In April 2010, Ordnance Survey made a number of their national mapping products freely available under the OS OpenData initiative. Vector and raster datasets at varying scales were released under a very permissive license which allows users to freely create derivative works, even for commercial purposes. Lutra Consulting released a WebGIS application to demonstrate the value and potential of combining OS OpenData, OGC services & standards and open source GIS software. The WebGIS application, Catchment Finder, uses the OGC Web Processing Service (WPS) to provide a simple method for users to generate hydrological catchments (or watersheds) for any point in the UK. Catchment delineation is based on the OpenData Landform PANORAMA dataset, a 50 metre resolution digital terrain model (DTM). Catchment Finder was developed using the following FOSS components: OpenLayers and Ext JS for all user-facing functionality. MapServer and TileCache to serve background mapping and processed results. GRASS GIS for server-side catchment delineation process. PyWPS to provide a mechanism for interaction between the browser and GIS processing taking place on the server. GRASS GIS sits at the core of Catchment Finder. National slope and aspect raster datasets were pre-calculated as inputs for the watershed analysis module in order to optimise calculation times. A WPS process was developed in python (using PyWPS and GRASS’ python bindings). The process chains together a number of GRASS commands in order to generate a vector layer representing the catchment outline which is then displayed in the web client via GML or optionally downloaded as a Shapefile. PyWPS (based on python) was chosen in preference to alternative WPS server implementations due to the typical flexibility and efficiency offered by python (a high-level programming language). Implementing specific GIS processing tasks as WebGIS applications simplifies the end-user’s tasks and therefore opens up GIS processes to non-technical people. Storing datasets and carrying out processing centrally helps remove the burden of managing large/national datasets. Any updates to underlying datasets can be carried out centrally with minimal impact. As Catchment Finder implements the OGC WPS standard, it is also possible for the service to be utilised by desktop GIS applications. At present, due to the low resolution of the underlying DTM, it is only possible to generate watersheds for larger watercourses.