1Generalise

Simplification retries generate intersecting edges.

The simplification retry operations – e.g. “process_douglas_peucker_simplification_retries” – applies a number of strategies in order to try to achieve good results:

1. Retry the original simplification in case modifications to nearby features allow it to succeed.

2. Simplify the edge that failed, and then find all other edges that intersect the result, then validate the result. This can achieve a better result when two edges are approximately parallel.

3. Split the edge into half and simplify each half independently. If a half-edge intersects no other nodes or edges and the result does not change topological relationships with any nodes or isolated edges, the result is used. If simplifying the half-edge would break topology, the algorithm is applied recursively to bisect the half-edge.

Intersecting edges were created because strategy #2, the intersecting edge simplifier, relies on the topology engine to determine whether the result is valid. Crossing edges are valid unless an “edges split edges” rule is defined. The reported session used only a “share nodes” rule, so crossing edges are considered valid.

An edges split edges rule is recommended for most uses of 1Generalise. However, the simplification failures processor has been changed to exclude the intersecting edge simplifier if there is no edges split edges rule. The recursive simplifier will generate a valid result.

Buildings with spikes.

Output from the buildings subflow resulted in 'triangular' buildings or obvious spikes. A number of scenarios have been resolved:

• Where one source building touches another at an angle

• Where one generalised building touches another at an angle

• Where a source building is not square; i.e. has angular sides within the building outline

• Some other rare cases