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