[OSM-talk] Segments, ways and order

80n 80n80n at gmail.com
Mon Jul 16 18:45:01 BST 2007

On 7/16/07, Nicola Ranaldo <ranaldo at unina.it> wrote:
> Segment order is important for many cases, one_way rendering, areas and so
> on.
> But it may be that a segment from node x to node y, for a one_way street,
> should appear as from y to x for one or more areas. Sometimes it's
> impossible
> to "reuse" the same segments. One solution could be in general create new
> segments with the same nodes id, but if this is admitted it may generate a
> lot of duplicates segments and seems to me a waste of resources in a
> topological model. Another solution could be in reuse an existent segment
> if
> it has the right order or to create the inverse if it does not match. A
> third
> solution could be in having software renderers compute the node order for
> areas at runtime with smart algorithms. In this case it will exists only
> one
> segment from x to y and will be reused for every way containing node x and
> y
> without care for the order. This maximizes the topological model but may
> create problems in areas with holes.

Renderers seems to be different, as mapnik and t at h do some reorder, while
> osmarender4 does not.

t at h uses the Frollo pre-processor to re-order segments and reverse their
direction.  Osmarender 4 can, and should, be used with the output from
Frollo to achive the same result.


This impacts on the model and on users practices.
> May be this definitively cleared? how does josm resolve this issue?
> Another issue i'd like to understand is if a way is a simple unconnected
> polyline or may split, or return on itself and so on. In this case
> segments
> reordering appears not so easy to unrerstand.
> May someone update http://wiki.openstreetmap.org/index.php/Data_Primitives?
> regards
>         Nicola Ranaldo
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