2) presence of object operations and behavior functions
in the data model;
3) inherent links between object classes.
Completeness of OOA in GIS is described by
(Helokunas, 1994):
- object-based system. Information is presented as a
geographic object (GO), not as layers or tiles as in case
of traditional GISs;
- object-centered system. As types of objects we have
geometrical primitives and their topological varieties, as
well as other geographical entities and phenomena;
- OO graphic user interface;
- usage of OO programming principles;
- object-oriented data base.
The main notion of OO GIS is an object. The paradigm
of an object is described by the following notions:
encapsulation, identity, classification, relationship,
inheritance, aggregation, grouping, polymorphism.
OO GIS contains varied objects, including entity model -
a geographical object (GO). Similar objects can be
grouped into object classes. The structure of classes is
defined by all characteristics of grouped objects. The
object classes can present a strict hierarchy, i.e. an
object of a certain class is automatically accounted for in
the upper classes. Objects of the hierarchy, as a rule,
inherit characteristics of the upper classes. Non-
hierarchical classification of objects and classes of
objects is also possible. In such case encapsulation of
object-related methods from other classes of objects is
being performed, in accordance with user-designed links
between objects.
There exist two types of object description in OO DBMS:
- structural object orientation. In such case the data
model supports designing of complex structural object
classes;
- behavior object orientation. In such case the data
model supports designing of user-defined object classes.
MAJOR TYPES OF OO GIS OBJECTS
Object classes used in GIS, can be divided into two
types (ESRI, 1994):
- abstract classes. Such object classes do not contain
any concrete specimens of an object They provide
general methods and attributes which are inherited by
sub-classes;
- concrete classes. Such object classes contain full
specimens of objects.
This is the system view on the classification of object's
classes.
OO DBMS presents the following major groups of
objects:
1. Theme objects and GOs. In OO DBMS geographical
information is presented with the help of theme objects,
which represent information of the real world. Theme
objects can be simple and complex, i.e. consisting of
other theme objects. Theme objects can possess various
attributes presenting theme characteristics. The
characteristics are as follows: simple attributes, complex
attributes; reference attributes.
One of the sub-types of theme objects are GOs. Such
objects are characterized not only by these attributes, but
also by containing geometrical objects. l.e,, GOs are
characterized by theme content and spatial
characteristics. Such objects are in fact abstract object,
as they may contain no data of theme content, but
encapsulate these from other classes.
2. Geometrical objects. Geometrical objects are used to
describe geometrical essence of geographical objects. A
geometrical object consists of elementary geometrical
objects (nodes, label point, vertex, etc.). Such
geometrical objects present a class consisting of sub-
classes, such as a point, a chain and a polygon. There
may be topological relations between these classes: a
polygon can have a chain border, one chain can belong
to a multitude of polygons, etc. A geometrical object can
be simple, i.e. comprising only a point, a chain or a
polygon. Also a geometrical object can be complex, and
comprise a multitude of geometrical objects. Such
description makes it possible to describe geometry of
any GO of free form with the preserved in the
geometrical objects, as a rule, in full, that is why such
objects can be called concrete.
3. Graphic objects and cartographic objects. Graphic
objects are designed to represent theme objects. A
graphic object is characterized by such attributes as
color, type of line, font, etc. Also a graphic object can
import image objects from data tables, raster and vector
objects of other GISs, multi-media objects, etc. A graphic
object can be part of a theme object. A component of the
graphic object is a cartographic object. Cartographic
objects contain methods defining GOs characteristics. In
particular, cartographic methods encapsulate CG
methods of GOs. Cartographic objects are also related in
their major part to abstract objects.
4. Other objects. There exist object classes designed to
organize interactive access to geodata, objects to
organize user interface, objects to provide import of
components from other informational systems, etc.
GENERALIZATION KNOWLEDGES ON ENTITY AND
OOA
CG process in GIS media can be divided into two stages:
1. Acquiring of spatial multi-detail model of a
geographical object in digital form (OOSMDM).
2. Cartographic representation of OOSMDM in the
required detail scale (OOCR).
Main requirements for creation of CG can be divided into
four groups (Buttrnfield, 1991):
1) phenomenon-based factors which are caused by the
conceptual nature of the modeled entity (the essential of
the phenomenon analyzed; peculiarities of the area;
relations among entities, etc.);
2) purpose-oriented factors (user needs and purposes,
contents; scale; technology of map compilation, etc.);
3)graphic media and format factors (visualization
purpose; technology of map compilation; types of
cartographic objects; methods of cartographic
representation; rules for map design, etc.);
4) computational factors (efficiency of introduction of the
information system).
Factor 1 and 4 are of most importance at the first stage
of CG. Factors 2 and 3 are accounted for during the
second stage, with influence of factors 1 and 4 also felt
here. Here factor 1 has the highest priority during
realization of interactive processes, during conflicts at
the 2nd stage of CG.
446
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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