Advances in gml for geospatial applications slide

Università degli Studi dell’Aquila
Facoltà di Ingegneria
Prof. Eliseo Clementini
Advances in GML for
Geospatial Applications
di Chang-Tien Lu · Raimundo F. Dos Santos Jr ·
Lakshmi N. Sripada · Yufeng Kou · 2007
Vittoriano Muttillo
214248

Introduction to GML
Geography Markup Language (GML) is an XML encoding designed for
use with geographic information
GML helps in the storage, exchange, and modeling of geographic
information containing both spatial and non-spatial attributes
GML uses the concepts provided in the Abstract Specification of the
Open Geospatial Consortium (OGC)
GML is a relatively new language still in development. Data
processing techniques need to be refined further in order for GML to
become a more efficient medium for geospatial applications
Support for more complex objects has been incorporated in GML 3,
whereas previous versions only accounted for simple features

Topics
 GML storage, schemas, and the differences between GML versions
GML parsers and query languages
Techniques for data transformation and visualization of GML
documents
Applications of GML for mobile devices, as well as GML Web Services
Various GML-related research

GML schemas and storage
Systems were not always based on XML grammar
Spatial data is heavy by nature
GML documents can be (and often are) very large, raising concerns
about processing and transport
The GML specification does not address (nor does it intend to)
functional aspects related to performance
The burden of efficient processing lies solely on user shoulders
GML can be used for storage of geospatial data in plain text files or
XML databases but many institutions implement their own storage
strategies in different ways

GML schemas (1)
The method of generating GML documents is irrelevant but
important is that the documents conform to the requirements in the
GML document specification
GML 1 used Document Type Descriptors (DTD) on Resource
Description Framework (RDF) schemas to define elements and
attributes. DTDs have some disadvantages. For instance they are not
written in XML, which makes them inconvenient to interpret.
GML 2 removed DTDs and RDF schemas
GML 3 extends the use of XML even further

GML schemas (2)
 A GML schema is an XML schema. A single interpreter can be used for
both the schema itself and the GML document.
 GML defines various XML schema types and elements such as features,
geometries, and topologies through a hierarchy of GML objects
 Designers can create different types of schemas by extending or
restricting the features from the GML base schema.
 Flexibility in using GML to represent a diverse range of spatial objects
 There have been initiatives towards the implementation of standard
application schemas to specific domains:
 Brodaric et al. describe the GeoSciML project as a tailored GML schema used
to manage scientifc data suited for geological mapping in “Standardizing
geologic data interchange: The CGI datamodel collaboration”
 Examples of GML documents and schema documents can be found in
the OGC GML specification

GML schemas (3)
 A spatial feature may be associated with one or more geographic
properties
 A feature is described by a set of properties
 Coordinates for feature are associated with a Spatial Reference system
(SRS)
 Element names corresponding to instances of GML classes start with an
uppercase letter (e.g., Curve), while property tags start with a lowercase
letter
 Envelope describes a region bounded by a pair of positions denoted by
its corners. The “coord” type has been deprecated with GML3.0. The
“pos” and “posList” elements are now used instead
A GML instance document “exampleRoad.xml ”

GML schemas (4)
The main body of GML instance document “exampleRoad.xml”
 CurveMember is a property of the CompositeCurve object
 Its children may have geometries with similar or different interpolations
 The two curve members in the example are Curve gml:id “c101” and Curve
gml:id “c102”
 Some of the elements that can be used as a value of gml:curveMember are
gml:LineStringSegment and gml:CubicSpline. These elements are new to GML
version 3

GML schemas (5)
A LineString is a type of curve that is composed of single segments.
It is defined by two or more coordinate tuples, with linear
interpolation between them
A CubicSpline is a spline constructed of piecewise third-order
polynomials which pass through a set of n control points
The control parameters record must contain vectorAtStart, and
vectorAtEnd which are the unit tangent vectors at controlPoint[1]
and controlPoint[n] where n = controlPoint.count
Only the direction of the vectors is
relevant, not their length

GML schemas (6)
Some of the other geometry elements supported by GML are:
 Point
 Linestring
 LinearRing
 Polygon
 MultiPoint
 MultiLineString
 GeometryCollection
 MultiPolygon
By using a few core schemas as defined in the GML specification,
new data types can be defined
Further details about GML standards, examples of GML documents,
and GML schemas can be found in the GML specification document

GML storage (1)
 Data can be stored in database format and converted into GML format
 There are several alternative approaches for the storage of
semistructured data or XML documents:
 a relational model
 an object-oriented model
 an object-relational model
 a specialized XML database
 full file storage on disk
 Oracle DBMS [1] and XML DB [2] are examples that support different
approaches
 Each approach must still be evaluated further in terms of the query
processing efficiency for large amounts of spatial and non-spatial data
[1] https://www.oracle.com/database/index.html
[2] http://www.oracle.com/technetwork/database/database-technologies/xmldb/overview/index.html

GML storage (2)
Various approaches to storing GML/XML documents

GML storage (3)
 The approaches to designing relational database schemas for XML
documents can conveniently be divided into two categories:
 Structure-mapping
 Model-mapping
 Under structure-mapping, the design of the database schema is based on
the DTD (Document Type Descriptor), or GML schema that describes the
structure of the GML documents
 With the model-mapping approach, a fixed database schema is used to
store any GML documents without the assistance of GML schema or DTD.
These mappings are performed on element types, attributes, and text
 Three implementation of document-storing based on relational DB are:
 LegoDB (Structure-mapping) [1]
 Monet (Model-mapping) [2]
 XParent (Model-mapping) [3]
[1] https://wiki.csc.calpoly.edu/csc560/wiki/LegoDB
[2] http://www.cs.ucla.edu/classes/spring04/cs240B/presentations/june3a.ppt
[3] http://www.cse.unsw.edu.au/~weiw/files/ICDE02-XParent-Final.pdf

GML storage (4)
• Summarizes of various approaches that can be used for storing GML
data
GML/XML data models

Differences between GML
version (1)
The OGC is in version 3.1.1 (3.3) of GML [1]
GML 1 serves as the first evolutionary step towards the better
approach that is available today
GML 2 provides facilities to handle simple features, such as linear
geometries restricted to one or two dimensions. It allows developers
to model the real world using the features present in its
specification
GML 3 incorporates more intricate structures than either of the
previous versions, including support for complex geometries, spatial
and temporal reference systems, topology, units of measurement,
metadata, gridded data, and default styles for feature and coverage
[1] http://www.opengeospatial.org/standards/gml

Differences between GML
version (2)
 GML 3 includes support for complex 3D geometries, 2D topology, temporal
properties, and dynamic features
 GML 3 remains compatible with the previous versions
 GML Schemas in version 3.x have been expanded significantly from previous
versions
 GML 3 is eight times as large as GML 2
Differences in GML versions

XML parser (1)
 A parser reads a GML document, validates it against the schema, and
creates a representation of the document
 GML does not need a special parser. In fact, XML parsers can be used for
parsing GML files since GML is based on XML specifications
 Some available XML parsers are
 Xerces2 [1]
 XSV [2]
 MSXML4.0 [3]
 A software application should be able to understand the meaning of
each element in the GML dataset, whether the element refers to a
feature, a property of a feature, or a feature collection
 Validation may not always be needed, but ideally this functionality
should be available
[1] http://xerces.apache.org/xerces2-j/
[2] http://www.stylusstudio.com/xml_schema/xsv.html
[3] http://www.microsoft.com/it-it/download/details.aspx?id=19662

XML parser (2)
 There are two standard APIs that are currently used by software
applications to parse GML documents:
 Document Object Model (DOM) [1]
 Simple API for XML (SAX) [2]
 The choice of a DOM or SAX parser for GML documents depends on the
resource usage and efficiency
 DOM builds a tree structure as it processes the data, which tends to
require a large amount of memory in the case of spatial databases
 A SAX parser traverses the document sequentially, treating the
document as a data stream. This tends to consume fewer resources and
hence can be used for larger datasets
 A parser ideally should combine the advantages of both DOM and SAX.
The Galdos GMLSDK [1] is an example that combines features from both
approaches
[1] http://en.wikipedia.org/wiki/Document_Object_Model
[2] http://en.wikipedia.org/wiki/Simple_API_for_XML

XML parser (3)
Comparison between DOM and SAX parsers

GML query language (1)
Many query languages have been proposed for querying GML
documents
Although GML may utilize the readily available query
languages developed for XML, these languages must be
extended with spatial operators if they are to be used for GML
The objects represented in GML are often more complex than
those typically encoded in XML, since geographic objects have
both spatial and non-spatial attributes.
The data model for a GML query language therefore has to
reflect this complexity, and the queries must follow suit

 XML query language models can be extended to include the spatial query
attributes of GML. This takes advantage of the existing XML query processing
capabilities and at the same time provides the additional capabilities
required for GML data processing.
 GML queries differ from XML queries as they tend to involve larger joins
over large datasets. In addition, storing, indexing, and querying spatial data
requires more abundant resources than those needed for relatively simple
alphanumeric data
Comparison between XML and GML query languages

Xquery [1] has been designed to meet the requirements of an XML
query language, as identified by theW3C XML query working group.
Vatsavai [2] extended XQuery as a base for a GML query language.
XQuery also allows extension functions that can include spatial
operations such as intersects.
An important consideration when developing such languages is to
decide whether to
 extend an already existing XML query language
 develop a new query language for GML
It is not clear that creating a completely new language would be
either efficient or successful.
[1] http://www.w3.org/XML/Query/
[2] http://www.cobblestoneconcepts.com/ucgis2summer2002/vatsavai/vatsavai.htm

Another research area in the field of query processing is
how to index GML data. Indexes contain data storage
information that can be used to speed up searches.
GML documents can either be stored as is or the data
can be stored in a database and converted to GML when
required.
Existing spatial indexing techniques can be used for
storing GML data in databases.
[1] http://en.wikipedia.org/wiki/R-tree

R-Trees [1], have a hierarchical structure that is suitable for fast
retrieval of spatial data, though such an implementation may
consume a great deal of computation power
R* Trees [1] and R+ Trees [2] insert objects in distinct paths, making
them exclusive to a node.
Z-order B-Trees [3] provide access control to objects while
permitting concurrency control, though this may impose a high
performance cost.
Quad Trees [4], may or may not be as suitable for spatial data as the
other approaches, though they still offer a fast search method.
[1] http://en.wikipedia.org/wiki/R*_tree
[2] http://en.wikipedia.org/wiki/R%2B_tree
[3] http://en.wikipedia.org/wiki/Z-order_curve
[4] http://en.wikipedia.org/wiki/Quadtree

Comparison of indexing techniques for spatial data

Using SVG for GML
visualization (1)
 There are several approaches to generating mapping products
in either web or local environments.
 SVG (Scalable Vector Graphics) is a common two-dimensional
vector graphics standard. SVG is a XML product that is well
suited for context-sensitive mapping based on the layers that
the system developer wants to make available externally.
Visualization approaches for geospatial data

Using SVG for GML
visualization (2)
 In the first, the GML file is converted into SVG format through a
parser in conjunction with styling tools. Once the SVG file is
available, it can be used by a visualization tool in rendering the map
 The second alternative is to utilize a spatial database (e.g., Oracle
Spatial or Arc SDE) and query the data based upon ad-hoc user
requests to convert the SVG data into a format that the application
can understand. This approach imposes a greater processing burden
on the script itself, and less on the visualization tool
SVG rendering process for GML data

GML and XML viewers (1)
 Visualizing geographical data is one of the primary goals of
Geographic Information Systems (GIS).
 Graphical optimization can be achieved with standard XML
viewers without any required changes to handle the newer
constructs released in GML 3. No adapters are needed. Rule
validation, for instance, can be enforced on GML constructs.
XML Viewers

Before a map can be made from a GML document, the geo-
spatial features must be extracted from the document and
transformed to a suitable graphical representation with styling
tools.
The process of interpreting the GML data in symbols, such as
line styles, area, and volume filling, is known as map styling.
X3D (XML 3D), for example, is one technology that can be
used for the presentation of GML data in a graphical format.
The graphical representation is then rendered into a viewable
image.

Making a vector map on the web with GML data involves
three steps:
 feature extraction
 map styling
 graphic rendering.
To view an SVG or X3D data file, it is necessary to have a
suitable graphical data viewer
While some of the viewers have been built into Internet
Explorer and Mozilla web browsers, several plug-ins are
currently being developed for other applications.

An effective visual tool for the representation of geographic
data should provide for the display and modification of maps .
Virtual Reality Modeling Language (VRML) describes 3D
objects and virtual environments, allowing the representation
of both static and dynamic data.
GeoVRML is an extension to VRML that provides geoscientists
with the ability to model 3-D geographic data that can be
distributed over the web

 Such data can be interactively visualized using a standard VRML97
browser configuration and these features of GeoVRML facilitate
widespread use of GML and geographic data by common users
 GeoVRML provides several capabilities, such as precision, scalability,
animation, and navigation, that have proven to be very important in
representing GML data in a 3D world.
 X3D, an open standards XML-enabled 3D file format, can also be a
useful language in assisting GML data visualization. If used in
conjunction with a rendering tool, it can translate GML directly into GIF,
JPEG, Postscript, and other formats.
Comparison between various visualization techniques

GML in a mobile devices (1)
GML documents usually require a significant amount of
memory due to their large size and a reasonable amount of
bandwidth for efficient transmittal
cGML, or compact GML, has been developed specifically for
mobile devices. The tags used in cGML are shorter in length
than those in standard GML, which reduces the bandwidth
and memory requirements by as much as 60%.
Other important standards and proprietary systems that have
been developed especially for use in mobile devices are C-
HTML, XHTML, Web clipping, Handheld Device Markup
language (HDML), Website Meta Language (WML), Mobile
Execution Environment (MExE), and Wireless Application
Protocol (WAP)

Another important issue currently limiting the use of GML on
mobile devices is the display of geographic information.
A GML document therefore has to be transformed into a
different format (e.g., SVG) to be displayed on the mobile
device.
An important consideration when designing wireless
geographic systems is that wireless users are usually doing
concurrent activities (e.g., driving a car, talking on the phone)
and hence have a limited attention span for visual information
display or for devices that require the use of hands such as a
trackball

 Due to these constraints, the information presented to a mobile user
has to be customized for the user’s task, as well as for the device itself.
 The heterogeneous nature of mobile devices, in conjunction with the
limitations imposed by the low-bandwidth and low-reliability wireless
networks, has proven to be a challenge in such situations.
 GML could play a major role in providing such services in constrained
environments.
Challenges to the deployment of GML to mobile devices

GML web services
 GIS web services have been grouped into three categories: data
services, processing services and registry or catalog services
 Web Map Service (WMS), Web Coverage Services (WCS), and Web
Feature Services (WFS) are examples of data services
GIS web services (data services)

Other web services
technologies
Other technologies that are used in the area of web services
are Simple Object Access Protocol (SOAP), Universal
Description Discovery and Integration (UDDI), and Web
Service Definition Language (WSDL)
SOAP is a XML-based protocol for information exchange
among programs in a heterogeneous Internet environment
using HTML and XML
UDDI is an XML-based registry that allows service providers or
businesses to advertise their web services on the Internet
WSDL is an optional XML-based language that is used by
service providers to list their services in the registry. GML
types (Application Schema) can be used in the types section of
a WSDL document to define geographic objects that are part
of the web service interface.

Summary
Summary of current GML technologies

Conclusion
 The efficient storage of GML documents is one of the problems that
must first be resolved. In addition, porting GML data into other formats
raises several questions.
 The choice of parsers depends on efficient memory usage as well as the
ability to retrieve random data, but XML parsers can be efficiently used
with GML.
 Query languages rely primarily on the efficiency of the data model and
its operators. The design of a GML query language has to account for
both spatial and non-spatial data.
 Integrating GML with mobile device applications is a relatively new area.
Adapting GML for low-bandwidth networks is a problem due to the
detailed nature of geographic data.
 Finally, web services and GML will play an important role in providing
GIS services to common users.

Advances in gml for geospatial applications slide

More Related Content

Similar to Advances in gml for geospatial applications slide (20)

More from Vittoriano Muttillo (6)

Recently uploaded (20)

Advances in gml for geospatial applications slide