[Tile-serving] [openstreetmap/osm2pgsql] Can stage 2 flex processing dissolve line geometries? (Discussion #1595)

[mboeringa]

Hi,

I have recently been exploring the possibilities of the new flex output based on Paul's development work for creating a "flex version" of 'openstreetmap-carto' (https://github.com/gravitystorm/openstreetmap-carto/pull/4431).

As one nice enhancement to the original 'openstreetmap-carto' style that Paul created, the new "flex version" of that style uses a Lua script that creates a new 'planet-osm-admin' table containing **_de-duplicated_** line geometries for OpenStreetMap admin_level boundaries, where the geometries of the lines always represent the highest rank admin_level that is present on a border line.

E.g. an "admin_level=2 " boundary line that is also an "admin_level=4" and "admin_level=8" boundary, will end up in the 'planet-osm-admin' table as "admin_level=2".

This allows symbolizing the lines in a more cartographically advanced way, e.g. by using open stippled lines like in true topographic maps, instead of having to properly stack solid lines for different admin_levels to "hide" less important border lines as is needed with polygon outline symbology.

While nice, the current implementation of the Lua style has one issue: common boundaries of the same admin_level between e.g. two bordering countries, are not being dissolved into a single line. E.g. see the attached image, where the boundary between Liechtenstein and Austria consists of multiple segments, as highlighted by a selected light blue segment. This may subsequently cause symbolization discontinuities between individual segments of a single border, a problem which is hardly noticeable at large scales (e.g. > 1:50k), but gets worse with smaller scales (e.g. 1:1M).

**This raises a question for me:**

Would it potentially be technically possible to use osm2pgsql's stage 1/2 flex processing to merge all the segments of a single boundary line, where the segments are all of the same admin_level (e.g. "admin_level=2"), and the "admin_level=x" relations associated with them are the same?

Or is this outside of the capabilities of current flex processing?

![afbeelding](https://user-images.githubusercontent.com/7635726/146638302-fc02f8cb-7edf-4a56-b681-b1028bc65d56.png)


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