[Talk-GB] Town Greens

[Robert Whittaker (OSM lists)]

On Fri, 3 Apr 2020 at 15:20, nathan case <nathancase at outlook.com> wrote:
> The two main components of the green, a wood and a grass area, are separately mapped as such.
>
> Where would you add the designation tag? To the boundary or to the two main landuse components? Or would you create a relation so that the designation tag and name (etc.) can be shared across the separate land uses?

Following https://wiki.openstreetmap.org/wiki/One_feature,_one_OSM_element
if there's a single legal "Town Green" entity, then there should be
one OSM element for the Town Green with designation=town_green. (You
can of course have other OSM elements for things within it, like the
separate wood and grass areas.) The "Town Green" object should follow
the legal boundaries of the town green. If it's all in one connected
piece, then you could have a single way following the edge of the
boundary sharing nodes with the edges of the wood and grass areas. (I
think this is what you've done with
https://www.openstreetmap.org/way/181597678 .) If there are multiple
disconnected pieces (e.g. two patches on either side of a road) then
you could use a multi-polygon relation to have a single object
covering all the areas.

You may also see a mapping style around where people try to avoid ways
sharing nodes, by having lots of line segments without tags, and then
grouping them together with multipolygons. In your example, they would
have three ways -- one for the part of the outer boundary next to the
wood, one for the part of the outer boundary next to the grass, and
one for the wood-grass joining line -- and then three multi-polygons
-- one containing the two outer boundary ways for the "Town Green"
object, one containing the two ways bounding the wood for the "wood"
object, and one containing the two ways bounding the grass for the
"grass" object. While conceptually neat, and not incorrect, I think
this over-complicates things and makes everything a pain to edit. I'd
just stick with three closed ways, sharing nodes round the boundary.

There's also a style in which you'd have one closed way for the wood,
and one for the grass, and then have a multi-polygon containing the
two closed ways. However, the fact that the two outer closed ways
share common line segments, violates the requirements at
https://wiki.openstreetmap.org/wiki/Relation:multipolygon .

Robert.

-- 
Robert Whittaker