[OSM-talk] Bridges / viaducts for railways

[Richard Fairhurst]

Oooh, I get to talk about something I really _do_ know about...

Martijn van Oosterhout wrote:

> On Feb 2, 2008 11:12 PM, Gervase Markham <gerv-gmane at gerv.net> wrote:
>> Yeah, it seems to me that putting max_heights on something where  
>> one of
>> the surfaces moves is fraught with possible problems...
>
> TBH this seems more of a hobbyist requirement anyway. If you're making
> a map for serious use you'd do it more like a network (like roads) and
> have routes between junctions that say what the maximum class of boat
> that can pass through this route. There is a standard classification
> for such things. No route planner for boats is seriously going to rely
> on checking the height of every bridge on the way.

For once in OSM's life, that's a Euro-centric view and not the case  
in the UK. ;)

You really do have to check every bridge and lock dimension if, for  
example, you're on the Thames: they vary widely and without any  
particular rhyme or reason, other than the general rule that  
dimensions get smaller as you go upstream. (It's even the practice at  
some bridges - e.g. Osney Bridge on the Thames in Oxford, Potter  
Heigham Bridge on the Norfolk Broads - that the owners of larger  
boats will approach at high speed, because that makes the boat sit  
slightly lower in the water and is the only way they'll get under. Of  
course, that makes it even more entertaining if they fail.)

On the narrow canals it's also true: 200-year old locks have an  
awkward habit of narrowing by an inch or two, which poses particular  
difficulty for historic narrowboats, and some locks were built (or  
rebuilt) very slightly shorter. So a 71ft 6in Grand Union narrowboat  
like ours (Hagley) may have difficulty in some Birmingham Canal  
Navigations locks.

> In the end we have to rely on the official classification for
> waterways anyway, because if a waterway is not officially classified
> as permitting a certain class of boat, then a council can just build a
> non-opening low bridge over it and you're SOL.
>
> Then again, this may be academic in the UK, I understand they don't
> have any waterways that could be classified in the smallest category.

We do have a few - the Aire & Calder and Sheffield & South Yorkshire  
systems, the Manchester Ship Canal, the tidal Thames, the Clyde, and  
I think maybe the lower reaches of the Weaver. But yes, the vast  
majority of our waterways are "hors categorie" - so there's a clear  
need for accurate max_height, max_width, max_length and max_draught  
tags.

cheers
Richard