[Tagging] Verifiability of geometry

Daniel Koć daniel at xn--ko-wla.pl
Sun Jun 16 18:15:22 UTC 2019


Hi,

There are still some problems with verifiability of objects geometry.
This has been discussed lately here:

https://github.com/gravitystorm/openstreetmap-carto/pull/3750

but we came to the conclusion that this is not the best place to go with
fundamental problems, so I come here to talk about tagging strategies.

Christoph (imagico) has proposed there a set of example rules that he
believes are self evident and invited to challenge them if someone
disagrees, so here I am:

  * polygons are a way to geometrically define a two dimensional entity
    through (and only through) an explicit delineation of its one
    dimensional boundaries.

I agree with that. That's why area mapping for bays is good as long as
you have a source for this. This is what we do with admin borders - we
get the nodes that we know and simply link them. Or we use some data
from authorities. No matter what source do you use, there can be
disagreement (look at admin borders problems which are not solved to
this moment, yet we don't simplify admin objects into nodes or lines
because of that).

  * for the decision what kind of feature to use to represent a certain
    real world feature in the OSM database mappers should put mapping
    and data maintenance efficiency above perceived desires of data
    users. The main criterion should be how to most efficiently
    represent verifiable information on the feature in question without
    storing either redundant or non-verifiable data.

Written rule does not support this interpretation, it's short and clear.
Data maintenace is not a rule, is not mentioned there even as an excuse
or exception and of course is not higher level rule than verifiability.
On the other hand people commonly use nodes or lines for representing
areas.

There is simply a clash between written rules and the common usage. It's
the open question how should it be solved.

  * there is no principal connection between the nature of a real world
    object and how it can or should be represented in OSM above the
    mapping efficiency criterion previously mentioned.

I don't agree here. There can be some simplification used (just beware
of oversimplification, especially for bigger objects), but this is
always worse than lack of simplification.

  * straits between concave coasts are one dimensional entities, they
    have a width but they have no length.

I see no support for this claim, so I completely don't agree with it:

- "Most commonly it is a channel of water" (Wikipedia, strait) - channel
has length and width

- "The shortest distance across the strait, 33.3 kilometres" (Wikipedia,
Strait of Dover) - the mentioned thing is only a part of the strait, not
the whole entity

- "A narrow area of water surrounded by land on two sides and by water
on two other sides." (description on the OSM wiki)

  * the verifiability of a node location representing a feature
    exclusively depends on if multiple independent placements of the
    node converge to a single location. This is a completely scale
    independent problem meaning variance of different placements can be
    anywhere from less than a meter to hundreds of kilometers. This has
    no bearing on the principal verifiability.

This sentence is very complex, I'm not sure what do you think.

When you have 4 different limits of the Gulf of Guinea for example (
https://commons.wikimedia.org/wiki/File:Limites_du_golfe_de_Guin%C3%A9e-fr.svg
), you will have 4 different central points which don't converge at all
(the distance between the middle of A and D is roughly 1000 km), so
using nodes does not help anything. At best one can put it in the A area
as the common part of all of them, but this is indirectly choosing A
area as proper, which is just hiding the problem, not getting rid of it.

The solution would be for example to ask local people from all 12
countries if they think this part of the coastline belongs to the bay
and take their claim above what others (non-local people) say. At best,
it will give you properly sourced shape. At worst, you may not get the
consistent answer - then you might just simply not map it at all.


-- 
"I see dead people" [Sixth Sense]

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