QGIS as OGC Data Client¶

WMS 지원 개요¶

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.

서버에 접근할 수 있는 URL을 알고 있고, 그 서버에 대해 접근이 가능하며, 서버가 데이터 전송 메커니즘으로 HTTP를 지원하면, WMS 레이어는 매우 간단하게 추가할 수 있습니다.

WMTS 지원 개요¶

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.

사용자가 원하는 수준과 가깝게 다양한 축척의 데이터를 표시하기 위해, WMTS 타일 셋은 여러 축척의 레벨로 생성되고 GIS 클라이언트가 타일 셋을 요청하여 활용됩니다.

이 다이어그램은 타일 셋의 개념을 보여줍니다:

Figure WMTS 1:

WMTS 타일 셋의 개념

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"

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, 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.

Selecting WMS/WMTS Servers¶

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

Begin by clicking the ^{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:

이름	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.
사용자이름	Username to access a secured WMS server. This parameter is optional.
비밀번호	Password for a basic authenticated WMS server. This parameter is optional.
GetMap URI 무시	Ignore GetMap URI reported in capabilities. Use given URI from URL field above.
GetFeatureInfo URI 무시	Ignore GetFeatureInfo URI reported in capabilities. Use given URI from URL field above.

표 OGC 1: WMS 연결 파라미터

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 Use proxy for web access. Make sure that you select the correct proxy type from the Proxy type drop-down menu.

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

WMS/WMTS 레이어 불러오기¶

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.

Your screen should now look a bit like figure_OGR_1, which shows the response provided by the European Soil Portal WMS server.

Figure OGR 1:

Dialog for adding a WMS server, showing its available layers

이미지 인코딩

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

옵션

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.

The Feature limit for GetFeatureInfo defines what features from the server to query.

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.

Finally you can activate Use contextual WMS-Legend if the WMS Server supports this feature. Then only the relevant legend for your current map view extent will be shown and thus will not include legend items for things you can’t see in the current map.

레이어 순서

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.

Transparency

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

Coordinate Reference System

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 투영법 다루기 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.

서버 검색¶

Within QGIS, you can search for WMS servers. Figure_OGC_2 shows the Server Search tab 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 Layers tab. QGIS has automatically 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.

Basically, this option is a front end to the API of http://geopole.org.

타일셋¶

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.

Using the Identify Tool¶

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.

포맷 선택

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.

GML 포맷 지원

The ^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"

Viewing Properties

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

메타데이터 탭

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 Literature and Web References), but here are a few handy definitions:

• 서버 속성

  • WMS 버전 — 서버에서 지원하는 WMS 버전.

  • 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.
• Layer Properties
  • Selected — Whether or not this layer was selected when its server was added to this project.
  • Visible — Whether or not this layer is selected as visible in the legend (not yet used in this version of QGIS).
  • Can Identify — Whether or not this layer will return any results when the Identify tool is used on it.
  • 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.
  • Available in CRS — The projections that this layer can be rendered in by the WMS server. These are listed in the WMS-native format.
  • Available in style — The image styles that this layer can be rendered in by the WMS server.

Show WMS legend graphic in table of contents and composer¶

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.

WMS Client Limitations¶

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

Editing WMS Layer Settings

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

WMS Servers Requiring Authentication

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 Selecting WMS/WMTS Servers for details.