Adding area features is in many ways similar to adding line features which is described above. Additional complication is that apart from outer boundary an area feature can have inner boundaries specifying internal regions which do no belong to the feature. For details on boundaries and internal regions of a feature see chapter 9 “Map structure”.
Every boundary is specified by a polyline. The system provides automatic polyline completion when adding an area feature.
Let us remind you about some requirements of correctness which boundaries of an area feature should satisfy. Each boundary should be simple, that is, it shouldn't intersect itself after its completion. Besides, boundaries should not intersect each other and every inner boundary should necessarily lie within the outer boundary and it cannot enclose other inner boundaries. Every boundary should contain not less than three points.
At figure 17-9 a dialog box of adding an area feature is shown.
Figure 17-9. Adding an area feature
This dialog box differs from the analogous box for line features (figure 17-6) in the presence of an additional pane Boundary which contains a list of feature boundaries and two buttons Add and Remove to the right of this pane. It should be noted that there are two buttons Remove and two buttons Add in the dialog box. The upper pair of buttons belongs to the list of boundaries, the lower one – to the list of vertices of the selected boundary.
Initially only the line “Outer” is present in the pane Boundary. If it is necessary to specify inner boundaries of features the button Add (the upper one) should be pushed. A line “Inner 1” will appear in the pane, after pushing the button again – “Inner 2” will be displayed and so on. The button Remove (the upper one) causes removal of the selected inner boundary. Outer boundary cannot be removed.
Adding boundary vertices is performed the same as adding polyline vertices for a line feature.
There is no need to repeat the coordinates of the first vertex to complete the boundary. The system always connects the last boundary vertex with the first one.
The button Reverse causes changing of the order of vertices of the selected boundary to the reverse order (the first vertex becomes the last and vice versa). The order of vertices can be important if the line type used for displaying the boundaries of the present feature is not symmetrical, that is, the right line side differs from the left one.
The button Circle allows the user to automatically build coordinates of a set of points which roughly form the circle boundary, and add them to the list of vertices. A dialog box “Creating circle” shown at figure 17-7 above is opened. Work with this dialog box does not differ from analogous actions for a line feature. After creating the circle the user can continue to add vertices of an area feature in the usual way.
Adding or editing an area features is completed after pushing the button OK. The system performs the check of feature boundaries correctness and in case of inadmissible boundaries displays a message box with description of the error. After pushing the button OK the system resumes feature editing.
Pushing the button Cancel cancels all changes made and closes the editing window.
 |  |
