4.6 Geography Markup Language (GML) (ISO 19136)
179
4.6.1 GML schemas
Feature and feature collection
A feature is an abstraction of areal world phenomenon. It is a geographie feature
if it is assoeiated with a loeation relative to the earth. The state of a feature is defined
by a set of properties, where eaeh property ean be thought of as a {name, type,
value} tripie.
A feature eolleetion is a eolleetion of features that ean itself be regarded as a feature. As a eonsequenee a feature eolleetion has a feature type and thus may have distinet properties of its own, in addition to the features it eontains.
Geographie features in GML include eoverages and observation as subtypes.
Geometry
The geometry of a geographie feature deseribes its loeation, shape or extent. The
geometry model of GML distinguishes geometrie primitives, aggregates, and eomplexes.
The geometrie primitives are the basic elements that are used to form the geometry of a geographie dataset. Primitives are open, that is, a eurve does not eontain its
end points, a surfaee does not eontain its boundary eurves, and asolid does not eontain its bounding surfaee.
The geometrie aggregates are arbitrary aggregations of geometry elements. They
are not assumed to have any additional internal strueture and are used to "eolleet"
pieces of geometry of a speeified type.
Geometrie eomplexes are closed eolleetions of geometrie primitives. That means
that they eontain their boundaries.
Figure 4.81 shows the hierarehy of the GML geometry types.
Coordinate Reference System
The Coordinate Referenee System (CRS) provides the meaning for loeation coordinates. A CRS may be assoeiated with any geometry of GML.
The CRS schema of GML deviates slightly from the ISO 19111 (Spatial refereneing by coordinates) and from the Topie 2 (Spatial Refereneing by Coordinates) ofthe
OGC's Abstract Speeifieation. Appropriate change proposals will be submitted to
the ISO/TC21I (Porteie 2003).
Topo/ogy
Topology deseribes the geometrie properties of objeets that are invariant under
eontinuous deformation. For example a square is topologieally equivalent to a reetangle and a trapezoid. In geographie modelling, the foremost use of topology is in
accelerating eomputational geometry.
179
4.6.1 GML schemas
Feature and feature collection
A feature is an abstraction of areal world phenomenon. It is a geographie feature
if it is assoeiated with a loeation relative to the earth. The state of a feature is defined
by a set of properties, where eaeh property ean be thought of as a {name, type,
value} tripie.
A feature eolleetion is a eolleetion of features that ean itself be regarded as a feature. As a eonsequenee a feature eolleetion has a feature type and thus may have distinet properties of its own, in addition to the features it eontains.
Geographie features in GML include eoverages and observation as subtypes.
Geometry
The geometry of a geographie feature deseribes its loeation, shape or extent. The
geometry model of GML distinguishes geometrie primitives, aggregates, and eomplexes.
The geometrie primitives are the basic elements that are used to form the geometry of a geographie dataset. Primitives are open, that is, a eurve does not eontain its
end points, a surfaee does not eontain its boundary eurves, and asolid does not eontain its bounding surfaee.
The geometrie aggregates are arbitrary aggregations of geometry elements. They
are not assumed to have any additional internal strueture and are used to "eolleet"
pieces of geometry of a speeified type.
Geometrie eomplexes are closed eolleetions of geometrie primitives. That means
that they eontain their boundaries.
Figure 4.81 shows the hierarehy of the GML geometry types.
Coordinate Reference System
The Coordinate Referenee System (CRS) provides the meaning for loeation coordinates. A CRS may be assoeiated with any geometry of GML.
The CRS schema of GML deviates slightly from the ISO 19111 (Spatial refereneing by coordinates) and from the Topie 2 (Spatial Refereneing by Coordinates) ofthe
OGC's Abstract Speeifieation. Appropriate change proposals will be submitted to
the ISO/TC21I (Porteie 2003).
Topo/ogy
Topology deseribes the geometrie properties of objeets that are invariant under
eontinuous deformation. For example a square is topologieally equivalent to a reetangle and a trapezoid. In geographie modelling, the foremost use of topology is in
accelerating eomputational geometry.