On Wed, Apr 29, 2009 at 5:31 AM, Lambertus <o...@na1400.info> wrote: > Maarten Deen wrote: > > Lambertus wrote: > >> Maarten Deen wrote: > >>> Not to diminish your work on that front, but I find the tilelayout on > >>> your site very strange. Of course it is a work of the splitter, but I > >>> would opt for a manual layout, guided by an initial automated process. > >>> > >> Please forgive me for relying on the automated Splitter layout > >> mechanism as I have no intention to manually divide the world into 500 > >> tiles (317 America + 182 Europe/Asia/Africa/Oceania currently). > >> Optimizing the tiles would result in even more, so you'd be talking > >> about e.g 750 tiles. > > > > I'm certainly not complaining about your work. It's the reason why I > > bought a Garmin Nüvi and not a TomTom. > > > See Garmin...this is why you need to publicize your map format :-) > > >> Needless to say: patches welcome ofcourse. The definition files for > >> Splitter are: > >> - > >> http://planetosm.oxilion.nl/~lambertus/america.list<http://planetosm.oxilion.nl/%7Elambertus/america.list> > >> - > >> http://planetosm.oxilion.nl/~lambertus/eurasia.list<http://planetosm.oxilion.nl/%7Elambertus/eurasia.list> > >> > >>> IMHO the strategy that the Mapsource tiles use is much more logical. > >>> Take one big tile, if that has too many nodes, split it in half > >>> horizontally, if that has too many nodes split it in half > >>> vertically... repeat until you have a sufficient small amount of nodes. > >>> > >> This strategy is afaik exactly what Splitter does. > > > > Then it does it in a strange way. I'm sure you've noticed that the edges > > of the tiles don't line up by just a few 1/100th of a degree in a lot of > > places. That is inconsistent with a strategy of dividing a tile in half > > if it has too many nodes. > > E.g. tiles 63240113 and 63240116. One has a north border of 51.679688, > > the other 51.635742. And then two tiles further east, 63240120 has a > > north border of 52.679688 again. > > > Well, maybe not exactly in half, but the idea is the same. Afaik, the > reason why Splitter doesn't split exactly in half is that there is less > chance that a polygon is present in too many tiles. > > > BTW: have you seen that Mapsource draws the maps as overlapping? I've > > attached a screenshot of how Mapsource displays maps 63240105 (yellow), > > 63240175 (blue, continuing to the top) and 63240179 (green). > > They all overlap eachother. According to their definitions, they should > > not overlap, but mapsource apparently disagrees. > > Any idea why that is? > > > I've seen this with topo maps made with older versions of Mkgmap as > well. This might be a bug in Mkgmap or a misunderstanding of the Garmin > map format but I don't think it's harmful though. >
Probably the reason for the overlap is nodes shared on a polyline which crosses a tile boundary, to prevent gaps. These shared nodes would extend the boundaries of each tile somewhat into the space of the neighboring tile. Otherwise, each tile would have to have a node exactly on the border, and the map resolution might not allow that. Shared nodes are required in routable maps (the shared nodes are marked as "external" to allow routing across tiles). Another reason I just thought of, which may explain why the overlap looks so large (larger than could be explained by shared nodes) is possibly the map projection used by MapSource. The projection might distort the map boundaries from a rectangle into something warped, but for simplicity MapSource only draws a straight line (not great circle) between the map boundary segments. Karl
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