OGC WMS 1.3 Demo and Reference Server

 

 

 About this Page

This page has been compiled as a quick reference and introduction to the OGC WMS interface and service standard. All map images on this page are dynamically requested by standard OGC WMS queries and rendered by the live OGC WMS Demo and Reference Server . This page does not describe all the features and options the standard offers. Please find the full technical description of the OGC WMS standard on the web sites of the Open Geospatial Consortium at: http://www.opengeospatial.org/standards/wms

 End Point

The end point to access this service is: http://metaspatial.net/cgi-bin/ogc-wms.xml. The OGC WMS Capabilities document contains all information required to make an educated request. It can be retrieved by sending a getCapabilities request to the server.

Current information about the status of this server can be found at the metaspatial wiki.

 Note

If this endpoint is called without any additional parameters the software running on this server (MapServer) will return the following text:

No query information to decode. QUERY_STRING is set, but empty.

This is documented and expected behavior. The server did not get any instructions what to do and therefore only informs us that it has nothing to do. Other software may return different or even no information at all.

In order to query maps from this server we have to first get the metadata. This metadata is contained in the ''OGC WMS Capabilities Document'' and comes in XML format. It is requested by calling:

http://metaspatial.net/cgi-bin/ogc-wms.xml?REQUEST=GetCapabilities&SERVICE=WMS&VERSION=1.3

This document contains all the information needed to make a valid OGC WMS GetMap request.

 Getting a Map Image

The GetMap request queries the server with a set of parameters describing the map image. The values of the parameters are taken from the Capabilities document (see above). A correctly formulated GetMap request will create the image shown below.

The URL of this link has been truncated for better readability.:

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 VERSION=1.3.0&
 REQUEST=GetMap&
 SERVICE=WMS&
 LAYERS=DTM,Overview,Raster_250K,Topography,nationalparks,Infrastructure,Places&
 STYLES=,,,,,,&
 CRS=EPSG:27700&
 BBOX=424735.97883597884,96026.98412698413,467064.02116402116,127773.01587301587&
 WIDTH=400&
 HEIGHT=300&
 FORMAT=image/png&
 BGCOLOR=0xffffff&
 TRANSPARENT=TRUE&
 EXCEPTIONS=XML
 

Use the link: GetMap to retrieve the map image form the OGC Demo and Reference Server. It will be rendered dynamically each time you request the image (given that no proxy interferes and delivers an earlier graphic rendition of the map data).

Map image returned by the OGC WMS server

 Layers

WMS can be structured into layers. Each layer can be queried individually or in combination with another layer. Layers will be drawn in the order requested, the first at the bottom of the image and each layer stacked on top. If an opaque layer is rendered at the top of the image it may cover all the other layers.

Every server has one root layer which contains the name of the server, here it is: OGC WMS Reference Server. This server has seven map layers:

  1. DTM (Digital Terrain Model)
  2. Overview (Overview 1:1m)
  3. Raster_250K (Raster 1:250k)
  4. Topography (selection of topographical data derived from OS VectorMap District 1:50k)
  5. National Parks (The national parks of Great Britain)
  6. Infrastructure (selection of topographical data derived from OS VectorMap District 1:50k)
  7. Named Places (selection of topographical data derived from OS VectorMap District 1:50k)

In the following request the layer "Overview" has been taken out of the request but otherwise the request is unchanged. It reveals the digital terrain model (DTM), a layer that was previously invisible.

The URL of this link has been truncated for better readability.:

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 VERSION=1.3.0&
 REQUEST=GetMap&
 SERVICE=WMS&
 LAYERS=DTM,Raster_250K,Topography,nationalparks,Infrastructure,Places&
 CRS=EPSG:27700&
 BBOX=424735.97883597884,96026.98412698413,467064.02116402116,127773.01587301587&
 WIDTH=400&
 HEIGHT=300&
 FORMAT=image/png&
 

Use the link: GetMap without Overview to retrieve the map image form the OGC Demo and Reference Server.

Map with translucent overlay

Note

The above link is a bit shorter because we ignored some parameters like TRANSPARENCY and BGCOLOR which are optional.

Most server implementations will always render all requested layers regardless of whether they are visible in the final map image or not. In the above example the Digital Terrain Model of Great Britain will always be rendered even if the overview map completely hides it. If you want to reduce the load on your server you might want to avoid this by combining layers in a more useful way or setting up a separate WMS for each content. Always remember that the OGC WMS interface standard gives you a lot of freedom to experiment and create an optimized set of services and layers for your content. But it needs a lot of experimentation and thought to get there. Strictly adhering to the standard will ensure that your service will be interoperable but it does not guarantee optimized performance. This is where your know-how and skillful means are needed.

Transparency / Translucency

The map image can be overlayed with maps from other servers. In those cases it may be helpful to request the top level image in a format which supports a transparency such as the alpha channel. Then we can see what is below this map image.

The following examples show how each layer can be transparent and reveal what is underneath.

Map with opaque overlay

This request additionally requests the DTM layer which adds the black to gray shading:

Map with translucent overlay

If we look at the examples in the section above we can see a green shade, this is the layer "National Parks". It is translucent. This means that it just adds a shade of green to the image - or rather all the layers "below" the "National Parks". This is a setting of the server which we cannot change. But many clients can change this setting in the image that they have received from the server on their own. This can be done with PNG but not with the JPEG format.

Scale Limits

It is important to understand that every layer can have scale limits when it is not displayed. For example it would not make sense to display the "Overview" and "Raster_250K" at the same time because they contain the same data but with different levels of generalization. So instead they have mutually exclusive scale limits so that they will never be returned in one image. The scale limits are set by the server and do not have to be configured by the client.

All this information is contained in the Capabilities document.

Zooming In and Out

The client can "zoom in" and "zoom out" of the map by calculating new values for the BBOX (bounding box) parameter. The same applies to all map navigation functions like panning or selecting a completely new area.

It is important to note that the server only returns an image to the client, but no additional information about the size, coordinate system, scale, etc. Therefore the server must make sure that it also returns images which may look "wrong" because the client has requested a 'squashed' image (for example with a width to height ratio other than 1:1). If the server would return anything other than exactly what was requested then the client will base its next request on wrong parameters.

The following image shows the same area as above but with the different parameters for the WIDTH which has been changed from 400 pixels in the example above to WIDTH=200 pixels here. The rest of the URL is unchanged

...
 WIDTH=200&
 HEIGHT=300&
 ...
 

The resulting image covers the same area as defined by the bounding box coordinates of the request but the image geometry is out of proportion.

Map image with changed width-to-height ratio

Use the link: Get squashed Map to retrieve the map image form the OGC Demo and Reference Server.

Reprojecting Maps

The OGC WMS standard allows to generate map images in different coordinate systems. So far we have been using the coordinate system "OSGB 1936 / British National Grid" which is referenced in the EPSG database under Code 27700 (see also the Note below). This coordinate system is useful to display the British Isles but it will not work well to display the whole world. Therefore the OGC WMS standard has built in the capability to switch between coordinate systems dynamically. Not every coordinate system can be used for every location on the earth, therefore every WMS needs to advertize which coordinate systems it supports in the Capabilities document. The OGC Demo and Reference Server offers the following list of coordinate systems:

<CRS>EPSG:4326</CRS>
 <CRS>EPSG:27700</CRS>
 <CRS>EPSG:4258</CRS>
 <CRS>EPSG:29903</CRS>
 <CRS>EPSG:2157</CRS>
 <CRS>EPSG:3035</CRS>
 <CRS>EPSG:4937</CRS>
 <CRS>EPSG:3034</CRS>
 <CRS>EPSG:4936</CRS>
 <CRS>EPSG:3038</CRS>
 <CRS>EPSG:3039</CRS>
 <CRS>EPSG:3040</CRS>
 <CRS>EPSG:3041</CRS>
 <CRS>EPSG:3042</CRS>
 <CRS>EPSG:3043</CRS>
 <CRS>EPSG:3044</CRS>
 <CRS>EPSG:3045</CRS>
 <CRS>EPSG:3046</CRS>
 <CRS>EPSG:3047</CRS>
 <CRS>EPSG:3048</CRS>
 <CRS>EPSG:3049</CRS>
 <CRS>EPSG:3050</CRS>
 <CRS>EPSG:3051</CRS>
 <CRS>EPSG:900913</CRS>
 

To request a map in a different coordinate system we simply have to change the Coordinate Reference System (CRS) and the Bounding Box (BBOX) parameters. In the image below you can see two requests side by side, on the left is the original EPSG:22770 and on the right hand side EPSG:4326:

Maps in EPSG:27700 and EPSG:4326 side by side

Use the link: GetMap 27700 to retrieve the map image in the original British coordinate system a seen on the left. Use the link: GetMap 4326 to retrieve the map image in the World Geodetic System '84 (WGS 84) as seen on the right.

If you compare the two URLs there are two major changes, the CRS and the BBOX

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 VERSION=1.3.0&
 REQUEST=GetMap&
 SERVICE=WMS&
 LAYERS=Overview&
 CRS=EPSG:27700&
 BBOX=-205339.46,9150.39,759739.90,1308621.29&
 WIDTH=228&
 HEIGHT=307&
 FORMAT=image/png&
 

In the second request the CRS value ist EPSG:4326 and therefore the BBOX also requires completely different parameters (latitude and longitude coordinates in decimal degrees). This means that in order to switch from one coordinate system to another we need to transform coordinates. The OGC Coordinate Transformation Service covers this functionality.

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 VERSION=1.3.0&
 REQUEST=GetMap&
 SERVICE=WMS&
 LAYERS=Overview&
 CRS=EPSG:4326&
 BBOX=48.5635,-8.3,62.8364,2.2999&
 WIDTH=228&
 HEIGHT=307&
 FORMAT=image/png&
 

Another well known coordinate system is EPSG:900913 which is for example used by Google Maps. It is designed to be usable around the whole world.

Maps in EPSG:3042 and EPSG:900913 side by side

Use the link GetMap in 3042 and GetMap in 900913.

The left hand side of the image shows the same mape in the Europen Terrestrial Reference System '89 (EPSG:3042) which has been desinged to cover all of Europe for scales larger than 1:500k. On the right hand you can see the EPSG:900913 projection and how it deforms the maps and makes the southern parts look considerably 'narrower' than they are in reality.

Note

Coordinate systems, coordinate transformation and projections are a pretty complex topic. EPSG refers to the "European Petroleum Survey Group" who were the first to collect and maintain a geodetic parameter set of Earth ellipsoids, geodetic datums, geographic and projected coordinate systems, units of measurement, and so on. The EPSG registry is nowadays located at http://www.epsg-registry.org/ and is managed by the International Association of Oil & Gas Producers (IOPG ).

Another excellent Online site to find the right code and all parameters required to do the math is located at http://spatialreference.org/.

Also note that as you can see in the map examples above it is almost impossible to give an exact scale for these map images as it is differrent at every place.

Lastly, if you are a developer, make sure that you understand the problem of the axis order confusion.

Getting Information about Objects "on" the Map

The OGC WMS standard offers an optional request type which allows the user to query the server for alphanumerical information. The name of the request is GetFeatureInfo and the functionality is limited to serve the needs of a simple "click into the map". Implementing the feature in a client is straightforward but requires some background understanding.

Note

As the name GetFeatureInfo already suggests the server will not return geographic feature(s) but only selected alphanumerical information. For more functionality please refer to the OGC WFS standard (link at the bottom) which allows to compose complex queries for features including a geographic selection, alphanumeric values and so on.

The GetFeatureInfo Request

Before querying for objects we need to see a map. We use the GetMap request to get a map image:

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 VERSION=1.3.0&
 REQUEST=GetMap&
 SERVICE=WMS&
 LAYERS=Overview,Raster_250K,nationalparks,Topography,Infrastructure,osm_points&
 WIDTH=400&
 HEIGHT=300&
 CRS=EPSG:27700&
 BBOX=427966.6666666667,106800,431833.3333333333,109700&
 FORMAT=image/png&
 EXCEPTIONS=XML
 

The result will look like this:

A map image from the New Forest National Park in Great Britain.

In the next step the user has to click into the map. Let's assume the user has clicked the position:

X=231
 Y=280
 

Then the client will create a GetFeatureInfo request and submit all the information needed by the server to decode this request:

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 VERSION=1.3.0&
 REQUEST=GetFeatureInfo&
 SERVICE=WMS&
 LAYERS=osm_points&
 QUERY_LAYERS=osm_points&
 WIDTH=400&
 HEIGHT=300&
 CRS=EPSG:27700&
 BBOX=427966.6666666667,106800,431833.3333333333,109700&
 INFO_FORMAT=text/html&
 EXCEPTIONS=XML
 &X=231&
 Y=280
 

Note that the URL contains information which we might not expect to need, for example the pixel size of the map. The size of the map is required for the server to decode the image coordinates of the click position. These are then transformed into the map data coordinate system using the BBOX (Bounding Box) parameter. With this information the server can query it's database for features.

If the server has found features it will return them in the requested format. In this example the server finds a pub called "The Crown Stirrup" and a bus stop named "Ye Old Crown and Stirrup" and returns them in an HTML file.

Result of a GetFeatureInfo request.

Use the link GetFeatureInfo to retrieve the data dynamically from the OGC WMS Demo and Reference Server.

Note

The OGC WMS standard does not specify what kind of a text or HTML file is returned as a result of a GetFeatureInfo request. The server will create the HTML file as it has been set up by the site operator.

GetFeatureInfo result as GML

If you need more structured information the best bet is to request for a GML file.

<?xml version="1.0" encoding="ISO-8859-1"?>
 <msGMLOutput
 	xmlns:gml="<a href="http://www.opengis.net/gml">http://www.opengis.net/gml"
 	xmlns:xlink="<a href="http://www.w3.org/1999/xlink">http://www.w3.org/1999/xlink"
 	xmlns:xsi="<a 	href="http://www.w3.org/2001/XMLSchema-instance">http://www.w3.org/2001/XMLSchema-instance">
 <osm_points_layer>
 	<gml:name>OpenStreetMap point objects</gml:name>
 		<osm_points_feature>
 			<gml:boundedBy>
 				<gml:Box srsName="EPSG:27700">
 					<gml:coordinates>430185.447238,107033.115034 430185.447238,107033.115034</gml:coordinates>
 				</gml:Box>
 			</gml:boundedBy>
 			<osm_id>16494895</osm_id>
 			<timestamp>2006-09-24T01:55:58Z</timestamp>
 			<name>The Crown Stirrup</name>
 			<type>pub</type>
 		</osm_points_feature>
 		<osm_points_feature>
 			<gml:boundedBy>
 				<gml:Box srsName="EPSG:27700">
 					<gml:coordinates>430172.215950,107071.994504 430172.215950,107071.994504</gml:coordinates>
 				</gml:Box>
 			</gml:boundedBy>
 			<osm_id>503420358</osm_id>
 			<timestamp>2009-09-20T16:44:15Z</timestamp>
 			<name>Ye Old Crown and Stirrup</name>
 			<type>bus_stop</type>
 		</osm_points_feature>
 	</osm_points_layer>
 </msGMLOutput>

To get the information in the GML format simply change the parameter INFO_FORMAT to read:

INFO_FORMAT=gml&
 

Use the link GetFeatureInfo with GML to get the GML file dynamically from the OGC WMS Demo and Reference Server.

Legends

The last feature that will be touched on in this short introduction are legends. The OGC WMS Standard specifies a request called GetLegendGraphic. It requests images for each layer of a WMS. The style of the returned image largely depends on how the software is implemented. The following example requests for a legend image of the national parks. The request looks as follows:

http://metaspatial.net/cgi-bin/ogc-wms.xml?
 version=1.3.0&
 service=WMS&
 request=GetLegendGraphic&
 sld_version=1.1.0&
 layer=nationalparks&
 format=image/png&
 STYLE=default
 

The result shows a legend item with the color of the areas covered by National Parks in the map (see above images) and a text string with the layer name.

Legend image for the National Parks layer.

Use the link GetLegend to get the legend image dynamically from the OGC WMS Demo and Reference Server.

Note

Every server will return different looking legends. This makes it hard to impossible to create a homogeneous legend by using generic requests to different servers. Therefore especially dynamic clients must be aware that there are many different ways this feature can be implemented.

Error Messages

In case the client causes an error by formulating a wrong request the server will return an error message. To demonstrate this we will request for a map with a layer named "Underview" (which does not exist on the server).

Typically the server will return an error message like this:

<?xml version='1.0' encoding="ISO-8859-1" standalone="no" ?>
         <ServiceExceptionReport version="1.3.0" xmlns="http://www.opengis.net/ogc"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.opengis.net/ogc
         http://schemas.opengis.net//wms/1.3.0/exceptions_1_3_0.xsd">
         <ServiceException code="LayerNotDefined">
                 msWMSLoadGetMapParams(): WMS server error.
                 Invalid layer(s) given in the LAYERS parameter.
                 A layer might be disabled for this request.
                 Check wms/ows_enable_request settings.
         </ServiceException>
 </ServiceExceptionReport>

Note that different software may return different text messages along the ``ServiceException code`. `

One problem may be that the client cannot display this message in the image display element. An HTML client in a browser would simply show the above message as a broken image, something like this:

Crossed out image: This is a typical symbol displayed by browsers for content that cannot be rendered as a graphic image.

To prevent this from happening we can change the format of the error message (the EXCEPTIONS parameter) to INIMAGE. In that case we can see the error message printed into an image:

Error message printed into an image file

Note that the image will have exactly the pixel size that was requested by the client.

Origin of the Data rendered by the Service

The OGC server uses OS OpenData produced and maintained by the Ordnance Survey Great Britain.

License

The data is licensed under the UK Open Government license.

Layers

The OGC WMS Demo and Reference Server hosts several raster and vector data layers containing point, line and polygon features. The service consists of 25 layers including the root layer (0) and two grouped layers, "Topography" and "Infrastructure". The "Topography" layer consists of 10 sub layers, "Infrastructure" of 6 sub layers. See details below.

DTM

The "bottom" layer is the DTM of the UK. The data ranges from zero-values representing elevation 0 which is rendered in black to the highest elevation of approximately 1400 meters rendered in white. Intermediate elevations are represented in shades of grey.

Overview

The "Overview" layer (source data in TIFF format) shows the British Isles in a cartographic design suitable for small scales down to 1:250k.

Raster 250K

The "Raster 250K" layer cartography is optimzed for display around 1:250.000.

National Parks

The National Parks are polygon features with a default dark green semi transparent filling. The cartographic representation can be modified using the OGC SLD standard.

Topography

The layers contained in the group "Topography" are based on the Ordnance Survey "Vector Map District" product. The "Topography" layers contains

  • Tidalwater
  • Foreshore
  • Woodland
  • Surfacewater
  • Surfacewater-linear
  • Rivernames
  • Lakenames
  • Contour Lines
  • Contour Lines 50m interval
  • Peaks
Infrastructure

The grouped layer "Infrastructure" contains the following sub layers.

  • Buildings (or rather, the outlines or building blocks)
  • Railway Tracks
  • Roads (wide representation)
  • Roads-filling (narrower representation to allow for cartographic repreentation as a filled line)
  • Railway Stations
Selected OpenStreetMap Point Objects

The point objects have been retrieved from OpenStreetMap by selecting the tag "Pub" (and similar), campsites and parking spaces. The symbols are represented at a scale greater than 1:50.000. This layer is also queryable so that it can show how a FeatureInfo request works.

Named Places

Named places are another layer containing Ordnance Survey's OpenSpace VectorMap District data also represented at a scale greater than 1:50.000.

Source Data Coordinate System

The source data is available for download at the Ordnance Survey OpenSpace web site in the coordinate system EPSG:27700 (OSGB 1936 / British National Grid).

The server is configured to support a European focused subset of EPSGs codes so that the data can be combined with other European services to show compatibility with INSPIRE etc.

Contact

Please contact the operator if you have questions regarding this service.

Links and Further Information

Please note that this little tutorial will only help you to get started. For full information please refer to the standards documents on the Open Geospatial Consortium web site. You can find an even shorter introduction that may help your boss to understand why this standard is useful here.