[OSM-talk] Large Rivers in general, mapnik rendering in Particular

[Andy Robinson (blackadder)]

David Groom wrote:
>Sent: 08 February 2008 10:40 AM
>To: talk at openstreetmap.org
>Subject: [OSM-talk] Large Rivers in general, mapnik rendering in Particular
>
>The proposed tag waterway = river,
>http://wiki.openstreetmap.org/index.php/Proposed_features/Large_rivers ,
>has
>been at proposal stage for over 18 months, which seems far too long for a
>tag which represents such an important feature.
>
>The main problem area seems to be that some people do not like the current
>proposal whereby a river is divided up in to separate closed areas.  The
>reason being that the "segment" crossing the river to close the area marks
>a
>boundary which does not actually exist.  Discussion on this could go on
>indefinitely, but it does really need a Mapnik "expert" to either (i) see
>if
>there is a way that Mapnik can render areas which are not closed (ie.
>comprised of two parallel ways), or (ii) if this is not , and will never
>be,
>possible then to state that fact , and we can then have a tag proposal
>which
>will render in both Mapnik and Tiles at home
>
>The main issue in practice is we now have no standard way of tagging
>rivers,
>and people are relatively free to do what they like, with the result that
>large portions of the River Thames disappeared from the Mapnik layer
>recently
>http://www.informationfreeway.org/?lat=51.49&lon=0.41&zoom=11&layers=0000F0
>B0F
>

As time goes on this is going to be an issue that comes up more and more
frequently. Thankfully OSM has a much simpler approach to data than a full
blown GIS approach where all edge features are tagged. In OSM we accept a
certain degree of simplification (roads are created as regular liner
features even if their width actually varies). I was looking at the
Rotterdam area yesterday:
http://www.openstreetmap.org/?lat=51.7578&lon=4.7882&zoom=13&layers=B0FT
and it was clear that rivers and other water courses are not ideally served
by regular linear rendering. With the exception of canals, which on the
whole have pretty regular width with length, rivers, streams and many other
water courses do not and we should therefore arguably always think of them
as areas. 

So where we have the required information we should always attempt to create
area rendering rather than regular liner lines. Requiring closed areas
though is not ideal for many reasons so achieving rendering between defined
objects, whether by relationship or otherwise would seem logical to me.

Cheers

Andy