Make multiple passes over road merging #1708
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The previous PR #1703, only made one pass over the road network. This meant that there were still many opportunities for further merging which were left untaken. Instead, we can loop over the merge step until all possible merges have been taken. Since each pass considers all mergeable points, but can only merge two lines in any given step, it should be possible to merge all points in `O(log N)` steps. This patch also re-orders the steps for merging, simplification and grouping. The merging and grouping operations used to happen together, and we had two of them sandwiching a simplification step. However, this caused problems when the simplification step re-introduced junctions that had just been merged. This was happening because `simplify` was being called on the MultiLineString, which tries to honour the locations of the intersections of its component lines. Instead, the `merge_line_features` post-processor can internally simplify each individual LineString between merging and grouping those LineStrings into non-overlapping MultiLineStrings.
…ng the simplification and junction merging steps.
rmarianski
approved these changes
Nov 20, 2018
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The previous PR #1703, only made one pass over the road network. This meant that there were still many opportunities for further merging which were left untaken. Instead, we can loop over the merge step until all possible merges have been taken. Since each pass considers all mergeable points, but can only merge two lines in any given step, it should be possible to merge all points in
O(log N)steps.This patch also re-orders the steps for merging, simplification and grouping. The merging and grouping operations used to happen together, and we had two of them sandwiching a simplification step. However, this caused problems when the simplification step re-introduced junctions that had just been merged. This was happening because
simplifywas being called on the MultiLineString, which tries to honour the locations of the intersections of its component lines. Instead, themerge_line_featurespost-processor can internally simplify each individual LineString between merging and grouping those LineStrings into non-overlapping MultiLineStrings.Connects to #1227.