Verifying the unlinks
An unlink can be any feature that identifies an intersection between two axial lines, that are to be disconnected. It can be a point near the intersection, or a line or polygon intersecting the two relevant axial lines. Below three different ways to define an unlink:
The ids of the unlinked lines are stored in attributes called “line1” and “line2”. The first time unlinks verification runs, it checks if the “line1” and “line2” attributes are present. If these attributes are missing the user is asked to run “Update IDs” to assign the id of the lines at the intersection identified by each unlink.
In the case of points, a buffer is used to identify the intersection (Fig. 11 a). The unlink threshold distance (default = 5m) can be changed in the “Layer verification settings” window (Fig. 9). To have greater control over the intersection of each individual unlink, the user can opt for polygon or line unlinks.
The verification checks for topological and line id problems in relation to the axial lines layer:
a) Multiple lines – unlinks near or intersecting more than two axial lines;
b) Single line – unlinks near or intersecting only one axial line;
c) No lines – unlinks not intersecting any lines, or further than the unlinks threshold (Fig. 9);
d) Same id – unlinks with duplicate line id on both attributes;
e) Unmatched id – unlinks with no line id attribute (NULL), or the id not matching the currently intersected axial lines.
Line id problems can occur when the unlinks or the lines are edited and/or moved and the intersection ids do not match the “line1” and “line2” attributes. If there are line id problems reported, the user should first confirm if the line id is wrong or if the unlink is in the wrong place. In this case it should be relocated to the correct intersection. Only then should the user run the “Update IDs” tool to update the line id attributes.
The verification of unlinks requires that an axial map layer is selected in the “Map” tab, and both layers must have the same data format and be in the same database (if using Spatialite or PostGIS). If there are problems with the unlinks, a report is created with a list of unlinks and line ids. The unlinks’ problem selection, filtering and editing process is identical to that of axial maps, described in the previous section.
