> The way I have handled dual carriageways (and motorways) is to assume that > both carriageways are plotted separately on OSM. So the figure that > results > should be twice the length estimated by DfT. However, for primary roads, > DfT > themselves show the total length of primary road, and the length of this > which is dual carriageway. So by adding their two numbers, I effectively > double up their figure for dual carriageways, to reach the same (in > principle) as the OSM total. Hence I can ignore "dual carriageway" tags on > the OSM stuff. I think.>
> calculation. Generally the figure for motorway is pretty close. I suspect > that significant differences in the motorway figure are down to errors in > the boundary position. It should be possible to compare adjacent > authorities > to see where these have resulted in a motorway appearing in the wrong > authority - but this is on my ToDo list for a later stage. The motorway figures aren't *that* close though. Your figures are almost all overstating the DfT's list. I count only a single authority that you've calculated the OSM length less than that of the DfT's. If it was purely due to boundary positions you'd expect that an overstatement in one authority would be balanced by an understating in the neighbouring authority. It does suggest that we're including things the DfT are not e.g. you mention sliproads. What would the figures be without highway=foo_link tags? Also, we might be double counting e.g. Shropshire. DfT list of motorways: 12.4km x 2 = 24.8km. OSM 55km That can't be right however we are counting, unless the OSM figures for Shropshire count that for Telford & Wrekin Unitary as well (which you've done separate analysis for). If that's correct then we're still over in both Shropshire County Council area (OSM ~ 29km vs 24.8km), and Telford & Wrekin (OSM 26km vs DfT 24.0km) - but it might be close if we remove sliproads? _______________________________________________ Talk-GB mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk-gb
