International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
regularity of soil distribution, the characteristics, representation
and application of soil maps. Section 2 designates specific
constraints in the phases of the generalization process. To
derive appropriate and correct maps, all kinds of generalization
constraints should be integrated efficiently in the process of
generalization. Therefore, in Section 3, we investigate the
integration of different kinds of generalization constraints and
their relationships. Then, two special cases in soil map
generalization are illustrated in Section 4, and a workflow
shows the whole process of soil map generalization with the
application of generalization constraints.
2. CONSTRAINTS TO SOIL MAP GENERALIZATION
Constraint-based generalization of geographic map has been
studied by many researchers (Beard 1991; Ruas 1998; Weibel
and Dutton 1998; Peter and Weibel 1999; Gadland 2003). The
concept of constraints used in the paper owes to previous
research conducted by Peter and Weibel (1999) and Ruas
(1998). Constraints designate the specification of final map or
database based on properties of a geographic phenomena or
feature. And constraints are used to detect conflicts, to control
the sequence or strategy of generalization, to compare
accomplished solution and evaluate the generalized result.
Generally, constraints are defined as various specifications
which control the specific aspects of an object, a group of
objects or a whole map insthe process of map generalization.
Take the research in Peter and Weibel (1999) as an example to
describe the constraints of map generalization. In their research,
the generalization constraints are classified into: graphic
constraints, topologic constraints, structural constraints and
Gestalt constraints.
- Graphical constraints define graphic perceptibility
thresholds of map objects based on human limits of
perception, such as minimal area, minimal distance between
two polygons.
- Topological constraints deal with basic topological
relationships such as connectivity, adjacency and
containment, which should be maintained when
generalizing data.
- Structural constraints include spatial structural constraints
and semantic structural constraints. Spatial structural
constraints mainly are responsible for the preservation of
typical shapes of individual map objects or patterns and
alignments of a group of map objects. Semantic structural
constraints deal with the preservation of the logical context
of patches.
- Gestalt constraints are related to aesthetic aspects for the
preservation of the patch characteristics as well as the
retention of the overall visual balance when multiple
patches or the whole dataset is considered.
Based on the classification system of constraint, Peter and
Weibel (1999) investigate the constraints to categorical data
generalization. Table 1 shows some titles of these constraints
relate to soil map generalization.
Constraints Constraints Constraints Related
Related to Patches Related to to Partitions or
Categories Groups of Patches
Minimum size Size ratio Neighborhood
(graphical) (structural) relations
Minimum distance | Shape/Angularity (topological)
(graphical) (structural) Spatial context
Separation Alignment / (structural)
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(topological) Pattern (Gestalt) Alignment/Pattern
Self-intersection Aggregability (Gestalt)
(topological) (structural) Visual balance
Amalgamation (Gestalt)
(structural)
Shape/Angularity
(structural)
Table 1 Constraints to Categorical Data (after Peter and Weibel
(1999))
Soil data is one of the important types of categorical data. These
constraints should therefore be taken into account in soil map
generalization (it is called generic constraint in the paper). In
our paper, we do not discuss these constraints in detail any
more, but investigate the specific constraints of soil map
generalization (it is also called thematic constraints) based on
the regularity of soil distribution, the characteristics and
application of soil maps. The following section discusses the
thematic constraints based on the four main phrases in the
process of soil map generalization: preparation and preprocess,
database generalization, graphic generalization and evaluation
of generalization results.
(1) Preparation and preprocess of soil map generalization
Obviously, the scale of the objective map should depend
on the scale of the original soil map or the resolution of the
present soil database. Furthermore, some more factors
should be taken into account to confirm the scale such as
the regularity of soil distribution in a specific geographic
region or area, the purpose of the objective map.
- Constraint 1: the scale of the objective map is
appropriate to represent the dominating regularity of
soil distribution in a specific geographic region. In
general, there are three kinds of main regularity of
soil distribution: the regularity of horizontal
distribution of soils, the regularity of vertical
distribution of soils, the regularity of zonal
distribution of soils. The first two regularities that are
influenced by hydrothermal conditions and large
geomorphology represent the geographic distribution
of soil in large zones. The third regularity that is
formed based on small or middle geomorphology,
thermal condition, parent materials and human
activities represents the geographic distribution of soil
in small zones. Thus, the first two regularities are
appropriate to be represented in middle-scale or
small-scale maps (generally, the scale should be
smaller than 1:300000), whereas the third one could
be represented effectively in large-scale or middle-
scale map (generally, the scale should be more than
1:100000).
- A Constraint 2: The scale of the objective map should
meet the requirements of the purpose of the map. If
the objective map is used as the reference for the
planning of anthropogenic soil and the planning of
soil amelioration, the scale should be between
1:10000 and 1:50000. If the objective map is used in
the statistics of soil resources, 1:500000 or smaller is
appropriate for the whole nation or a province, and
the scales between 17100000 and 1717300000 are
better for a county.
- A Constraint 3: the level of soil categories described in
the objective soil map should be in harmony with the
scale. Based on CST, the large-scale soil maps
describe the soil genus level, and the middle-scale and
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