Verifying the axial map
The verification of the axial map checks for possible geometry and topology problems with the model. These may include:
a) Small lines – lines with a length below the minimum threshold (default: > 1 m)
b) Coinciding points – lines with both end points coinciding
c) Polylines – lines made up of more than one segment
d) Duplicate geometry – two lines with identical geometry
e) Short lines (undershoots) – lines whose endpoints are close than the minimum threshold to another line without intersecting (default: > 1 m)
f) Orphans – lines that are not intersecting any other lines
g) Unlinked orphans - orphans that result from the presence of an unlink
h) Islands – groups of intersecting lines that are disconnected from the main map
Clicking the “Verify” button in the “Verify layer” tab starts the verification of the selected layer, and when the process concludes, any problems found will be listed in a report. The problems listed in the report can be filtered by type of problem in the “All problems” drop-down menu. The number in brackets indicates the total number of occurrences of a problem.
When verifying the axial map, if an unlinks layer is selected, the unlinks are taken into account and can reveal topological problems due to unlink disconnections. This unlinks layer must also be verified and fixed for the axial map verification results to be completely correct. (See Verifying the unlinks).
The “Settings” button gives access to the distance thresholds used in the verification process (Fig. 9). The “Axial crossing threshold” value is used to identify short lines (default = 1m). The “Minimum axial length” value is used to identify small lines (default = 1m). These values can be changed to increase or reduce the verification tolerance to suit the level of accuracy of the model.
Selecting an entry in the problems report automatically selects and zooms into the corresponding line or group of lines. Once located, the user can use the editing mode and modify or remove the line(s) (Fig. 10). The lines are identified by their unique ID, which is the primary key of a geodatabase layer, an existing layer attribute called “ref”, “id”, “pk”, “pkid”, “sid”, “uid” or “fid”, or the internal QGIS feature id when none of the previous is available or valid.
Notes: The verification process is faster if the data is in a geodatabase because it makes use of optimised SQL queries. This can be important when working with very large maps. The verification process is meant for axial maps only. It does not identify any possible problems in segment maps or road centre line maps, for example