WMS - Operations¶

Usage¶

Through a highly configurable interface the WMS standard makes map images (but not source data) available in standard image formats. Government agencies publish all kinds of official map cartography using this standard. Other large organizations use this standard to enable independent departments to interact more easily internally. Anybody using this standard can use it to overlay map images from many different sources regardless of the underlying software.

WMS provides a standard interface for requesting a geospatial map image. The benefit of this is that WMS clients can request images from multiple WMS servers, and then combine them into a single view for the user. The standard guarantees that these images can all be overlaid on one another using a common geospatial coordinate reference system. Numerous servers and clients support WMS.

Relation to other OGC Standards¶

• OGC Web Map Tile Service Interface Standard (WMTS): The WMTS standard is a better fit For highly scalable systems (many simultaneous requests) that only need static maps. It complements the WMS standard with cachable static map tiles. WMS servers can be used as data sources or rendering engines for WMTS services.

• OGC Web Feature Service (WFS): The WFS standard is a better fit for extended query functionality of spatial data. It provides programmatic access to the geographic feature data. WMS and WFS often go together. An organization publishing both WMS and WFS often use the same data source.

• OGC Styled Layer Descriptor Interface Standard (SLD): The SLD standard allows the user to modify the cartographic appearance of a map image. It is an optional feature of the OGC WMS standard.

• OGC Symbology Encoding (SE): The SE standard describes how to define map symbology. It is used to modify the cartographic appearance of map images. It is an optional feature of the OGC WMS and SLD standards.

This section provides detailed information about the types of WMS requests a client is able to perform to a WMS server.

Request¶

A WMS server responding to a GetCapabilities request returns metadata about the service, including supported operations and parameters, and a list of the available layers.

An example of a GetCapabilities request is:

https://ows.terrestris.de/osm/service?
REQUEST=GetCapabilities&
SERVICE=WMS&VERSION=1.3.0

This is a link to a GetCapabilities request.

There are three parameters (and values) being passed to the WMS server, SERVICE=WMS, VERSION=1.3, and REQUEST=GetCapabilities.

• The SERVICE parameter tells the server that a WMS request is forthcoming.

• The VERSION parameter tells the server what version of the WMS is being requested.

• The REQUEST parameter tells the server that the operation requested is the GetCapabilities operation.

The WMS standard requires that requests always includes these three parameters. The table bellow summarizes the parameters and values required to perform the request.

Response¶

The response is a Capabilities XML document with a detailed description of the WMS service. It contains three main sections:

GetCapabilities Layer Style Section¶

The GetCapabilites response contains a Layer section, which details about the style available to that layer. In the example bellow the available style is default.

<Layer queryable="0" opaque="0" cascaded="0">
  <Name>nationalparks</Name>
  <Title>National Parks</Title>
  <EX_GeographicBoundingBox>
    <westBoundLongitude>-4.43064</westBoundLongitude>
    <eastBoundLongitude>1.99728</eastBoundLongitude>
    <southBoundLatitude>50.3532</southBoundLatitude>
    <northBoundLatitude>55.5917</northBoundLatitude>
  </EX_GeographicBoundingBox>
  <BoundingBox CRS="EPSG:27700"
         minx="246828" miny="56378.4" maxx="652374" maxy="633117"/>
  <Style>
    <Name>default</Name>
    <Title>default</Title>
    <LegendURL width="110" height="22">
      <Format>image/png</Format>
      <OnlineResource xmlns:xlink="http://www.w3.org/1999/xlink"
         xlink:type="simple"
         xlink:href="..."/>
    </LegendURL>
  </Style>
</Layer>

Request¶

A WMS server responding to a GetMap request returns a map image for a specified area and content.

The core parameters specify one or more layers and styles to appear on the map, a bounding box for the map extent, a target spatial reference system, and a width, height, and format for the output. The information needed to specify values for parameters such as layers, styles and Spatial Reference Systems (SRS) can be obtained from the Capabilities document.

The response is a map image, or other map output artifact, depending on the format requested.

An example of a GetMap request is:

https://ows.terrestris.de/osm/service?
REQUEST=GetMap&
SERVICE=WMS&
VERSION=1.3.0&
LAYERS=OSM-WMS&
STYLES=&
CRS=EPSG:4326&
BBOX=51.49451,-0.11377,51.53267,-0.06971&
WIDTH=400&
HEIGHT=300&
FORMAT=image/png&
TRANSPARENT=TRUE

This is a link to a GetMap request.

The getMap request accesses a server offered by Terrestris. The request retrieves a map created from OpenStreetMap data

The coordinate reference system (CRS) is EPSG:4326, which is the World Geodetic System 1984 (WGS84) coordinate reference system. The image is returned in a PNG transparent format with width 400 and height 300 pixels.

The table bellow summarizes the available parameters and values.

Another WMS request examples is as follows:

http://localhost:8080/geoserver/wms?
request=GetMap
&service=WMS
&version=1.1.1
&layers=topp%3Astates
&styles=population
&srs=EPSG%3A4326
&bbox=-145.15104058007,21.731919794922,-57.154894212888,58.961058642578&
&width=780
&height=330
&format=image%2Fpng

The request specifies the topp:states layer to be output as a PNG map image in SRS EPGS:4326 and using default styling population.

A WMS request can also be sent via HTTP POST as an XML document, as follows:

<?xml version="1.0" encoding="UTF-8"?>
<ogc:GetMap xmlns:ogc="http://www.opengis.net/ows"
            xmlns:gml="http://www.opengis.net/gml"
   version="1.1.1" service="WMS">
   <StyledLayerDescriptor version="1.0.0">
      <NamedLayer>
        <Name>topp:states</Name>
        <NamedStyle><Name>population</Name></NamedStyle>
      </NamedLayer>
   </StyledLayerDescriptor>
   <BoundingBox srsName="http://www.opengis.net/gml/srs/epsg.xml#4326">
      <gml:coord><gml:X>-130</gml:X><gml:Y>24</gml:Y></gml:coord>
      <gml:coord><gml:X>-55</gml:X><gml:Y>50</gml:Y></gml:coord>
   </BoundingBox>
   <Output>
      <Format>image/png</Format>
      <Size><Width>550</Width><Height>250</Height></Size>
   </Output>
</ogc:GetMap>

Response¶

The response of a GetMap request is an image.

If the request is wrong the server will return an error message.

Time¶

The TIME parameter allows filtering a dataset by temporal slices as well as spatial tiles for rendering. The TIME attribute for WMS GetMap requests is described in version 1.3 of the WMS specification.

TIME=<timestring>

yyyy-MM-ddThh:mm:ss.SSSZ

Request¶

A WMS server responding to a GetFeatureInfo request returns the underlying data, including geometry and attribute values, for a pixel location on a map. It is similar to the WFS GetFeature operation, but less flexible in both input and output. The one advantage of GetFeatureInfo is that the request uses an (x,y) pixel value from a returned WMS image. This is easier to use for a naive client that is not able to perform true geographic referencing.

The standard parameters for the GetFeatureInfo operation are:

Example formats are as follows:

An example request for feature information from the topp:states layer in HTML format is:

http://localhost:8080/geoserver/wms?
request=GetFeatureInfo
&service=WMS
&version=1.1.1
&layers=topp%3Astates
&styles=
&srs=EPSG%3A4326
&format=image%2Fpng
&bbox=-145.151041%2C21.73192%2C-57.154894%2C58.961059
&width=780
&height=330
&query_layers=topp%3Astates
&info_format=text%2Fhtml
&feature_count=50
&x=353
&y=145
&exceptions=application%2Fvnd.ogc.se_xml

An example request for feature information in GeoJSON format is:

http://localhost:8080/geoserver/wms?
&INFO_FORMAT=application/json
&REQUEST=GetFeatureInfo
&EXCEPTIONS=application/vnd.ogc.se_xml
&SERVICE=WMS
&VERSION=1.1.1
&WIDTH=970&HEIGHT=485&X=486&Y=165&BBOX=-180,-90,180,90
&LAYERS=COUNTRYPROFILES:grp_administrative_map
&QUERY_LAYERS=COUNTRYPROFILES:grp_administrative_map
&TYPENAME=COUNTRYPROFILES:grp_administrative_map

The result will be:

{
"type":"FeatureCollection",
"features":[
   {
      "type":"Feature",
      "id":"dt_gaul_geom.fid-138e3070879",
      "geometry":{
         "type":"MultiPolygon",
         "coordinates":[
            [
               [
                  [
                     XXXXXXXXXX,
                     XXXXXXXXXX
                  ],
                  ...
                  [
                     XXXXXXXXXX,
                     XXXXXXXXXX
                  ]
               ]
            ]
         ]
      },
      "geometry_name":"at_geom",
      "properties":{
         "bk_gaul":X,
         "at_admlevel":0,
         "at_iso3":"XXX",
         "ia_name":"XXXX",
         "at_gaul_l0":X,
         "bbox":[
            XXXX,
            XXXX,
            XXXX,
            XXXX
         ]
      }
   }
],
"crs":{
   "type":"EPSG",
   "properties":{
      "code":"4326"
   }
},
"bbox":[
   XXXX,
   XXXX,
   XXXX,
   XXXX
]
}