# [OSM-talk] Parking aisle as boundary of car park not showing

Dave Stubbs osm.list at randomjunk.co.uk
Thu Sep 25 13:36:11 BST 2008

```On Thu, Sep 25, 2008 at 11:16 AM, Frederik Ramm <frederik at remote.org> wrote:
> Hi,
>
> On 25.09.2008, at 12:03, Andy Allan wrote:
>> And it means that areas preserve their actual dimensions
>> instead of growing outwards beyond their actual boundary - an
>> increasingly proportionately large problem as we add ever smaller
>> details to OSM.
>
> But is that not a problem for roads as well? What use is it to have
> the car park exactly where it is in reality when the *road* next to
> it will grow outwards beyond its actual boundary because people
> thought in unnecessary to actually map the extent of the road surface?
>

It is a problem for roads, but that's just an argument for mapping road areas.

My main problem with mapping areas to the centre line (I'm not really
counting landuse here, its possibly an exception) is the farcical
situations to which it leads. The buildings example is the best: it's
not uncommon at all to have a road abutted on both sides by buildings.
If we map them to the centreline then we possibly increase their size
by 50%, and happily record that they share a wall. Obviously they
don't, and if you somehow figure out there's also a road in there, you
might be able to reconstruct the real situation given a road width
(which very few people actually record).

So we say not to do it for buildings, but only for parks and car parks
(I've seen this advice given). So now we have these sticking out into
the road compared to a building right next to them. And there's lots
of roads with grass verges; how big do they get before the object
doesn't abut the road, and where is this width accounted for?

The situation is confused greatly as soon as you try to map the park's
fence or wall (if it exists): where's the road, inside or outside? Or
we have a post box on the side of the road on the pavement: these are
almost universally mapped as nodes where they are, rather than as a
way node -- if we mapped to the centre line these post boxes are now
inside the park? Or if the park diverges from the road at some point,
do we maintain the offset and get the angles right, break the angles,
or introduce an arbitrary curve?

I get that the roads are being mapped as abstractions, and I get that
it makes a lot of sense to stay abstracted, but on the whole I'd say
the number of questions raised by this approach makes it a lot easier
in the long run to just give up. We already break the abstraction for
so many features precisely because coming up with tagging schemes to
cope is just too hard, and because users find it so much easier to
just plonk something on a satellite photo.

Dave

```