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QGIS como cliente de datos OGC

The Open Geospatial Consortium (OGC) is an international organization with membership of more than 300 commercial, governmental, nonprofit and research organizations worldwide. Its members develop and implement standards for geospatial content and services, GIS data processing and exchange.

Describing a basic data model for geographic features, an increasing number of specifications are developed by OGC to serve specific needs for interoperable location and geospatial technology, including GIS. Further information can be found at http://www.opengeospatial.org/.

Importantes especificaciones OGC implementadas por QGIS son:

OGC services are increasingly being used to exchange geospatial data between different GIS implementations and data stores. QGIS can deal with the above specifications as a client, being SFS (through support of the PostgreSQL / PostGIS data provider, see section PostGIS Layers).

Cliente WMS/WMTS

Información general de la implementación WMS

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

A WMS server acts 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 format would typically be JPEG or PNG.

WMS is generically a REST (Representational State Transfer) service rather than a full-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 server on the market and they all have their own interpretation of the WMS standard.

Las capas WMS se pueden añadir sencillamente, siempre que conozca la URL para acceder al servidor WMS, si tiene una conexión útil a ese servidor, y el servidor entiende HTTP como mecanismo de transporte de datos.

Información general de la implementación WMTS

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 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, 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.

Para mostrar los datos en una variedad de escalas cercanas a lo que el usuario podría querer, los conjuntos de teselas WMTS se producen en varios niveles de escala diferentes y están disponibles para el cliente SIG para pedirlos.

Este diagrama ejemplifica el concepto de conjunto de teselas:

Figure WMTS 1:

../../../_images/concept_wmts.png

Concepto de conjunto de teselas WMTS

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 must open the WMS/WMTS interface and add the following string to the URL of the WMTS tile service:

"?SERVICE=WMTS&REQUEST=GetCapabilities"

Un ejemplo de este tipo de dirección es

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.

  1. The RESTful WMTS service takes a different form, 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 of this type of address for the case of an Austrian basemap is http://maps.wien.gv.at/basemap/1.0.0/WMTSCapabilities.xml.

Nota

You can still find some old services called WMS-C. These services are quite similar to WMTS (i.e., same purpose but working a little bit differently). You can manage them the same as you do WMTS services. Just add ?tiled=true at the end of the url. See http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification for more information about this specification.

When you read WMTS, you can often think WMS-C also.

Seleccionar servidor WMS/WMTS

The first time you use the WMS feature in QGIS, there are no servers defined.

Begin by clicking the mActionAddWmsLayer Add WMS layer button on the toolbar, or selecting Layer ‣ Add WMS Layer....

The dialog Add Layer(s) from a Server for adding layers from the WMS server appears. You can add some servers to play with by clicking the [Add default servers] button. This will add two WMS demo servers for you to use: the WMS servers of the DM Solutions Group and Lizardtech. To define a new WMS server in the Layers tab, select the [New] button. Then enter the parameters to connect to your desired WMS server, as listed in table_OGC_1:

Nombre

A name for this connection. This name will be used in the Server Connections drop-down box so that you can distinguish it from other WMS servers.
URL URL of the server providing the data. This must be a resolvable host name – the same format as you would use to open a telnet connection or ping a host.

Nombre de usuario

Username to access a secured WMS server. This parameter is optional.

Contraseña

Password for a basic authenticated WMS server. This parameter is optional.

Ignorar URI GetMap

checkbox Ignore GetMap URI reported in capabilities. Use given URI from URL field above.

Ignorar la URI GetFeatureInfo

checkbox Ignore GetFeatureInfo URI reported in capabilities. Use given URI from URL field above.

Tabla OGC 1: Parámetros de conexión 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 Settings ‣ Options and click on the Network & Proxy tab. There, you can add your proxy settings and enable them by setting checkbox Use proxy for web access. Make sure that you select the correct proxy type from the Proxy type selectstring drop-down menu.

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

Truco

En las direcciones URL del servidor WMS

Be sure, when entering the WMS server URL, that you have the base URL only. For example, you shouldn’t have fragments such as request=GetCapabilities or version=1.0.0 in your URL.

Cargando capas WMS/WMTS

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 lower left of the WMS dialog.

La pantalla ahora debe lucir un poco como figure_OGR_1, que muestra la respuestra proporcionada por el servidor WMS de DM Solutions Group.

Figure OGR 1:

../../../_images/connection_wms.png

El diálogo para añadir un servidor WMs, mostrará las capas disponibles nix

Codificación de la Imagen

The Image encoding section lists the formats that are supported by both the client and server. Choose one depending on your image accuracy requirements.

Truco

Codificación de la Imagen

Normalmente, encontrará que un servidor WMS le ofrece la opción de codificación de la imagen en JPEG o PNG. JPEG es un formato de compresión con pérdida, mientras que PNG reproduce fielmente los datos crudos raster.

Use JPEG if you expect the WMS data to be photographic in nature and/or you don’t mind some loss in picture quality. This trade-off typically reduces by five times the data transfer requirement compared with PNG.

Use PNG if you want precise representations of the original data and you don’t mind the increased data transfer requirements.

Opciones

The Options area of the dialog provides a text field where you can add a Layer name for the WMS layer. This name will appear in the legend after loading the layer.

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

El Límite del objeto espacial para GetFeatureInfo define los objetos espaciales del servidor a consultar.

If you select a WMS from the list, a field with the default projection provided by the mapserver appears. If the [Change...] button is active, you can click on it and change the default projection of the WMS to another CRS provided by the WMS server.

Orden de la capa

The Layer Order tab lists the selected layers available from the current connected WMS server. You may notice that some layers are expandable; this means that the layer can be displayed in a choice of image styles.

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.

Truco

Ordenar capas WMS

WMS layers rendered by a server are overlaid in the order listed in the Layers section, from top to bottom of the list. If you want to change the overlay order, you can use the Layer Order tab.

Transparencia

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

Truco

Transparencia de capa WMS

La disponibilidad de imagen WMS transparente depende de la codificación de la imagen utilizada: PNG y GIF reconoce la transparencia, mientras JPEG deja sin reconocerlo.

Sistema de referencia de coordenadas

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 Working with Projections will appear. The main difference with the WMS version of the dialog is that only those CRSs supported by the WMS server will be shown.

Conjunto de teselas

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 Tilesets tab 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 by selecting Settings ‣ Panels (KDE and Windows) or View ‣ Panels (Gnome and MacOSX), then choosing Tile scale. This gives you the available scales from the tile server with a nice slider docked in.

Utilizar la herramienta de Identificar objetos espaciales

Una vez que haya añadido un servidor WMS, y si alguna capa de un servidor WMS es consultable, puede entonces utilizar la herramienta mActionIdentify Identificar objetos espaciales para seleccionar un píxel del lienzo del mapa. Una consulta se hace al servidor WMS por cada selección realizada. El resultado de la consulta se regresara en texto plano. El formato de este texto es dependiente del servidor WMS particular utilizado.

Selección de Formato

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 may be stored in the project for the layer.

Usar formato GML

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

Ver propiedades

Once you have added a WMS server, you can view its properties by right-clicking on it in the legend and selecting Properties.

Pestaña de Metadatos

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 gleaned by reading the WMS standards (see OPEN-GEOSPATIAL-CONSORTIUM in Referencias bibliográficas y web), but here are a few handy definitions:

  • Propiedades del servidor

    • Versión WMS — La versión WMS implementada por el servidor.

    • Formatos de Imagen — La lista de MIME-types que el servidor puede responder con la hora de elaboración del mapa. QGIS reconoce cualquier formato las bibliotecas Qt subyacentes con que fueron construidas, que es típicamente al menos image/png y 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.
  • Propiedades de la capa

    • Seleccionar — Sea o no esta capa seleccionada cuando su servidor fue añadido a este proyecto.

    • Visible — Whether or not this layer is selected as visible in the legend (not yet used in this version of QGIS).
    • Poder Identificar — Sea o no esta capa regresará algunos resultados cuando la herramienta de identificar se utilice en él.

    • 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.
    • Conteo en Cascada — Los servidores WMS pueden actuar como proxy para otros servidores WMS para obtener datos ráster de una capa. Esta entrada muestra el número de veces que se remitió la solicitud de esta capa para ver a los servidores WMS para obtener un resultado.

    • 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.
    • Disponible en SRC — Las proyecciones que esta capa puede representar por el servidor WMS. Éstos se enumeran en el formato nativo de WMS.

    • Disponible en estilo — Los estilos de imagen que esta capa puede representar por el servidor WMS.

Mostrar leyenda gráfica WMS en la tabla de contenido y diseñador de impresión

The QGIS WMS data provider is able to display a legend graphic in the table of contents’ layer list and in the map composer. The WMS legend will be shown only if the WMS server has GetLegendGraphic capability and the layer has getCapability url specified, so you additionally have to select a styling for the layer.

If a legendGraphic is available, it is shown below the layer. It is little and you have to click on it to open it in real dimension (due to QgsLegendInterface architectural limitation). Clicking on the layer’s legend will open a frame with the legend at full resolution.

In the print composer, the legend will be integrated at it’s original (dowloaded) dimension. Resolution of the legend graphic can be set in the item properties under Legend -> WMS LegendGraphic to match your printing requirements

The legend will display contextual information based on your current scale. The WMS legend will be shown only if the WMS server has GetLegendGraphic capability and the layer has getCapability url specified, so you have to select a styling.

Limitaciones del cliente WMS

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

Editar la configuración de la capa WMS

Once you’ve completed the mActionAddWmsLayer Add WMS layer procedure, there is no way to change the settings. A work-around is to delete the layer completely and start again.

**Autentificación necesaria en servidores WMS **

Currently, publicly accessible and secured WMS services are supported. The secured WMS servers can be accessed by public authentication. You can add the (optional) credentials when you add a WMS server. See section Seleccionar servidor WMS/WMTS for details.

Truco

Acceso garantizado a capas OGC

If you need to access secured layers with secured methods other than basic authentication, you can use InteProxy as a transparent proxy, which does support several authentication methods. More information can be found in the InteProxy manual at http://inteproxy.wald.intevation.org.

Truco

QGIS WMS Mapserver

Since Version 1.7.0, QGIS has its own implementation of a WMS 1.3.0 Mapserver. Read more about this in chapter QGIS como Servidor de Datos OGC.

WCT Cliente

wcs A Web Coverage Service (WCS) provides access to raster data in forms that are useful for client-side rendering, as input into scientific models, and for other clients. The WCS may be compared to the WFS and the WMS. As WMS and WFS service instances, a WCS allows clients to choose portions of a server’s information holdings based on spatial constraints and other query criteria.

QGIS has a native WCS provider and supports both version 1.0 and 1.1 (which are significantly different), but currently it prefers 1.0, because 1.1 has many issues (i.e., each server implements it in a different way with various particularities).

The native WCS provider handles all network requests and uses all standard QGIS network settings (especially proxy). It is also possible to select cache mode (‘always cache’, ‘prefer cache’, ‘prefer network’, ‘always network’), and the provider also supports selection of time position, if temporal domain is offered by the server.

Cliente WFS y WFS-T

In QGIS, a WFS layer behaves pretty much like any other vector layer. You can identify and select features, and view the attribute table. Since QGIS 1.6, editing WFS-T is also supported.

In general, adding a WFS layer is very similar to the procedure used with WMS. The difference is that there are no default servers defined, so we have to add our own.

Cargar una capa WFS

As an example, we use the DM Solutions WFS server and display a layer. The URL is: http://www2.dmsolutions.ca/cgi-bin/mswfs_gmap

  1. Click on the wfs Add WFS Layer tool on the Layers toolbar. The Add WFS Layer from a Server dialog appears.
  2. Haga clic en [Nuevo].

  3. Ingrese ‘DS Solutions’ como nombre.

  4. Introducir la URL (véase más arriba).

  5. Haga clic en [Aceptar].

  6. Seleccione ‘DM Solutions’ de la lista desplegable Conexiones de servidor selectstring.

  7. Haga clic en [Conectar]

  8. Espere a que la capa de capas este poblada.

  9. Seleccione la capa Parks en la lista.

  10. Haga clic en [Aplicar] para añadir la capa al mapa.

Tenga en cuenta que cualquier configuración de proxy que pueda haber establecido en sus preferencias también son reconocidos.

Figure OGR 3:

../../../_images/connection_wfs.png

Añadir una capa WFS nix

You’ll notice the download progress is visualized in the lower left of the QGIS main window. Once the layer is loaded, you can identify and select a province or two and view the attribute table.

Only WFS 1.0.0 is supported. At this time, there have not been many tests against WFS versions implemented in other WFS servers. If you encounter problems with any other WFS server, please do not hesitate to contact the development team. Please refer to section Help and Support for further information about the mailing lists.

Truco

Encontrar servidores WFS

Puede encontrar servidores WFS adicionales al utilizar Google o su buscador favorito. Hay un número de listas con URLs publicas, algunos de ellos son mantenidos y otro no.