On 19 April 2011 14:50, Andy Allan <gravityst...@gmail.com> wrote: > On Tue, Apr 19, 2011 at 2:31 PM, Peter Miller <peter.mil...@itoworld.com> > wrote: > > > > Andy Allan just asked me a question privately about changes I have made > to > > layers in Wandsworth which has prompted me to do a post here saying what > I > > have been up in order to rationalise use of the layers in East Anglia, > > London and now Kent. > > > > The ITO Map 'Layers' view highlighted a huge amount of weird layer tags > in > > the area. London seemed to be particularly bad. There were parks at > layer-5 > > and lakes at layer 1; railways at layer=1 or -1 even though they were on > the > > ground. > > So my concern was that data is being removed for no particularly good > reason. For example, at > > http://osm.org/go/euum@dsaa-- > > the two central carriageways were tagged layer=-1 to show they are > below the nearby sliproads, but Peter has removed these layer tags. > I'm assuming his map layers view has some logic that layers tags only > apply to ways that cross but I don't believe that to be true. He's > also removed the layer tags from stretches of the railway, for example > at >
To my mind the lower road is at 'modified ground level' which is layer=0 which is optional. It it is in a cutting the the 'cutting' tag would be appropriate. because the layer tag saying nothing about relative height to a parallel way, only about the z ordering at crossing points. Consider the path that the top and bottom of the Grand Canyon both of which are at 'ground level'. > > http://osm.org/go/euunor2Ku-- > > which again, those of us who know that area know the railway is on a > different layer to the surrounding roads. While there is an argument > in both cases that there could be additional methods of tagging the > situation (such as adding embankment or cutting tags) I still don't > see that removing the layer tags is doing anything other than removing > the correct information that was there previously. > Since the railway crosses the Old York Road then I believe that there should indeed be a bridge (with a layer tag). Adding a layer tag for the full section of track and not having a bridge is not the right answer. If the whole section of railway is up on a concrete platform then it may be more appropriate to use a viaduct for the whole section but that does not seem to be the case from Bing. If it is raised up on a bank then an embankment may be appropriate. However... the layer tag is not the right tag to use and doesn't give any of that information. On balance I think bridge is right for the section over the road. I failed to add that bridge section - sorry about missing that one. Make that 101 bridges! It may well be good to add an embankment tag to the section between the bridges. Regards, Peter > Cheers, > Andy >
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