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when I draw a line over a trace or bing image I'm guessing that using too many segments wiil use up memory and slow down map on osm system and on a gps/pda/ smart phone. what is best advice from an expert note I've edited my question(02/02/11) to make it clearer |
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There is some advice about this on these pages, with example pictures: Editing Standards and conventions and Good practice Basically, you should try to add enough nodes to make the curves look reasonably smooth. This means if you have a tight curve, the nodes will have to be closer together than they would on a wider curve. So you should avoid having close to right angles between nodes (unless there actually is a right angle on the road/path). There is no fixed rules about how many nodes to use, you have to use some judgement for this. I think you should just try to make it fairly close to what is on the ground. I don't think you have to worry about the memory used unless you are adding hundreds of nodes for a short road. Also remember GPS traces may be inaccurate, so make sure you are not drawing curves that are not actually there. Its helpful to average out several traces, as well as using aerial imagery where available. Note that Vclaw gave this answer before Bing was available to OSM.
(27 Aug '12, 01:05)
andy mackey
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May I add a plea for people not to follow Bing imagery to the exclusion of everything else. The georeferencing of the imagery is not quite exact, as can be shown by comparing the position of a mapped feature against the imagery at two different zoom levels. Certainly one GPS trace can sometimes be inaccurate, but often the average of many traces (not necessarily all your own) can be very accurate indeed. Please note that I am not criticising Bing. I am more than ready to accept that photogrammetry is difficult. When I asked this question Bing wasn't available for use in OSM. Now Bing is available it will give you the shape of roads BUT I would agree it should be checked for alignment with a SEVERAL GPX traces before doing any drawing from it. In Potlach2 you can drag the image into alignment by holding down the spacebar and left mouse button.
(27 Aug '12, 01:00)
andy mackey
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