On 2011-10-13 12:07, Bartosz Fabianowski wrote:
As it is, it *seems* to realise that the coastline file is as I'd flagged it, but then reports that it can't read such a file. Confusing.
That is rather silly behavior indeed :). Well, hopefully one of the gurus will drop in a fix sometime. If it got me, it'll certainly get someone else. No point in having the list continually jammed with people asking what's the matter!
P.S: I can now confirm that Bartosz's fix (specifying a separate coastline file) appears to have completely fixed my flooding issues.
I am quite convinced my analysis of the problem is correct. I know what is going wrong. And the only way to fix it that I can see is to use coastlines based on a larger extract that the one you are processing into a map. Hence, European coastlines when generating a map for the EU are not really a hack or a workaround. They are the correct solution.
Hmm - well I agree with you Bartosz that your analysis is correct. And the workaround is a good one, but quite honestly, it ought to be possible to handle coastlines directly. As I see it, the coastline problem (and a possible similar issue over other monster polygons like country boundaries) are the same. I did post about this before, but possibly we handle polygons (and polygon splitting) in too naiive a fashion. I would argue that whenever a splitter works on a map, it must keep any polygons complete in the output file: but to reduce all the points outside the tile's region-of-interest to a simple set of lines that skirt the boundary of the tile but located infinitesimally outside the region-of-interest, (possibly marked up as "invisible"), but preserving the closed nature of the polygon. So for instance, with my map of Wales, the splitter that generated "Wales" from the planet file would notice that the UK's coastline runs off the eastern side of the tile at the southern border of Gwent near the Second Severn Crossing (a bridge over the Severn Estuary into England). The splitter would follow the coastline right around the UK, noticing that it re-enters the "Wales" tile at the River Dee near Chester in the North East. So the splitter would replace all that UK coastline with a single (invisible) line joining the Severn Estuary in the south-east with the River Dee Estuary in the north-east, running just outside the eastern side of the "Wales" tile. That way, of someone took the Wales tile, and split it again to isolate just (say) the south west corner of Wales, then the coastline follower could easily repeat the same trick, as the invisible line alone Wales's eastern border maintains the continuity. It would be the same issue with country boundaries, city boundaries and other such things that might get chopped by a splitter. Notice also that a "stitcher" would be able to use this information to join two adjacent tiles and correctly maintain the polygon continuity. Steve **