[OSM-talk] Openstreetmap-carto switching to ocean polygons

SandorS sandors39 at gmail.com
Wed Mar 6 20:35:02 UTC 2019

Joseph, I am not sure you will get some response to “...it needs testing...” especially not from GIS and vector map-makers. To do such analyses one should start from the source data, should have a robust tool-set and be willing to invest lot of work and time to compare, measure, run applications and so on. If the “switching” issue is only a low scale rasterization and display efficiency related then the whole issue IMO is irrelevant (arguments). However, in GIS and vector map-making, especially from the scientific point of view, the coastline land polygons vs. ocean polygons issue is essential. This issue was many times up to discussion and they were various data versions of these polygons available, some used in the OSM main map layers.
Some years ago I made a pretty detailed analyses of the coastline large land/water polygons and their generalized (scale level) versions and published the results here https://lists.openstreetmap.org/pipermail/dev/2016-June/029295.html . Last several days I have invested to repeat some of the work done in the link to see the present state of the polygons in focus. Many of the mentioned anomalies in the former article are still there though by reduced extent. Some short related notes:
-The former polynomial polyline smoothing is replaced with a cartographic vector smoothing model. Much more realistic.
-Inland waters are moved to lakes and that allows radically faster water/ocean polygons creation by remaining land polygons inversion.
-The land polygons data contain large number of small polygons of land-on-land type. 
-There are still several replicated consecutive nodes on land polygons.
-The global ocean (the large water) polygons still contain anomalies along the World frame (inversion problems).
-Simplified water polygons are created by simplifying and inverting land polygons. This is never the same as simplifying water polygons.
-The split polygon versions contain large overlaps and these for GIS and vector map-makers present more trouble than good.
Finally I would strongly underline the following note. Using generalized version of water polygons without synchronization with other object classes (lakes, rivers, forests...) causes serious logical and aesthetic cartographic problems. For instance, large objects in one class disappear while small objects in other classes still are present (collapse synchronization), objects with radically different details are connected or slightly overlap and so on. An example of simplified water polygon and not simplified lake connection can be seen here   
https://osm.org/go/Z1a9HCF6g-?layers=TD&way=100084105 . This link also illustrates the mentioned erroneous small polygons in the coastline land polygons that are missing islands in lakes or rivers. 

Sent from Mail for Windows 10

From: Joseph Eisenberg
Sent: 26 February 2019 04:10
To: talk at openstreetmap.org
Subject: [OSM-talk] Openstreetmap-carto switching to ocean polygons

There is a new PR on the Openstreetmap-carto Github page which will switch this “standard” style (used on the main OSM page) to water polygons instead of land polygons. 

However, some users reported problems with the simplified water polygon shapefiles when this change was attempted in 2016. We believe the problem is no longer present, because the German map style is using these shapefiles without problems, but it needs testing. If you were one of the affected users, please try out the change on your server. 

The branch is available at: https://github.com/jeisenbe/openstreetmap-carto/tree/ocean-polygons

New PR: 

Old issue: 

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