On Wed, 26 Nov 2008, Frederik Ramm wrote:
If you go to http://tools.geofabrik.de/osmi/?view=geometry there's a view
(Europe only atm) that will show you all nodes placed more than 10km apart
(the green lines).
The following is an exercise in computing the maximum allowed distance
between nodes so that the line still gets rendered.
[EMAIL PROTECTED] downloads a rectangular buffer around the z12 tile to be rendered. This
buffer extends the tile size by 17% of the tile height on the N and S side of
the tile, and 50% of the tile width at the E and W side of the tile (as
configured in general.conf).
We're looking for the shortest line between two points where both points are
outside of the buffer (thus the line is not downloaded) but the line itself
intersects the z12 tile inside the buffer (thus its omission gets noticed).
The tile 2218.624 that you mention is at lon=14.94140, lat=77.15716. It is
roughly 0.0878 degrees wide and 0.0195 degrees high, so the buffer extends to
the left by 0.0439 degrees and to the top by 0.0033 degrees.
I make the back-of-the-envelope assumption that the shortest straight line
we're looking for will pass through the buffer at a 45° angle, and it touches
the above mentioned tile corner; so it computes that it intersects the buffer
at lon=14.8975, lat=77.1132 and at lon=14.9447, lat=77.16046. The great
circle distance between these points is about 5.4km, so even allowing for the
error incurred by using planar geometry, it should not be possible to
construct a 3922.84 metre segment that gives you trouble.
Of course this calculation is only valid for the given latitude.
I have no idea if what I calculated above is correct and would be interested
to hear other approaches to the question "what's the maximum way length to
ensure [EMAIL PROTECTED] compliance".
(I'm attaching a PNG sketch but don't know if it will make it through.)
To be a bit nitpicking: If the extension to one side is 50% and 17% to the
other, than 45 degree is not correct. Anyway that does not influence the
result a lot. To get a valid lower value you could use the two
extension values directly, as every line must be longer than the sum of
these two values. On the other hand you also need to take half of the
width of the lines into account, as well as the bezier curve hinting :-)
Ciao
--
http://www.dstoecker.eu/ (PGP key available)
_______________________________________________
Tilesathome mailing list
[email protected]
http://lists.openstreetmap.org/listinfo/tilesathome
