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Updating geo-information in a heterogeneous networked environment – Experiences and evaluation of OpenGIS Web Feature Services  T.J. Brentjens GIS Technology, OTB/TBM, TU Delft The Netherlands  M.E. de Vries GIS Technology, OTB/TBM, TU Delft The Netherlands  C.W. Quak GIS Technology, OTB/TBM, TU Delft The Netherlands  C. Vijlbrief GIS Technology, OTB/TBM, TU Delft The Netherlands  P.J.M. van Oosterom Kadaster, Apeldoorn, The Netherlands.
Though Internet GIS has been very popular for nearly a decade and is becoming more popular all the time, it is most often limited to simple map viewing or retrieval. Today there are many Internet servers, often conforming to the standard OpenGIS Web Map Server (WMS) protocol, mainly delivering (raster) images, which can be viewed by clients. There are also a few ‘closed’ Internet GIS applications, which allow editing of features by clients based on proprietary communication protocols. In practice this means that both the server and the client have to be of the same vendor and no heterogeneous situations are feasible; e.g. GeoShop (van den Berg et al 1997). However with the availability of the standard OpenGIS Web Feature Server (WFS) protocol, it is now possible, for the first time ever, to realize Internet based geo-information processing environments which include multiple servers offering data layers and different client types specifying the updates. This will be illustrated via a case study ‘notary drafts cadastral parcel boundary’, a relatively simple distributed editing prototype. The WFS protocol will be analysed for more advanced edit scenarios and a number of improvements of the WFS protocol are suggested.
1. Introduction This paper evaluates the strengths and weaknesses of the OpenGIS Web Feature Services (WFS) protocol for creating distributed heterogeneous, yet interoperable geo-information systems. The evaluation is based on our experiences with the development of a WFS environment for editing cadastral data by notaries: using a simple Web client notaries can sketch new or changed cadastral parcel boundaries or edit ownership information (in their offices or in the field, with a PDA) and submit these changes to the central cadastral geo-database via an OpenGIS Web Feature Service that supports transactions. In order to realize interoperable systems, standards must be used. The OpenGIS Consortium (OGC) has issued a number of Web service interface specifications in order to standardize the requests and responses between a Web service and a Web client. The scope of the Web Feature Service specification is not only the retrieval of geo-information over the Web, but also the editing of geo-data (both the spatial and the thematic attributes). Section 2 will give a short overview of the WFS protocol (and protocols used by WFS, such as GML and filter encoding). In section 3 our case study will be introduced. The interoperability aspect of the systems is discussed in section 4, where also a number of alternative server and client implementations are tested in cooperation with our case study prototype server and client. Based on these experiences an evaluation of the WFS protocol is given in section 5, accompanied by suggestions of future extensions/improvements of the WFS protocol. The conclusions can be found in the last part of this paper. 2. Short WFS overview Two classes of Web Feature Services are defined by OpenGIS: Basic WFS (needed for retrieving features) and Transaction WFS (needed for editing geo-data) (OGC, 2002). The WFS protocol allows a client to retrieve geospatial (vector-) data encoded in Geography Markup Language (GML) from multiple Web Feature Services. GML is an XML encoding for the modelling, transport and storage of geographic information, including both the spatial and non-spatial properties of geographic features (OGC, 2003). The current version WFS (1.0) is based on GML 2.1.2. This has some disadvantages as also GML 3 is available with more functionality (e.g. topology, metadata, 3D primitives,…) and GML 3 will also become a ISO standard (in contrast to GML 2.1.2). |