A futher important property of object oriented system refers the
extensibility: an existing system can be extended without intro-
ducing changes to it. The behavior of an object may be extended
by simply including additional methods. Besides this pre-
defined attributes and methods of an object may also be re-uesd
and inherited into a new specilized object.
à Dui tating F | Data S ith Obiect
Oriented Model
The analogy of classes in object oriented model and in FDS is
quite evident. Point features from the point class, line features
from the line class and area features from the area class corre-
spond to objects or instances. Based on the Fig.1 three levels
may be dealt with as follows:
At central level (in the sense of user) terrain features are refered,
thus the feature level. Thematic description starts from the
feature level upwards and in association with calssifications,
whereas geometric description (including shap and planar topo-
logy) starts from the feature level downwards.
It should be noted that Fig.1 does not contain classes for com-
posite features. A composite feature has attributes that are them-
selve feature identifiers from other classes.
An example of composite feature is given as follows. Defining
class house which is composed of some area features, the class
meadow which is composed of some other area features, and the
class footpath composed of some line features. Then the class
garden - its every feature with attributes refering an area feature
from the class house, an area feature from the class meadow and
a line feature from the class footpath - is a class composed of
composite features. In this case, house feature, meadow feature
and footpath feature are parts of garden feature (aggregation).
Composite features and its classes are important for describing
geographical information. They are not considered in the basic
figure of FDS explicitly because they vary from one application
to another. Based on the extensibility of object oriented model
insertion of new classes to an existing class hierachy or network
does not effect changes of the exsiting system and thus may be
carried out in an any time later.
Special emphasis will be made for the geometric description of
terrain features. As shown in Fig. 1 links are drawn between node
and point feature, between arc and area or line featurre.
The relationship between point and node class is actually a bila-
teral class-composition hierachy: every point feature possesses
an attribut its value reprensents an identifier to a node from the
node class, and vice vesa. Note that a node can but not necessa-
ryly possess the meaningful attribut value as identifier to a point
feature.
Coordinates are objects of class composed of pairs of real
numbers. The relationship between node and coordinate pair
leads to another class-composition hierachy, not necessaryly bi-
lateral.
But more important are here functions or methods associated
with objects and classes: by defining the point feature class,
functions may be defined simultaneously that must be carried
out in association with messages passing to, for example in the
process of creation, updating and deleting of a point feature.
153
For example, two functions:
Fp1=(send a message to node class for starting the function Fn,
the message contains the identifier and the coordinate pair of new
created point feature);
Fp2=(write the value of the passed identifier to the attribute of
actual point feature for refering a node object)
are associated with point feature class. Another function:
Fn=(find the existing node or create a new node with the given
coordinate pair passed by and send a message to the point feature
with passed identifier and start the function Fp2, the message
contain the found or new created node identifier)
is associated with the node class.
The creation function of a new point feature associated with the
point feature class starts the function Fpl immediately after a
new point feature is created. By tracing the way of message
passing the new point feature get the reference to its node.
The relationship between area feature and arc is also a class-com-
position hierachy and bilateral aggregations: every area feature
has a set of attributes their values refer arc objects from arc class
for representing the contours of an area. Conversely every arc
has two attributes their values refer two areas features (left and
right area) from area class. Methods associated with area feature
may be more complicated. Because a series of geometric-topo-
logical conditions must be satisfied for arcs such that they con-
struct a area feature. It is not possible to specify them without
more detailed specifications.
Other relationships between the feature level and the geometric
level can be refined at the same way which will be not treated
here in more detail.
The consistency of planar topological structure within geome-
tric data and hence correct inter-relationships between the terrain
features can only be guaranted if manipulations of geometric-to-
pological level are involved by messages exchanged between
feature level and primitive level. Of course powerful functions
for handling geometric data and topology are required.
3. Concluding Remarks
Conclusively said FDS can serve as object oriented data model
of geographical information in a meta-level, which integrates
spatial (geometric) data with its planar toplogical structure, non-
spatial data and inter-relationships between the spatial and non-
spatial data in a whole system.
To implement a new-generation GIS with object oriented model
FDS can be well-used but it must be refined in two aspects: on
the one side, a more detailed meta-structure is required for defi-
ning thematic, geometric and topological data that will be eva-
luated by applications (state model); and on the other side, the
behaviors of object features and classes (functions or methods)
must be specified for implementation (behavior model).
Besides this, object oriented programming languages such as
C++, Object LISP etc play an important role because they
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