Egil Hjelmeland wrote: Sorry, I forgot to change the subject line. > 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. >> >> >> > >
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