Updating geo-information in a heterogeneous networked environment – Experiences and evaluation of OpenGIS Web Feature Services

Besides the GeoServer WFS server also other server software is being tested, i.e. the Ionic RedSpiderWeb WFS. Also this test was successful after solving some minor issues. Figure 8 shows the situation in which our WFS client accesses both the GeoServer WFS and RedSpiderWeb WFS. In this case the cadastral data is available at GeoServer WFS server and some background topographic data is available at the RedSpiderWeb WFS server.

5. Evaluation of WFS
The unique aspect of Transaction WFS is that, now for the first time ever, it is possible to edit data in a heterogeneous distributed environment. However, though ‘simple’ editing does go well (sketching new parcel boundaries without maintaining the topology of the final and approved parcels), we would like to share a number of observations related to editing in more complex real world situations, such as handling topology (van Oosterom, 1997). These observations are usually related to current limitations of the WFS protocol, but once identified they may be solved in future versions of WFS:

  1. GML3/Topology: WFS is not yet advanced enough to support transactions on complicated geographic data sets based on GML3 (e.g. with topology). Incorporating GML 3.0 in WFS would allow WFS to deal with topology, temporal aspects and default styling.
  2. New object identifiers: When the client creates new features, these should be assigned unique identifiers. The client needs identifiers in order to be able to refer from one object to another object; e.g. a boundary may refer to the parcel on the left and on the right side. Of course, the client can generate locally unique identifiers (for new boundaries and parcels), but there is no guarantee that these are also unique at the server (in a multi-user environment). Two possible solutions are:
    • A new WFS request type is added ‘GetNewIds’ in which a client can request one or a range of new unique object-identifiers.
    • The WFS request type ‘Transaction’ has built in functionality to translate the local id’s of new features into global id’s and send back a report to the client with these translation details. Note that this translation also the referring local id’s (foreign keys in RDBMS terminology) should be replaced by global id’s; e.g. in the boundary feature not only the local id of the boundary itself should be replaced by a global id, but also the left and right references (local id’s) to parcels should be replaced by global id’s in the left and right references.
  3. Multiple attribute identifiers: The current WFS (and Filter encoding) specification assumes one attribute to be the feature identifier. In real world applications the identifiers may be composed of several attributes; e.g. in case of a cadastral parcel this could be: municipality, section parcel number and time (parcel version). A solution for this would be that the WFS request ‘DescribeFeatureType’ (perhaps a better name for this request would be ‘GetFeatureTypeDescription’) returns in its description the definition of identifiers (possibly composed of multiple attributes); both for the features own identifiers (primary key) and identifiers of other features types used in references within this feature (foreign key).
  4. Area Locking: Though it is possible to lock all features overlapping with a specified lock area via the WFS request type ‘LockFeature’ (and specifying the actual area via the filter encoding), this does not truly locks an area. Other users may for example insert new features (which could overlap with some of the locked features). Certain application may require a true locking of the area (and not only locking the features within the area).
  5. True (atomic) transactions: To a client it is unknown (and up to a certain extend it does not matter) whether a server is based on a DBMS or on files to manage the data. One important aspect of an atomic (DBMS) transaction is that either all actions within one transaction (inserts, deletes, updates) succeed or none of the actions succeed. This in order to bring the system from one consistent state into the next consistent state (which may require several basic actions at the level of insert/delete/update). However, WFS has defined the status of partial successful (in addition to completely successful and fail), to allow for web feature servers that don’t support atomic transactions (file based) to give reports which actions succeeded and which failed. Note that the web feature server cannot advertise whether transactions are dealt with as atomic transaction (thus as one entity) or not. This should be enhanced in the future versions of the WFS request ‘GetCapabilities’.
  6. Error reporting: This should be enhanced and common error messages - especially in the case of edit operations - should be standardized, in order to give useful feedback to end-users.
  7. Integrity constraints in transactions: Validation of (changes in) features should prevent that a data set will contain invalid features, that is, features that violate topological rules or other spatial or non-spatial restrictions. The WFS specification defines some operations and mechanisms that can be used for validation of single features. It is not so easy however to enforce integrity constraints that concern combinations of features (as in the case of topologically structured data or certain rules implied by business logic).
  8. Clients defining new feature types: Currently, transaction WFS allows clients to add, delete and update feature instances of feature types known at the server. In DBMS terms this is related to the Data Manipulation Language (DML) operations. One could image situations in which a client wants to define a new feature type (from scratch or based on inheritance from an existing feature type). Again in DBMS terms, this would then be related to the Data Definition Language (DDL) operations. Therefore, the future WFS specification should consider including a new request: ‘DefineFeatureType’, through which a client can submit a GML schema defining a new feature Type.
  9. Transferring constraint knowledge to client: The server may check certain integrity constraints after the client posts a transaction and as a result the transaction may fail. However, for the client it is unknown what the constraints are (except for the conditions implied by the GML schema defining the individual feature types). It may be quite frustrating for a client trying to update data and getting back (unexpected) errors. In fact, dealing with all kinds of constraints in a data model is a generic problem. Using a Model Driven Approach (MDA) constraints have to be modeled first (similar to object classes, attributes and relationships), for instance in UML (Unified Modeling Language) class diagrams and OCL (Object Constraint Language). The actual implementation is a derivative of the model. For more information on MDA, UML and OCL, visit the website of the Object Management Group (http://www.omg.org/, [49]). One interesting question is: is it possible (and meaningful) to translate constraints in the data model to constraints related to the structure of valid transactions (e.g. a parcel split always implies at least deleting one old parcel, inserting a new boundary and two new parcels).

6. Conclusions and future work
The case study presented in this paper shows that the retrieval and combination of geo-data from multiple, heterogeneous data sources in one Web client is relatively easy with OpenGIS WFS services. One reason is that the WFS specification clearly describes the requests and responses that a WFS service should support. This way it is possible to 'decouple' client and server and e.g. build an application-specific Web client that still can communicate with (open source or commercial) server software developed by others. Another reason is that WFS uses standard web technologies as HTTP and XML (GML and WFS-requests/responses). Common web technologies can be used, like Servlets/Java Server Pages for application logic and SVG for cartographic visualization.

Not only web-based clients, but also thicker clients (like GIS software for analysis or viewers) can use data from WFS servers. This makes exchanging and sharing geo-data a lot easier: whether the data is stored in a local file system or in a remote database has become (almost) transparent to the end-user.

Besides retrieving, also editing of data in an interoperable web environment has been tested in this case study. In the case study ‘notary drafts cadastral parcel’ a relative simple edit procedure has successfully been realized in our prototype. Based on these experiences and requirements of more complex cadastral editing, an evaluation of the WFS protocol has been given, together with a number of suggestions for future extensions/improvements of the WFS protocol. For developing fully functional Internet-GIS based applications, application logic can be divided between the web feature server, the client and other (mediating) services like application services. An interesting research topic is how and where to the check integrity constraints (and other application logic) in WFS based distributed systems.

References
  • Berg, C. van den, Tuijnman, F., Vijlbrief, T., Meijer, C., Oosterom, P. van, Uitermark, H., Multi-server internet GIS: Standardization and practical experiences. In Goodchild, M.F., M.J. Egenhofer, R. Fegeas, and C.A. Kottman, editors (1999), Interoperating Geographic Information Systems, Boston, USA, (International Conference and Workshop on Interoperating Geographic Information Systems, Santa Barbara, California, USA, December 3-4 and 5-6, 1997) pages 365-378.
  • Brentjens, T., OpenGIS web feature services for editing cadastral data. Analysis and practical experiences, MSc thesis, TU Delft, section GIS technology, May 2004.
  • OGC, 2001, Vretanos, P. (editor), Filter Encoding Implementation Specification, version 1.0, Reference number: OGC 02-059, OpenGIS Consortium Inc., USA.
  • OGC, 2002, Vretanos, P. (editor), Web Feature Service Implementation Specification, version 1.0, Reference number: OGC 02-058, OpenGIS Consortium Inc., USA.
  • OGC, 2003, Cox, S., Daisey, P., Lake, R., Portele, C., Whiteside, A. (editors), OpenGIS Geography Markup Language (GML) Implementation Specification, version 3. Reference number: OGC 02-023r4, OpenGIS Consortium Inc., USA.
  • Oosterom, P. van, 1997, Maintaining consistent topology including historical data in a large spatial database. In Chrisman, N. (Ed.), Proceedings of Auto-Carto 13, Bethesda: ACSM & ASPRS, pp. 327-336.
  • SVG, Scalable Vector Graphics, http://www.w3.org/TR/SVG/intro.html, last visited 13 April 2004.
  • XML Schema, http://www.w3.org/XML/Schema, last visited 21 April 2004.
  • XSLT, XML Stylesheet transformations, http://www.w3.org/TR/xslt, last visited 16 June 2004.


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