I think it does not hurt to define the exact meaning of a line-segment in OSM. And I think that great circle (in wgs84) is the natural choice, in stead of defining line to be straight relative to some arbitrary projection.
Since the API (for performance reasons) can not return line segments with endpoint outside the bounding box, we (at least for some time) have to live with adding redundant nodes for every x km for great-circle lines. Then I suggest that the purists may add a tag "redundant"="y" to the redundant nodes. To Frederik's concern about mappers getting confused about what a straight line is: I guess that there is only a tiny fraction of the mappers that ever will come across very long line segments. I suppose more than half of them can do it right in the first place if it is properly described on the Wiki. (Particulary state that a line of lattitude is not a Great circle, except for equator). And the other cases can be corrected by others who understands the concept of a great circle. That is the beauty of wiki-style mapping. Best regards Egil > Message: 6 > Date: Sat, 09 Jan 2010 09:58:20 -0500 > From: Greg Troxel <g...@ir.bbn.com> > Subject: Re: [OSM-talk] New "Highways" view in OSM Inspector > To: Frederik Ramm <frede...@remote.org> > Cc: OSM Talk <talk@openstreetmap.org> > Message-ID: <rmiskafl3zn....@fnord.ir.bbn.com> > Content-Type: text/plain; charset="us-ascii" > > > We were discussing what exactly a straight line was. There is no such > thing as a "straight line in the database", because, as you correctly > state, the database only stores the end points of a line. If you draw a > line from point lat=10;lon=10 to lat=30;lon=30, then it is unclear > whether that line visits point lat=20;lon=20. Some might think yes, some > might think no. > > I think this is exactly the key question. > > When there is a line segment in the database, in WGS84 lat/lon, with > points (lon1,lat1) and (lon2,lat2), then we need to have a definition of > what that representation means. Obvious candidates are: > > 1) linear in lon,lat space > > 2) great circle in wgs84 > > 3) linear in google spherical mercator > > 4) linear in WGS84 UTM > > 5) linear in your own country's local grid, or US state plane coordinate > system > > 6) we don't define it, and if any of the above are different in any > discernible way, you need more points. In the 10,10 30,30 example > above, we are clearly in this state. > > > _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
