[Tagging] [Talk-us] Trunk VS primary

Joseph Eisenberg joseph.eisenberg at gmail.com
Sat Dec 21 11:36:06 UTC 2019


> I would expect that a road shown as e.g. trunk in Massachusets would be quite similar in
characteristics to a road shown as trunk in Montana.

Characteristics always change strongly between rural areas and urban
areas: in most places a highway=primary will have several lanes in a
large city, but in a rural area it will usually be only one lane each
way. In a city the speed limit will often been 40 kmh (25 mph), but in
a rural area it might be 80 kmh (55 mph) on the same class of road.
Similarly, near-wilderness areas like Alaska can be expected to have
different road characteristics than more densely-populated rural
areas.

> try to solve the problem of roads in low-zoom maps at the rendering level, by modifying the list of displayed road classes until a target density of displayed roads is reached

That's the right solution for maps that are designed for a particular
region or country, but it is not feasible for automatically generated
maps with global coverage (like the 4 styles shown on
openstreetmap.org). The big problem is what happens at the borders
between areas: it could be quite confusing if the color or zoom level
of a certain highway tag suddenly changed at the border between Spain
and France for example.

- Joseph Eisenberg

On 12/21/19, Wolfgang Zenker <wolfgang at lyxys.ka.sub.org> wrote:
> * Joseph Eisenberg <joseph.eisenberg at gmail.com> wrote:
>> Above it was said that the highway=trunk vs highway=primary
>> distinction is mostly for routing applications. But allowing a proper
>> rendering is also a main goal of the road tagging system.
>
>> While it's true that road class is useful for routing when there are
>> two alterate routes, a main reason to tag highways with a certain
>> class is to be able to render maps properly at different zoom levels.
>
>> When you are making a high-scale, low-zoom-level map of a large area
>> (say, the whole State of Alaska, all of England, or all of Australia),
>> you will want to only render highway=motorway + highway=trunk, because
>> showing all highway=primary would lead to rendering many smaller roads
>> which are not reasonable to show at that scale in most places.
>
>> [..]
>
> What makes the problem of road classification so hard is that we want it
> to do different things at once. For rendering we have on the one hand
> the requirement that we want to show all the "relevant" roads for a
> given zoom level, on the other hand, as a map user I would expect that
> a road shown as e.g. trunk in Massachusets would be quite similar in
> characteristics to a road shown as trunk in Montana.
>
> I suggest to keep the road classification consistent at least within
> a country and try to solve the problem of roads in low-zoom maps at
> the rendering level, by modifying the list of displayed road classes
> until a target density of displayed roads is reached. That might
> become easier to do when we move to vector tiles.
>
> Wolfgang
> ( lyx at osm )
>
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