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4 Geometry standards
Although a less abstract way of describing the structures may be desirable in some
cases, the terminology ofthe conceptual schema language UML will be used in order
to be consistent with the UML-models of the ISO 19100 standards. In most cases,
the models consist of a hierarchy of classes. A high level class describes a graphical
element in a very general sense. For example, a GM _ Object can be any geometry object of the dataset like a point, a curve, a surface, or asolid. A low level class describes a specific geometry like an arc or a spline. The whole model contains an abstract view of the real world and it's only purpose is to address the classes that could
possibly be present in a dataset. If areal dataset is created, it uses the classes supplied by the standard. The elements, which form the real dataset, are the instances of
the abstract classes. F or example, the instances of the class GM _ Surface may be the
property parcels 1, 2, ... , n in the dataset.
4.2 Positions
The ISO 19107, the ISO 19123, and possibly others in the future have an integrated concept of dealing with positions in the n-dimensional space. The ISO 191251 supports only two dimensions and uses a different method.
The concept based on the ISO 19107 will be described first. An n-dimensional set
of coordinates is called a DirectPosition like (x, y, z). Obviously a point object (class
GM_Point) has one set of coordinates and thus one DirectPosition. Curves, surfaces,
and so lids have more than one set of coordinates and consequently more than one
DirectPosition.
The name DirectPosition is derived from the direct identification of the position.
Alternatively a position can be defined by the position of another "point object"
(class GM_Point). This is called an indirect identification of a position. An example
would be a parcel boundary which is mostly a line string. The positions of the line
string are the corner stones of the parcel which are point objects themselves. Thus,
the coordinates ofthe line string are the coordinates ofthe point objects.
In order to better handle numerous DirectPositions in a dataset, the positions of a
curve, a surface, or a solid are collected in an ordered manner that is called a point
array (class GM_PointArray). In order to avoid redundancy and unnecessary use of
storage volumes, the point array only stores pointers and no coordinates. The pointers may point to DirectPositions or point objects.
The pointers within a point array (dass GM_PointArray) are alternatively called
controlPoints.
The dass storing the flag with direct or indirect reference is called GM_Position.
The ISO 19123 concept of a coverage relates a set of attributes to a position
within a bounded space. From this perspective, the basic element of a coverage is a
pair of data collections; one data collection denotes a position like (x, y) and the
other data collection denotes the attributes at this position like temperature and soil
type. The ISO 19123 refers to the ISO 19107 for the details of positions.
4 Geometry standards
Although a less abstract way of describing the structures may be desirable in some
cases, the terminology ofthe conceptual schema language UML will be used in order
to be consistent with the UML-models of the ISO 19100 standards. In most cases,
the models consist of a hierarchy of classes. A high level class describes a graphical
element in a very general sense. For example, a GM _ Object can be any geometry object of the dataset like a point, a curve, a surface, or asolid. A low level class describes a specific geometry like an arc or a spline. The whole model contains an abstract view of the real world and it's only purpose is to address the classes that could
possibly be present in a dataset. If areal dataset is created, it uses the classes supplied by the standard. The elements, which form the real dataset, are the instances of
the abstract classes. F or example, the instances of the class GM _ Surface may be the
property parcels 1, 2, ... , n in the dataset.
4.2 Positions
The ISO 19107, the ISO 19123, and possibly others in the future have an integrated concept of dealing with positions in the n-dimensional space. The ISO 191251 supports only two dimensions and uses a different method.
The concept based on the ISO 19107 will be described first. An n-dimensional set
of coordinates is called a DirectPosition like (x, y, z). Obviously a point object (class
GM_Point) has one set of coordinates and thus one DirectPosition. Curves, surfaces,
and so lids have more than one set of coordinates and consequently more than one
DirectPosition.
The name DirectPosition is derived from the direct identification of the position.
Alternatively a position can be defined by the position of another "point object"
(class GM_Point). This is called an indirect identification of a position. An example
would be a parcel boundary which is mostly a line string. The positions of the line
string are the corner stones of the parcel which are point objects themselves. Thus,
the coordinates ofthe line string are the coordinates ofthe point objects.
In order to better handle numerous DirectPositions in a dataset, the positions of a
curve, a surface, or a solid are collected in an ordered manner that is called a point
array (class GM_PointArray). In order to avoid redundancy and unnecessary use of
storage volumes, the point array only stores pointers and no coordinates. The pointers may point to DirectPositions or point objects.
The pointers within a point array (dass GM_PointArray) are alternatively called
controlPoints.
The dass storing the flag with direct or indirect reference is called GM_Position.
The ISO 19123 concept of a coverage relates a set of attributes to a position
within a bounded space. From this perspective, the basic element of a coverage is a
pair of data collections; one data collection denotes a position like (x, y) and the
other data collection denotes the attributes at this position like temperature and soil
type. The ISO 19123 refers to the ISO 19107 for the details of positions.