[OSM-talk] Mapping everything as areas

[Roy Wallace]

On Mon, Nov 30, 2009 at 10:23 AM, Steve Bennett <stevagewp at gmail.com> wrote:
> On Mon, Nov 30, 2009 at 10:09 AM, Roy Wallace <waldo000000 at gmail.com> wrote:
>> interpolation. But approximation with trapezoids or whatever is a bit
>> fudgy....e.g. what if you *do* want to represent an instantaneous
>> change in width?
>
> I can think of several options, and I'm sure you can too :)

Of course, but they're all "a bit fudgy" :P

>> My point is that this general suggestion seems to be a way to *map
>> areas*, by *tagging ways*. Is this actually better than to *map areas*
>> by *tagging areas*? If so, how?
>
> I can think of a few reasons it would be better:
> * No parallel data structures. Ie, there is just a way, with markup,
> rather than a way and an area. (I don't think simplying have an area
> without a way is viable, as it imposes too great a burden on routing
> software.)

Yup, valid point.

> * Conceptually cleaner.

I'm not entirely sure what "conceptually cleaner" means.

> From the point of view of a map, a road really
> is a line...that happens to have some width and shape. Mapping it as
> an area makes it primarily a chunk of asphalt...that you happen to be
> able to drive along to get somewhere.

Hmm...I think these are *both* valid interpretations of a road. I
don't think it's a strong argument against mapping areas explicitly.

> The ideal situation would probably be to have users be able to enter
> either ways or areas, and have the server software understand the
> relationship between them, and convert between them.

Agreed. The issue remains of whether the server should convert
everything to 1) "split ways with width tags, implying linear
interpolation between segments with different widths" or 2) "a way and
an area, possibly linked with a relation". I prefer the second, as it
more clearly encapsulates the two aspects of a road that you have
pointed out above: 1) a line you drive along and 2) a chunk of
asphalt.

> So if you enter a
> way, it automatically creates an implicit area around it with some
> default width.

A default width is not necessary. The absense of a width tag should
imply the width is unknown/unspecified. Or, in other words, the
default is the road is a *line* (0 width, no area).

> Nice client software might let you manually tweak that
> area. This aspect of GUIs is very hard to get right though: when there
> are automatic/implicit data structures that are occasionally
> customised.

Should be less of a problem when "area" is only implied after
additional user input.

> What happens if you customise the shape of the road, then
> re-route the way? There's never a very clean answer to that question.
> (Possibilities are, keep the area - even if it's out of sync; discard
> the area information; attempt to preserve some of the area information
> - even if it now makes no sense...)

Not entirely sure what you mean, but in general, I would say give the
mapper control (e.g. discard information only when requested).