Database Server Models for WMS and WFS Geographic Web Services - Geographical Information Systems: Trends and Technologies - page 146

a simple visualization with a web browser to a dynamic and interactive
cartographic tool. The dynamic component is provided for example by
pan-zoom, modulations of displayed layers or some widgets such as
tooltips: e.g., Google Maps or Bing Maps. To provide such facilities, the
GetFeatureInfo operation requires a set of parameters. The server can
therefore build a structured result based on the database schema. The query
can be reformulated by: “What is there here?” Here is defi ned by a couple
of coordinates. The output format is an array in HTML or a XML tree.
Current results of a WMS service are generally images. The intrinsic
structure of a result reduces possible manipulations. An end-user does
not manipulate structured data but an image. To be able to access data, the
end-user must select an object on the image. The GetFeatureInfo operation
provides this selection. Nevertheless, some limits appear in the sense that
the provided schema is the set (or a sub-set) of available data for this/
these object(s). No link is performed with the environment of this object.
Furthermore, an end-user must be aware of the database schema in order
to select only a sub-set of available data.
WMS operations are graphic-result-oriented. WFS specifi cations want
to deal with data access. In the following, we present operations based on
the WFS specifi cations and in particular the GetFeature operation.
WFS
Information provided in this part is based on the standard documentation for
WFS version 2.0.0 (OGC 2010b). WFS is an OGC specifi cation for data access.
It describes responses of a Web server to geographical data manipulation
operations. These operations are based on the CRUD manipulations of
geographic data based on alphanumeric/spatial constraints: creation (C),
read (R), update (U) and deletion (D).
The formalism used to model data exchanges for the WFS specifi cation
is GML (Geography Markup Language). GML (OGC 2007a) is a XML dialect
designed to encode, to manipulate and to exchange geographical data.
Specifi ed by the OGC, its main goal is to guarantee interoperability in the
location-based data fi eld.
The WFS specifi cation defi nes fi ve operations to send queries to a
geographical data server and to get answers from it: GetCapabilities,
DescribeFeatureType, GetFeature, Transaction and LockFeature. Figure 2
presents the basic operations of the WFS specifi cation.
The most important operation, in our context, for a WFS server, is the
GetFeature operation. This operation delivers data instances typed by
features, identifi es properties that should be delivered and provides the
results of spatial and non-spatial queries.