Re: [Tagging] R: Request for new tag "natural=upland" (as way) orenabling "way" for "place" tags

[Christoph Hormann]

On Thursday 09 June 2016, Amacri wrote:
> > On a general note - when things are mapped as nodes this is
> > frequently done with the implicit notion that this is a location
> > with a certain tolerance margin.  You might think of mapping
> > something with a linear way as a method to specify an anisotropic
> > tolerance, well localized in one direction but poorly in another. 
> > However that is not what you actually do when you map it as a way -
> > on the contrary you much more specifically localize it.
>
> I would ask you to elaborate your concept that I might not understand
> well.

If you in the simplest case map something either as a node or as a two 
node way extending a distance on either side of the node in the other 
case the way does not indicate a higher tolerance in localization in 
direction of the way.  Instead it indicates a larger extent of the 
feature in that direction and generally a lower tolerance in 
localization since you specify two coordinate pairs instead of just 
one.

In other words: when you map something as a way this always says 
something about the shape of the thing you map, it does not represent 
the shape of the probability distribution for the location of an 
otherwise shapeless feature.

> As you mention, documenting the feature by highlighting anisotropic
> tolerance of its extension, as per analysis of historical maps (e.g.,
> official maps from one side but being the only source for data from
> the other side,

A historic map that is old enough so you may legally use it without 
permission of the creator is usually not an appropriate source for 
geometry data and labels in maps are never a suitable source of 
geometry data - after all label placement is subject to a lot of 
influences completely unrelated to the geographic reality.  So this 
line of argumentation does not really convince.

As i said before - you best think about what you know about the thing 
you want to map in terms of verifiable, observable facts.  Based on 
this you can best decide how to represent such a feature in the 
database.

-- 
Christoph Hormann
http://www.imagico.de/

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Re: [Tagging] R: Request for new tag "natural=upland" (as way) orenabling "way" for "place" tags

[Martin Koppenhoefer]

sent from a phone

> Il giorno 09 giu 2016, alle ore 23:43, Amacri  ha scritto:
> 
> A polygon provides additional specifications than a line and implies
> knowledge of surface information that an historical map could not document
> itself and that a mapper shall not arbitrarily add.


this boils down to the question whether it is better to completely omit the 
extension or to do a best effort approximation, which can still be refined 
later if necessary. I'd clearly prefer the latter. I don't think "arbitrarily" 
is a good description of the work a mapper will do in order to draw the outline.


cheers,
Martin 
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[Tagging] R: Request for new tag "natural=upland" (as way) orenabling "way" for "place" tags

[Amacri]

> On a general note - when things are mapped as nodes this is frequently
> done with the implicit notion that this is a location with a certain
> tolerance margin.  You might think of mapping something with a linear way
> as a method to specify an anisotropic tolerance, well localized in one 
> direction but poorly in another.  However that is not what you actually do
> when you map it as a way - on the contrary you much more specifically
> localize it.

I would ask you to elaborate your concept that I might not understand well.

As you mention, documenting the feature by highlighting anisotropic
tolerance of its extension, as per analysis of historical maps (e.g.,
official maps from one side but being the only source for data from the
other side, with no more information apart from some knowledgeable locals
confirming the existence of that name in the same zone of the mountain
reported in the source map), was the reason for considering to use a curve
rather than a point, not only highlighting a length but also its curve shape
and direction, where the curved line would represent the most probable
distribution of an uncertain named area.

A polygon provides additional specifications than a line and implies
knowledge of surface information that an historical map could not document
itself and that a mapper shall not arbitrarily add. On the other hand, a dot
loses the placement information offered by the shape of the text in the
source map, reducing the quality of information reported in OSM.

Regards

Alberto



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