WFS User Guide thinkwhere Glendevon House Castle Business Park Stirling FK9 4TZ www.thinkwhere.com Tel +44 (0)1786 476060 Fax +44 (0)1786 47609
Table of Contents WHAT IS A WEB FEATURE SERVICE?... 3 Key points to note... 3 Performance... 3 Feature Limits... 4 Scale Thresholds... 4 Working with WFS and WMS/WMTS... 4 Spatial Restrictions... 5 ADDING WFS INTO ARCGIS... 6 ArcMap 10.X... 6 ArcMap Catalog... 7 Interoperability Connection... 7 WFS (Web Feature Service) parameters... 8 WFS layer... 12 Filtering features... 13 Applying styles... 14 2
WHAT IS A WEB FEATURE SERVICE? A Web Feature Service (WFS) is an Open Geospatial Consortium (OGC) standard protocol for exchanging geospatial data over the internet using HTTP. In contrast to a Web Map Service (WMS) which provides a rendered image of the data to the GIS client, or a Web Map Tile Service (WMTS) which provides pre-cached map tiles, a WFS provides access to the raw data in Geography Markup Language (GML). A WFS therefore gives the client direct access to both feature geometries and attributes and therefore allows for a full range of functions to be carried out against the data. For example the data can be queried, used to produce derived data, or the loaded data can be saved locally. Web feature services can be consumed by any OGC compliant software package or a compatible API, without the need to store, update and maintain large local copies of data in-house. thinkwhere s web services are delivered by our own fully-managed Cloud-based platform and spatial data infrastructure: themapcloud, architected using industry leading Open Source technologies and frameworks. The platform is hosted on a Cloud infrastructure offered by Amazon Web Services (AWS) and supports highly reliable, scalable and performant web services. Key points to note There are some important considerations to note in order to use a WFS effectively. Performance As with WMS/WMTS, one of the key advantages of WFS is that the data is stored and maintained in a central data repository and is updated on the server without the need for local data management, storage and quality assurance. However, while a WFS is far more flexible than a WMS/WMTS in terms of providing access to the raw feature geometries and attributes, far greater volumes of data are transmitted over the internet from server to client. Users should therefore expect that the data received through a WFS will be slower to load than when working with a WMS/WMTS or with locally stored data. Performance of WFS is heavily impacted by the number and complexity of feature geometries and the number of attributes and layers being served. The speed of the end-user s internet connection is also a critical factor in determining service performance. 3
Feature Limits Due to the potential volumes of data being transmitted, it is necessary to limit the number of features that can be returned from any one WFS request. This is necessary to ensure adequate performance of the service. A feature limit is set up on the server and is configured so as to balance the demands for performance and usability. For datasets with more complex feature geometries and/or more attributes, a lower feature limit may be required, while simpler datasets, for example those with point geometries and few attributes, are likely to have higher feature limits. Scale Thresholds Users should note that where the number of features requested by the client exceeds the server feature limit for that dataset, the data provided to the client will only be a subset of that requested. Furthermore, the client will not flag this to the user. To ensure that all features within the current map extents are drawn, the WFS must be added to the map at an appropriate scale relative to the density of the features in that area, the feature limit of the WFS on the server and the user s monitor size/resolution. For example, with a feature limit of 2,000 and a monitor size of 25, users wishing to display OS MasterMap for a rural area should be at a scale of 1:5,000 or larger to ensure all features are drawn. In areas of higher feature density e.g. urban areas it is suggested that the user be 1:1,000 or larger. It is therefore suggested that an appropriate scale visibility threshold be set for the WFS layer within the GIS client and adjusted as appropriate depending on the set up and the nature of the data in the area of interest. Working with WFS and WMS/WMTS The key benefit of using WFS is that the service returns actual features with geometry and attributes. However, given the slower load times of WFS layers and the use of server-side feature limits, it s important to consider whether a WFS is required for a given task or whether a WMS/WMTS would be more appropriate. In this regard, the user will need to consider whether the feature geometries/attributes are required, and the scale at which the data is to be viewed. As noted above, it is recommended that appropriate scale visibility thresholds are set up for WFS layers on the client and that a WMS or WMTS is used at smaller scales for map navigation/context. In this way, the WMS/WMTS and WFS can be used seamlessly together. For example: Map navigation / Locating an area of interest (AOI) WMS/WMTS can be used as a contextual dataset when panning and zooming the map to locate a particular area of interest to give better data loading performance. Once zoomed in on the AOI, a WFS can be used for querying and analysis of the data. Data capture/digitising Again, WMS/WMTS data can be used as a background mapping to provide contextual information at smaller scales while the WFS layer is used for snapping or copying and pasting data features when at larger scales. Georeferencing For the task of georeferencing raster data WMS/WMTS is generally sufficient, however vector data provided by a WFS would be required if the user wished to snap to a particular feature. 4
Spatial Restrictions Commercial products may be spatially restricted to meet the end user licensing requirements. WFS spatial restrictions checks are performed on the server. These checks ensure that the server returns the product that is within the allowed geographic boundaries to ensure that the licensing requirements are met. This way, if the client's request contains bounding box even marginally outside the geographic boundaries set for the product, the server will not return any features. Please ensure that you are within the allowed area when requesting a WFS dataset. 5
ADDING WFS INTO ARCGIS ArcMap 10.X Because a given dataset may contain a very large number of features, you should zoom into a larger scale in the desired area before importing the data. We recommend using background WMS/WMTS mapping for contextual information if required. To use WFS content in ArcGIS for Desktop, you need to install the Data Interoperability extension and add an Interoperability Connection to the WFS service (WFS connections are enabled without licensing the extension, but installation is required.) http://www.esri.com/software/arcgis/extensions/datainteroperability/supportedformats 6
ArcMap Catalog 1. Open Catalog in ArcMap. Go to Windows menu Catalog. 2. Catalog window will open. 3. Click on ' Interoperability Connection' 'Add Interoperability Connection'. Interoperability Connection 1. Select WFS format from the drop-down. 2. Click on Parameters... A pop-up window will appear. 7
WFS (Web Feature Service) parameters 1. Input WFS URL. 2. Set WFS version to 1.1.0. 3. Tick 'Use HTTP Authentication' and input the username and the password. 4. Click on the 'Feature Types'. A pop-up window will appear. Select Feature Types 8
1. Select the layer(s) you wish to add. 2. Click OK. The layer will be displayed in the input field. 9
WFS (Web Feature Service) Parameters 1. Tick 'Use Search Envelope'. 2. Select Search Envelope Coordinate System (a pop-up window will appear, use 27700 in the filter). 3. Leave the rest of the setting default and click OK. 10
Interoperability Connection Properties 1. Select the required Coord. System & click OK Adding the connection as a layer 1. Drag and drop the connection into the Layers panel. 11
2. Alternatively, click the Add data button, then click the drop-down menu and click Interoperability Connections. WFS layer The layer will be added to the Layers Panel and displayed in a map window with default styling. 12
N.B. If the number of features in the viewing area exceeds the feature limit for that dataset, not all features present in the data will be displayed on the map. Users must be aware that the client will not flag this to the user and so great care must be taken to ensure that the WFS is only used at larger scales so that all features will be returned. It is therefore recommended that an appropriate visibility scale threshold is set up for the layer that reflects the feature limit set on the server, the relative density of features and the users monitor size. Filtering features Adding filter to loaded WFS data Filtering could be applied to the loaded dataset. 1. Right click on the WFS layer in Table of Contents and select Layer Properties. 13
2. In the Layer Properties window navigate to Definition Query tab 3. In Definition Query window add your query and press Ok. Applying styles ArcMap will apply default styling (Single symbol simple marker/line/ fill with random colour). For complex datasets as OS Master Map the styling should be applied to make data look meaningful. There are ready made styles for complex datasets like OS MasterMap available. To apply styling 1. Right click on the WFS layer in Table of Contents and select Layer Properties. 14
2. In the Layer Properties window go to Symbology tab. 15
3. Click Import. 4. A pop up window will appear. Navigate to your style file. 5. Click Ok. 16
6. In Import Symbology Matching Dialog select the Value Field. 7. Click Ok. 17
8. Click Ok, so chosen styling will be applied to the layer. 18