This is a brief overview of how I edit Open Street Map (OSM) to add adjoining land-cover areas as multipolygons with shared boundary ways to reduce duplication and overlapping ways.
The area I’m editing, is replete with examples of this type of mapping: https://www.openstreetmap.org/#map=14/44.0199/-73.1530
The tools used are:
Documentation on MultiPolygons in the OSM wiki:
I also have a video about debugging gaps in multipolygons: https://youtu.be/87nRQHuatOE