The operation of simplification of an area or line feature consists in the elimination of those vertices which have insignificant influence on the feature's overall shape.
When performing this operation, all the vertices of a line feature are examined consequently, or all the vertices of each boundary of an area feature. For every vertex the distance from it to the line connecting the previous vertex to the next one is calculated. If this distance is less then a specified simplification threshold, then the given vertex is deleted.
Performing simplification can be desirable if a line feature or an area feature's boundary is defined with redundant details and have many redundant vertices. Reducing the number of vertices allows to decrease size of the stored data and to accelerate the feature's displaying.
To execute simplification one should select one or more features which belong to the area and/or line spatial categories. On performing the context menu command Simplify a dialog box is opened which is shown on the figure 18-15.
Figure 18-15.Dialog box “Feature simplification”
The value of the simplification threshold is specified in map units. The number of vertices before simplification shown in the pane relates to all the selected features in total. After the button Calculate is pushed, it is shown in the same pane how many would all the selected features have after the simplification with the given threshold. The greater the value of threshold is the fewer vertices are to be retained after simplification, minimal values being 2 vertices for a line feature and 3 vertices for each boundary of an area feature.
The possibility to simplify many selected features at once allows using this operation for all features of a certain type or, for instance, for features of a small size only. A special theme can be built if necessary, including only those features which are to be simplified.
In some cases, for area features of complicated shape, simplification can cause violation of simplicity of a boundary, i.e. appearing of the boundary self-crossings or its crossings with other boundaries of the same feature. A message about simplicity violation will be shown after pushing the Calculate button. In such cases it is impossible to perform simplification, but one can try to specify a different (greater or less) value for the threshold or to edit the problem feature manually.
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