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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
An example of a simple XSLT declaration is shown in the
Figure 2. XSLT declarations are expressed in the form of
templates. The template in the example selects all elements
representing buildings (*Rakennus' in Finnish) from the source
tree that match to the selection phrase (expressed in a language
called XPath) (W3C, 1999a), then filters out all elements for
which the given test phrase inside the 'xsl:if element does not
hold. All elements inside the template not belonging to the xsl-
namespace are written to the result tree. For instance, in the
example the Building-element forms part of the target common
vocabulary, so the effect of the transformation in this case
would be a change in the naming system (from the Finnish to
English terms) and a change in the collection criteria (only
buildings with area (‘pinta-ala’ in Finnish) larger than the
threshold value will be included). After the instruction:
'xsl:apply-templates', the process continues down the XML tree.
The transformation goes on until no more matching elements
are found.
<xsl:template match="Rakennus">
<xsl:if test="pinta-ala > 200">
<Building>
<xsl:apply-templates/>
</Building>
</xsl:if>
</xsl:template>
Figure 2. A sample XSLT template
5. CASE: GIMODIG PROTOTYPE
5.1 General
The XSLT-based data integration transformation described is
being tested in a European Union funded project GiMoDig. The
Finnish Geodetic Institute acts as a coordinator for the project.
The other participants are the University of Hanover and the
NMAs of Finland, Sweden, Denmark and Germany (Sarjakoski
et al., 2002).
The objective of the GiMoDig project is to develop and test
methods for delivering geospatial data to a mobile user by
means of real-time data-integration and generalization. The
project aims. at creation of a seamless data service providing
access, through a common interface, to the primary topographic
geo-databases maintained by the NMAs in various countries. A
special emphasis is put on providing appropriately generalized
map data to the user depending on a mobile terminal with
limited display capabilities.
In the GiMoDig system architecture each participating NMA
provides geospatial data through the WFS interface, encoded in
a country-specific XML-format (GML Application Schema).
These datasets are processed by a middleware service on the
Data Integration layer to integrate the pieces of data coming
from individual countries into a common application schema
and coordinate system. The middleware service employs the
XSLT technology extensively in the process. The service is
implemented as a Java servlet environment and the XSLT
Processor used is a product called Xalan from the Apache
community (Xalan, 2004).
179
5.2 Global Schema and Service Architecture
In the GiMoDig project there are four different national data
models involved. Each of the participating countries has
organized the topographic map data in an individual way. For
the purposes of the data integration a common data model,
named GiMoDig Global Schema has been developed
(GiMoDig, 2003b), (Aftlerbach et. Al, 2004). This data model
consists of 17 different Feature types. These types are selected
based on the data availability in the national databases on one
hand, and on the requirements of the selected mobile use cases
on the other. The list of the Global Schema Feature types is
shown in Table 1.
Feature type Geometry type
Administrative Boundary Line
Water (except inland) Area
Watercourse Area or Line
Lake / Pond Area
Marsh / Swamp Area
Park Area
Building Area
Contour Line (Land) Line
Cropland Area
Named Location Point
Built-Up Area Area
Railway Line
Road Line
Trail / Footpath Line
Airport / Airfield Area
Forest Area
Grassland Area
Table 1. The GiMoDig Global Schema Feature types
The GiMoDig prototype service is built according to the system
architecture illustrated in the Figure 1. As the aim in the project
is to develop as open service architecture as possible, the access
interfaces to cach of the service layers are based on
internationally accepted standards. Consequently, the four
national data services implement the OGC's WFS interface
(OGC, 2002).
The Data Integration service also implements the WFS interface
providing a single access point to all of the participating
national data services. As such, the Integration Service plays
the role of a Cascading WFS. Query interface for the Data
Processing layer (Generalization Service) has been developed
in the project, as one does not exists as a result of the
international standardization efforts. On the Portal layer the
widely recognized Web Map Service (OGC, 2001) interface is
used, together with the newer OpenLS Presentation Service
interface (OGC, 2004).
