Listing projection(s) from a WMS server

The Web Mapping Service (WMS), which can be found at provide data in several coordinate systems and you can then specify which one you would like. However, you can't reproject or transform the WMS into some other system that the service provider does not provide, which means that you can only use the coordinate system that is provided. The following is an example of a WMS getCapabilities request (http://gis.ktn.gv.at/arcgis/services/INSPIRE/INSPIRE/MapServer/WmsServer?service=wms&version=1.3.0&request=getcapabilities), showing a list of the five available coordinate systems from a WMS service:

Getting ready

The WMS service URL that we will use is http://ogc.bgs.ac.uk/cgi-bin/BGS_1GE_Geology/wms?service=WMS&version=1.3.0&request=GetCapabilities. This is from the British Geological Survey, titled OneGeology Europe geology.

Tip

For a list of WMS servers that are available worldwide, refer to Skylab Mobile Systems at http://www.skylab-mobilesystems.com/en/wms_serverlist.html. Also, take a look at http://geopole.org/.

We will use a library called OWSLib. This library is a great package for working with OGC web services such as WMS, as follows:

Pip install owslib

How to do it...

Let's go through these steps to retrieve the projections that a WMS server provides and print the available EPSG codes to screen:

1. Create a new Python file named ch02_03_show_wms_srs.py in your /ch02/code/working/ directory, and add the following code:

   #!/usr/bin/env python
   # -*- coding: utf-8 -*-
   
   from owslib.wms import WebMapService
   
   url = "http://ogc.bgs.ac.uk/cgi-bin/BGS_1GE_Geology/wms"
   
   get_wms_url = WebMapService(url)
   
   crs_list = get_wms_url.contents['GBR_Kilmarnock_BGS_50K_CompressibleGround'].crsOptions
   
   print crs_list

2. Now, run the ch02_03_show_wms_srs.py script and you should see the following screen output:

   $ python ch02_03_show_wms_srs.py
   ['EPSG:3857', 'EPSG:3034', 'EPSG:4326', 'EPSG:3031', 'EPSG:27700', 'EPSG:900913', 'EPSG:3413', 'CRS:84', 'EPSG:4258']
   

How it works...

Determining information on the WMS projection involves using the OWSLib library. This is quite a powerful way to get all kinds of OGC web service information from your client. The code simply takes in the WMS URL to retrieve the WMS information. The content of the response is called, and we are able to access the crsOptions attribute to list out all the available EPSG codes.