29.03.2023 | Frederik Ramm
This weekend we’ll implement a change that affects the handling of boundary-straddling multipolygons in our OSM extracts. (See this 2017 blog article for some background.)
We’ll stop completing cross-border multipolygons except landuse polygons and a hand-picked list of natural polygons (e.g. water, grassland, wetland).
This has become necessary because of the propensity of OSM mappers to create huge multipolygons like “the Iberian penisula” or “the Alps”, artifacts that not only unnecessarily increase the data volume of any given PBF but also have unexpected consequences – for example, for a while anyone processing the rivers of the Switzerland extract would find a stretch of the River Danube in Vienna, because it happened to be part of the outline of the “Alps” multipolygon.
We hope that by restricting multipolygon completion to landuse and a small list of natural polygons we’ll be able to curb these unexpected side effects of polygon completion.
As a result of this change, the .osc.gz files generated on Friday night will contain “delete” operations for ways and nodes that were heretofore contained in the extracts due to multipolygon completion, but are not any longer.
Some data extracts, notably those for small islands or archipelagos, will shrink by more than 10%, but for most extracts the size will not be affected dramatically.
