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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
GML also give user the flexible ability to define new feature
types for application purpose. So to build an application schema
is thus a two-part process: The profile acts as a restriction of
GML Lo produce types and elements consistent with the complete
GML 3 but potentially lacking in some optional particles. The
application schema then uses these types as a common base, and
uses them in new types and elements by extensions or inclusion.
GML 3 selection & restriction GML profile extension &
inclusion : 3
— application schema
GML schema determines content and structure of GML
document, so different GML schema definition manner affect
the content, size, data presentation manner and the convenience
of GML parsing. But WFS specification did not give out a
uniform GML application schema define manner, a web feature
service provider can define their own application schema,
DescribeFeatureType operation return the schema, the only
requirement of the WFS specification is that the returned GML
document must conform to the schema return by
DescribeFeatureType operation. WFS specification let service
providers define schema, so it gives service provider lots of
freedom and lets the feature services more flexible. But this
flexibility also brings complexity for parsing and overlaying
GML data from different services providers. Different service
providers have different GML application schema, so the GML
data structures from different provider are not same. Now there
is no effective method can map one schema to another
automatically. We hope OGC can provide a uniform GML
application schema define manner in the next version of WFS
specification.
6.WEB FEATURE SERVICE CLIENT DESIGN
The client of Web Feature Service based on web service can be
created in any language at any platform. User can send request to
different web feature services, and the WFS client receives the
response of the request from different services and overlay them
for user. If the return data has same Reference Coordinates
System, the client can overlay directly, otherwise the client need
to change data to the same Reference Coordinates System first
and then overlay. WFS client not only overlay data come from
different web feature services, it also can overlay data come
from other OGC web services (such as Web Map Service) and
data come from local storage. WFS client can provide basic
operation functions: zoom in, zoom out, span, query etc. The
data come from WFS are written in GML, so WFS client can
parse it and storage it to memory data structure in the client side.
Using memory data structure, the basic functions such as zoom
in, zoom out, span and query can operate memory data, so it can
improve the response speed of the client and cut down net
workload. If the client overlay two kind of data, one comes from
a web feature service, one comes form a web map service, if user
wants to zoom in, there is no need to send a new request to web
leature service, the client can process memory data directly, but
for a web map service, the client needs to send a new request to
web map service and overlay the result with the WFS data. The
design of WFS client is show in figure2. A WFS client
implementation using Java shows in figure3.
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7.WEB FEATURE SERVICE APPLICATION SAMPLE
Web Feature Service gives out an effective manner for spatial
data sharing and interoperability based on Web (XML). It can
query data, and it also can provide data manipulate functions
such as add, delete, and update. We are using Web Feature
Service to build data exchange system in Ministry of Land and
Resource, this system can implement the capability of two level
(country level and province level) data exchange and can satisfy
the update requirement of 1:10000 land use status quo data. Web
Feature Service provides a real time online data update manner.
When the province land use data changed, Update Management
will send out update message to Country Data Management, and
Data Management decides when to get the new data using web
feature service. The architecture design of the system is show
in figure4.
Country land use status quo data
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status quo datae Managernente
Fig. 4 Data Exchange System Design
