Operations of splitting features, opposite to operations of combining, can also be applied only to line, area and image features. The result of this operation will be splitting a feature to several components each of which becomes an independent feature of the same layer and type. After selecting split feature for editing and selecting the menu item Split a nested menu will be displayed on the screen. The menu will contain operation variants: splitting By Feature, By Polygon or By Polyline. Thus, in the first case the user will have to specify the splitting feature and in two other cases – enter the splitting polygon or splitting polyline. A line, area or image feature can be selected as a splitting feature. Spatial types of split and splitting features or figures should not necessarily coincide, for example, one of them can be an area feature, another – a line one.
Below four different cases of performing splitting operations are described. For easy presentation it is assumed that splitting is performed using the feature. Actually, when performing this operation there is no essential difference between using a polyline or a line feature, and also between using a polygon or an area (or image) feature.
Splitting a line feature using a line splitting feature (or polyline). In this case the system determines all the points of intersection of two features, and every fragment of the split line feature between two neighboring points of intersection becomes a separate line feature.
Splitting a line feature using area or image splitting feature (or polygon). All the points of split feature intersection with all boundaries of the splitting one are determined (that is, intersection points with outer boundary and with inner ones, if any), and every fragment of the line feature between two neighboring points of intersection becomes a separate line feature.
Splitting an area or image feature using line splitting feature (or polyline). The system splits the internal part of the split feature to non-intersecting polygons formed by boundaries of the split feature and polyline of the splitting feature. Every polygon becomes a separate area (or image) feature.
Splitting an area or image feature using area or image splitting feature (or polygon). In this case when building non-intersecting polygons all the boundaries of both features are used. Every built polygon becomes a separate area (or image) feature.
At figure 18-4 examples of performing operations of splitting line and area features using a polyline are given.
a) Splitting a line feature
b) Splitting an area feature
Figure 18-4. Splitting of features
There is one limitation when performing splitting of area or image feature by a polyline (or line feature). It is required that the splitting polyline formed at least one completed contour with one of the boundaries of the split feature. In other words, the feature should be split from boundary to boundary. It is impossible to cut out an area inside the feature by the operation of splitting. Similar action can be performed by another operation which is described in next subsection. Besides, it should be noted that polyline fragments ending inside the split feature are ignored when performing the splitting operation.
At figure 18-5 examples of splits satisfying and not satisfying this requirement are shown.
a) admissible splits b) inadmissible splits
Figure 18-5. Admissible and inadmissible splits
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