[OSM-talk] Very long ways

Mike Collinson mike at ayeltd.biz
Wed Mar 28 09:05:37 BST 2007

At 02:17 PM 28/03/2007, Lars Aronsson wrote:
>As I gain experience from working with JOSM, I'm seeing problems with long ways that are incompletely loaded into JOSM.  The other day I tried to split E4 in Sweden and E75 in Finland into shorter pieces, and I'm not sure I didn't ruin some of it.  When I'm creating new ways, I'm trying to break them into pieces of 40-140 segments each.
>Are there any guidelines for how long ways should be?  Either by number of segments or by geographic size?
>When I was looking in planet.osm for extremely long ways [*], I found coastlines.  For example, way id="4318034" has 5575 segments and these tags:
>    <tag k="natural" v="coastline" />    <tag k="created_by" v="JOSM" />    <tag k="source" v="SRTM" />
>It seems inevitable that these large objects will be hard to handle.  Is this a good design?  Should clients (like JOSM) be required to handle long ways? 

FYI, the way you quote is almost certainly my import of low resolution Philippine coastline - expedient rather than good design!  I'd prefer to see coastlines as a separate layer.

More generally from my editing rather than rendering or technical db point of view:

Number of segments:  Probably irrelevant?   The more, the slower it is to download.  If they are in your current edit area, you still need to download them whether it is one or several ways. If they extend outside your area, it is pointless to download them but the crucial factor is the geographic size, not the number.

Geographic size:  The bigger, the more statistically likely the way will be incomplete in any given edit and cause problems.  I just sat down and worked out a rule of thumb given my JOSM edit habits. I can't say they will work for anyone else!  I generally try and download an area bigger than I intend to edit.  I'd should say the rough guide is about half the geographic width of the JOSM window when you do your initial download (a sort of Nyquist frequency).  

I came up with 12km for long distance features such as railways and 1.5km for urban ways.  

1.5km seems good but 12km too small for countries like Canada and Australia.  Doubling the allowed OSM download area would give 24km, which might allow a way break at the next turn-off or bridge and would allow for less crowded labelling on 1:250,000 and 1:500,000 maps.



I'd should say the rough guide is about half the geographic width of the JOSM window when you do your initial download (a sort of Nyquist frequency).

At the moment OSM won't let me download if the slider is set to over 2.1 km, so that gives me 24 km of window so 12km optimum length.  When dealing with 500km roads I find that a pain in the behind.  Allowing double the download window for working in sparsely populated areas of the world would allow 24km, not great but workable.

I'd say 24km max and preferably half that, 12km, would be good in sparsely populated areas and for long distance features such as railways and motorways.  24 km is roughly the width of a JOSM screen when the slide bar is set to 2.1km, the maximum that you can download from OSM at them moment.

For dense urban areas where your edits are probably going to be very local and you don't want to download the whole city,  I tend to download with the JOSM slider set to 263m --> 3km window --> 1.5km way length.

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