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--></style><title>Re: [Talk-us] Park Boundary
tagging</title></head><body>
<blockquote type="cite" cite>Greg Troxel <gdt@ir.bbn.com>
wrote:</blockquote>
<div>> I will concede that my view is contradictory to what's
documented. But</div>
<div>> I think there's a fundamental semantic confusion lurking, in
that</div>
<div>> boundaries are linear features, and properties of land
belong as area features. </div>
<div><br></div>
<div>Apollinaris Schoell <aschoell@gmail.com> answered:</div>
<blockquote type="cite" cite>welcome to osm. forget clean semantic and
strict definitions.</blockquote>
<blockquote type="cite" cite>Yes it doesn't make any sense for someone
with a understanding of traditional systems and technology. take the
path vc. footway discussion as an example. It's still not unified and
about every couple months someone starts the discussion again with no
progress. almost all tags in osm are a "mess" but data
consumers have learned to live with it. Don't change a running system
...</blockquote>
<div><br></div>
<div>I'm not sure about a "fundamental semantic confusion"
lurking, and I hereby ask Greg to share with us an existence proof of
such a thing regarding boundaries. Consider "boundary"
and you might think of the very local one-dimensional act of
"crossing it" or "there is this side of it and that
side of it." To my mind's understanding of geometry, OSM's
wiki documenting "boundary" as areas seems 100% correct:
name a boundary that does not actually describe the inside vs. the
outside of an area. You can't do it, can you? We live on
the surface of a planet, making what maps describe essentially
two-dimensional. Sure, there are objects on Earth we might agree
it is convenient to describe as one-dimensional. You might find
a fence on a boundary that runs only part way around it, meaning that
the fence has endpoints. But though that allows us to agree the
fence is one-dimensional, the boundary isn't, it is two-dimensional:
it describes an area. What semantic confusion?</div>
<div><br></div>
<div>Well, OK: our wiki page for Boundaries (plural) says
"The original accepted usage was to apply this tag to areas.
However, there now seems to be a consensus of using the boundary key
also on linear ways, with relations used to aggregate these
ways." (Like that fence?) So guess what I'm going to
bring up now!</div>
<div><br></div>
<div>I'm glad Apo chimed in here, as it highlights something he did in
his State Parks upload a few years back s germane to this discussion.
In his upload of Anza-Borrego Desert State Park, he uploaded two
relations: one (relation 184199) to describe the boundary,
another (relation 181095) which described the area inside.
Specific tags are:</div>
<div><br>
<font face="Helvetica" color="#000000">Relation 184199:<br>
type=boundary<br>
boundary=national_park<br>
admin_level=4</font><br>
<font face="Helvetica" color="#000000"></font></div>
<div><font face="Helvetica" color="#000000">Relation 181095:<br>
type=multipolygon<br>
leisure=park</font><br>
<font face="Helvetica" color="#000000"></font></div>
<div>(Of course, both relations also have additional, identical tags,
like "name=Anza-Borrego Desert State Park").</div>
<div><br></div>
<div>This seems odd from a "what the wiki says"
perspective: supposedly, only when boundary=administrative is
defined is the associated "combo tag" of admin_level also
"allowed" to be used. Yet here, Apo instead uses
boundary=national_park + admin_level=4 (+ type=boundary to describe
the type of relation). In mapnik, it works: this
combination produces admin_level=4 green-dashing AROUND the park:
its boundary. It seems what is going on is admin_level is paid
attention to (by JOSM, by mapnik...) even when boundary=administrative
is not present, but boundary=national_park is present. OK, now
we know that.</div>
<div><br></div>
<div>The other relation draws the AREA of the INSIDE of the park:
the leisure=park tag in this second relation renders the light-green
park "fill color." (I'm pretty sure I'm getting this
right; I don't want to go yanking out tags and playing around, but
maybe I should. Maybe Ian sets up a sandbox server where Dane
allows mapnik style sheet changes for some experimentation. That
is an off-list email conversation I'm part-having).</div>
<div><br></div>
<div>In short, Apo has discovered that using two relations, one to
"draw the boundaries" (of the members), the other to
"fill in the areas" (of the members) is both effective (two
relations, two items drawn as we might like to see them) and efficient
(a couple of relations with a dozen or so tags, most
shared/duplicated, yes, but not a terrible amount of excess storage.
Oh, yes, plus polygons as members of both relations, but that should
go without saying). Crucial is that two relations draw two
things: edge and inside.</div>
<div><br></div>
<div>In simple cases where no multipolygon is required, Apo puts these
(germane, there are others) tags on the simple closed polyline way
(polygon):</div>
<div><br></div>
<div>leisure=park</div>
<div>boundary=national_park</div>
<div>admin_level=4</div>
<div><br></div>
<div>No need to say either "type=boundary" or
"type-multipolygon" since there is no relation needing its
type specified, just a polygon. So leisure paints the
fill-color, and boundary and admin paint the color and dashing of the
"edge." And it appears mapnik gives us a pass using
admin_level without boundary=administrative, as long as another
rendering boundary= key-value pair is included -- though
boundary=national_park seems to be the only other one that renders in
a particular color.</div>
<div><br></div>
<div>I think I understand Greg's perspective that a boundary is the
"crusty edge" of "something" but I still say that
that something is a two-dimensional entity we can always say is an
area (otherwise you are "being too local" and ignoring away
something that would truly show it as an area). I mean always:
I don't think there are any one-dimensional boundaries anywhere on
Earth (I'm listening, though -- and there is that consensus of key
boundary on linear ways...where?). Plus, if you want mapnik to
show you this distinction between "the edge" and "the
inside area" for parks, Apo has shown us a pretty, succinct and
effective method for doing so.</div>
<div><br></div>
<div>Many questions still remain, but this is too long already.
And I'm not sure I've clarified anything, just given some concrete
tagging examples which allow mapnik a distinction between "crusty
edge" and "gooey filling."</div>
<div><br></div>
<div>SteveA</div>
<div>California</div>
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