[OSM-talk] [tagging] absent tags (was: noname streets)

[Vincent Zweije]

On Mon, Jun 09, 2008 at 11:58:28AM +0100, Richard Fairhurst wrote:

||  Dave Stubbs wrote:
||
||  > The good point about name=__none__ is that I can bet large amounts of
||  > money that no street is actually named "__none__" -- the bad points
||  > are that renderers that don't know about it are going to write it in
||  > the street name
||
||  So maybe named=no (or unnamed=yes)?

The problem is really more general.

You want to be able to say (a): "tag T is absent", as opposed to (b)
"nobody has bothered to enter tag T yet".

For distinguishing these two, we need to find some way to explicitly
specify (a), because (b) has to be modeled by the tag just being not there
(think about it).

I can see several ways, in personally increasing preference:

 1. define a special value for the tag (e.g. "" or "__none__")

    disadvantage: takes away a technially valid value, so may not be
    generalizable to tags other than "name".

 2. add another (meta) tag specifying which tags are undefined:

    absent_tags=name,any_other_tag,...

    disadvantage: this is a multi value tag, which complicates the
    model. Witness the gratuitous potlach value-combining with semicolons.

 3. specify a derived tag, as with localised names:

    name:absent=yes

    disadvantage: might accumulate to a lot of extra tags in the database,
    but it only needs to be added if there really is uncertainty about
    the situation.

So... how about doing T:absent=true? The interpretation of which being:
tag T is not there in the real world, as opposed to only not there in
the database.

Ciao.                                                             Vincent.
-- 
Vincent Zweije <zweije at xs4all.nl>    | "If you're flamed in a group you
<http://www.xs4all.nl/~zweije/>      | don't read, does anybody get burnt?"
[Xhost should be taken out and shot] |            -- Paul Tomblin on a.s.r.
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