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14 Feature Portrayal Service FPS
The new Feature Portrayal Service specification describes a service that is used to remotely visualize features from a Web Feature Service. Clients indicate the WFS and
feature types to be used, and provide styling information. This is useful when the data from the WFS cannot be visualized locally, possibly because the data is not familiar, or
because visualization of the symbology is not known.
14.1 Portrayal Image Retrieval using a Feature Portrayal Service
A Feature Portrayal Transparent Workflow
Integrated Client WFS
CSW FPS
GetCapabilities Request Capabilities Document
GetRecords Request GetRecords Results
GetCapabilities Request Capabilities Document
GetPortrayal Request GetRepositoryItem
SE Document GetFeatures
Features Render Features
Portrayal
Figure 19 Image Retrieval Through an FPS
The above diagram illustrates the sequence of requests and responses that are used to retrieve a Portrayal from a Feature Portrayal Service.
The first transaction is a GetCapabilities request to the WFS server containing the features that are to be displayed. To use a FPS an integrated client must create or locate
an appropriate Symbology Encoding document. In the above example a Catalog Service is queried to obtain a SE document. After retrieving the Capabilities document from the
FPS, a GetPortrayal request can be constructed. This request consists of the URL of the
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65 WFS, the URL of the SE document, and the typename of the features to be rendered. The
request is then sent to the FPS to retrieve a Portrayal. The Feature Portrayal Service retrieves the Symbology Encoding document, as well as
the features from the WFS and renders them. This visualization is returned to the client to be displayed to the user.
14.2 Issues and Tradeoffs with Feature Portrayal Service
The only issue encountered was the varying time of responses from Feature Portrayal Services. Depending on the WFS used, a GetPortrayal request sometimes took as long as
six minutes. The large size of the MSD3 feature type schemas was definitely a factor, but performance also determined the size of the bounding box used in the request.
14.3 Implementation Comments
14.3.1 Intergraph
Once the Intergraph integrated client is given the WFS endpoint either by catalog query or user key-in containing the data to be portrayed, the user is allowed to supply a
symbology encoding document that defines the portrayal for each feature type served by the WFS. Then the user selects the appropriate Feature Portrayal Service and supplies
addition information such as the height, width, and geographical bounding area. Then a GetPortrayal request is made to the service and a image is returned.
Once the image is returned to the client, it is written to local memory and stored in a warehouse connected to the commercial GeoMedia platform. At this point the recently
acquired image is added to the MapView legend using standard commercial rendering capabilities.
14.3.2 Refractions Research
Feature Portrayal Service support was used in Refractions GeoDSS client to support two workflows; the visualization of WFS 1.0 with GML3 content, and the display of
information using MIL2525B Symbology.
An important aspect of this implementation is the seamless access of the catalog for the discovery of Symbology Encoding documents. A user control preference EMS or
MIL2525B is used to determine which request is made. After this single configuration operation all subsequent use of FPS is automatic.
14.4 Recommendations
The approach used for the Feature Portrayal Service has been successful. The separation of concerns represented by the use of the SE document and the OWS Context document
represent an exciting possibility for moving the SLD specification forward.
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The FPS represents a service of pure process it does not hold any intrinsic resources of its own. This is the important difference between an FPS and a WMS. It is hoped that
this example of transparent service chaining will be expanded to work with other services.
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15 Data Aggregation Service DAS
A Data Aggregation Service is used to both remodel schemas from one data model to another as well as aggregating two or more WFS datasets. The output is another WFS
endpoint that is accessed with the same GetCapabilities, DescribeFeatureType, and GetFeature requests. This section documents the process of using a Data Aggregation
Service directly from an Integrated Client.
15.1 Direct Access to a Data Aggregation Service
