Database Reference
In-Depth Information
an open interchange format for geospatial data as well as a modeling language
for geographic information. In GML, real-world objects are called
features
and
have a spatial component (geometry) and nonspatial properties. The most re-
cent GML version, 3.1, is being standardized as ISO 19136. While earlier GML
versions used Document Type Definitions (DTDs), the later versions are based
on XML-Schema. GML version 3.x also includes support for two-dimensional
complex geometries and topology, three-dimensional geometries, spatial and
temporal reference systems, and visualization.
Because GML is based on XML-Schema, it allows users to create their own
application schemas by making use of GML (core) schema components such
as geometry, topology, and time, and follow the simple, structured rules of
the GML encoding specification. GML application schemas are very flexible in
that they allow users to tailor and extend predefined GML data types (mostly
geometrical and topological) to specific needs in an application domain. GML
also serves as data exchange format for geospatial data, an aspect that is
particularly important to achieve a high degree of interoperability among
geospatial data repositories through geospatial Web services.
10.2.3.2
Metadata Standards
There are many metadata frameworks for spatial data and applications that
have geospatial components. Most of these frameworks and initiatives are
driven by individual science communities. Metadata frameworks can be found
at all data management levels, ranging from metadata associated with tra-
ditional database and GIS schemas to approaches where metadata is simply
encoded as part of a file format containing the (geo)spatial data.
The most widely used geospatial metadata standard in GIS products is the
standard developed and maintained by the Federal Geographic Data Com-
mittee (FGDC).
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The FGDC developed the Content Standard for Digital
Geospatial Metadata (CSDGM) in 1994, which is often simply referred to as
the FGDC metadata standard.
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This standard has components to describe
the availability, fitness for use, and access and transfer information of geospa-
tial datasets. According to the CSDGM version 2 published in 1998, Section 1
has entries to describe the geographical area a geospatial dataset covers; Sec-
tion 3 describes the spatial data model that is used to encode the spatial data
(vector/raster) or other possible methods for indirect georeferencing; and Sec-
tion 4 describes the information about the spatial reference system.
In addition to the CSDGM, several other metadata standards have been
developed over the past few years for different application domains in the
geosciences and environmental sciences. For example, the Ecological Metadata
Language (EML) developed by the National Center for Ecological Analysis
and Synthesis (NCEAS) has been widely adopted in the ecological data man-
agement community.
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EML has been designed as a collection of modules
and has an extensible architecture. For the data module, EML has detailed
structures to describe tabular, raster, and vector data. In EML, the metadata
