data for a layer. This entry shows how many times the request for this layer is forwarded to peer WMS servers for a result.
  • Fixed Width, Fixed Height — Whether or not this layer has fixed source pixel dimensions. This version of QGIS assumes all WMS layers have this set to nothing. Deficient layers may be rendered strangely.
  • WGS 84 Bounding Box — The bounding box of the layer, in WGS 84 coordinates. Some WMS servers do not set this correctly (e.g. UTM coordinates are used instead). If this is the case, then the initial view of this layer may be rendered with a very ‘zoomed-out’ appearance by QGIS. The WMS webmaster should be informed of this error, which they may know as the WMS XML elements LatLonBoundingBox, EX_GeographicBoundingBox or the CRS:84 BoundingBox.

  • SRC Disponível — As projeções que esta camada pode ser apresentadas pelo servidor WMS. Elas estão listadas no formato nativo WMS.

  • Estilo Disponível — Os estilos de imagem que esta camada pode renderizar pelo servidor WMS.

Limitações do cliente WMS¶

Not all possible WMS Client functionality had been included in this version of QGIS. Some of the more notable exceptions follow.

Editando configurações de camada WMS

Once you’ve completed the ^{Add WMS layer} procedure, there is no ability to change the settings. A workaround is to delete the layer completely and start again.

Autenticação requerida nos servidores WMS

Atualmente públicas acessíveis e seguros os serviços-WMS são suportados. Os servidores-WMS seguros podem ser acessados ​​por meio da autenticação pública. Você pode adicionar as credenciais (opcional) quando você adiciona um servidor-WMS. Consulte a seção: ref: ogc-wms-servers para mais detalhes.