Operations of combining features can be applied to line, area and image features. A line feature can be combined with another line feature, area feature – with another area feature or arbitrary polygon, image feature – with other image feature or polygon.
To perform the operation of combining it is necessary to switch over to the mode Edit and select a line, area or image feature for editing. The selected feature will be called the first feature below. Then it is required to right-click to display the context menu of the displaying pane and select the menu item Combine.
If the line feature was selected, only one item: With Feature will be enabled in the appeared nested menu. After that the cursor takes the form of 
, and the user should click the second feature with which the selected feature should be merged. The second feature should also be a line one.
The system checks the possibility of performing the operation and displays a dialog box “Combining features” (figure 18-1).
Figure 18-1. Dialog box “Combining features”
The dialog box shows the status of the operation: “The operation is possible” or “Cannot perform the operation”. In the second case description of the reason for which combining is impossible is also given.
Actually combining two line features is always possible not depending on whether these features have common points and belong to one and the same layer and type or not. Feature combining means creation of a new line feature which contains all vertices and edges of the two initial features plus, if necessary, one additional edge connecting the pair of the closest end vertices of the initial features. If one of the end vertices of the first feature coincides with the end vertex of the second one, the connecting edge is not built. The new feature will belong to the same layer and type that the first combined feature. Examples of combining line features are shown at figure 18-2.
Figure 18-2. Combining line features
If an area or image feature was selected as the first feature to be combined, two menu items: With Feature and With Polygon will be enabled in nested menu of combining operations.
When selecting the operations of combining With Feature the user should indicate the second area feature for combining (or image feature accordingly). The condition of admissibility of this operation is presence of the common region of two features being combined. In other words, features should have a common internal area and it is not enough that their boundaries touch each other. The new area (or image) feature built as a result of the combining operation will consist of all points belonging at least to one of two features that are being combined.
If the operation of combining With Polygon is selected, the system switches to the mode analogous to the mode of adding an area feature. The user should build a polygon clicking its vertices and complete the operation by a double click. Right after completing the operation of adding a polygon, the system combines the feature which was selected earlier with this polygon. The new area (or image) feature will consist of all points belonging either to the selected feature or the polygon. The conditions for admissibility of the operation are firstly correctness of the built polygon and secondly the presence of a common area between the polygon and the feature. At figure 18-3 examples of combining area features are given.
Figure 18-3. Combining of area features
There is additional condition for correctness of combining an image feature with another image feature or with a polygon – the boundary of a new feature should not be outside the rectangular image feature on the basis of which the feature is created. If this limitation is broken the part of the feature outside the image is displayed by red color and the system displays a message about incorrectness of the feature.
The checkbox Remove source features in the dialog box “Combining features” indicates if it is necessary to remove the source features after performing the combining operation or these features should be kept. If source features are removed, the result of combining will be one (combined) feature. If they are not removed – the result of combining two features is appearance of three features (both source features and a combined one), and when a feature is combined with a polygon there will be two features (the source and the new one).
The checkbox Combine links with records determines if the new feature should inherit all the links with table records which two initial features had (see chapter 32 “Links between maps and tables”). If the box is cleared the new feature will not have links with the records.
 |  |
