[Talk-ca] PEI NRN import 21I16 - and questions about ways

[Richard Weait]

On Wed, May 12, 2010 at 6:46 AM, Robert Shand <bob at shand.org.uk> wrote:
> Hello Everyone,
>
> I've completed the initial steps of the NRN import for part of NW PEI.

Hi Bob,

Yay!

[ ... ]

> Take a look
> at http://www.openstreetmap.org/?lat=46.8381&lon=-64.1547&zoom=13&layers=B000FTF
>
> Here we see the renderer has places lots of the markers for the road
> number/name.

No need to worry.  The frequent tertiary ref shields is a mapnik
rendering setting in the shield symbolizer.  If you pop over to the
osmarender layer you'll see that tertiary refs appear as numbers (not
shields), that they appear less frequently, and that they don't appear
until z14.

[mapnik geeky aside]

The rule for those shields is approximately this:

ShieldSymbolizer name="ref" fontset_name="bold-fonts" size="10"
fill="#fff" placement="line" file="&symbols;/ter_shield3.png"
type="png" width="31" height="17" min_distance="40" spacing="750"

So the shields will appear on ways as short as 40 pixels and repeat
every 750 pixels on long ways.  Merging ways will reduce the number of
shields you see when the uncombined ways are < 750 pixels long.

[end mapnik geeky aside]

> Taking a look at the data in JOSM we can see that the road 151 (like many of
> the others) is comprised of many ways
> http://www.shand.org.uk/osm/NRNImport/talk-ca/way1.png
>
> http://www.shand.org.uk/osm/NRNImport/talk-ca/way2.png
>
> However if I try to combine the ways I'm getting a conflict on the
> geobase::uuid
> http://www.shand.org.uk/osm/NRNImport/talk-ca/conflict.png
>
>
> So my question boils down to how do we want to handle this.
>
> We shouldn't be mapping for the renderer, but the numerous markers on the
> map do look odd.  In my mind the numerous ways should be combined into one
> way, however if I do that I naturally have to lose some of the geobase data
> like the uuid.  If we get newer higher quality geobase data matching up the
> roads might more painful without the correct uuid.
>
> What are peoples thoughts?

I'm not certain of The One Truth regarding this.  But here are some
related thoughts.  Perhaps others will join the discussion as well.

- combining the ways is not inherently bad.
-- non-import ways tend to be longer.
- combining the ways is more labour for you.
- combining the ways only to fix the shield issue in mapnik _is_
tagging for the renderer.
-- we could submit a ticket to increase min_distance
--- the current min_distance setting might work really well in denser
road networks than the one we see in this example.
-- we could render in a Canadian style and "fix" min_distance for ourselves.
--- rendering in a Canadian Style requires a server, bandwidth and hosting.
- combined ways should drop the uuids, rather than merge them (?)
-- the combined uuids will not correlate to a single uuid in any other system.
-- the combined uuids may hint at what happened to the original uuids,
but not at where the uuids were split.
-- to recreate the original uuids, one would have to dive in to the OSM history
--- if you have to dive into the history to find where the uuids were
split, why not rediscover the uuids at the same time?
- combined ways may later need re-dividing if relations, bridges or
other features are added.

What do others think?