[OSM-dev] coastline self-intersections - added to the error checker
bsupnik at xsquawkbox.net
Mon Sep 7 19:17:18 BST 2009
Martjin has added self-intersection checks to the coastline error
checker. :-) That's what those purple dots are. Of course since we've
never looked for them before there are now about 6000 new errors.
If anyone ever finds a false positive self-intersection, please email
me, I will investigate. The intersection math is floating point so it
could fail for very near "grazings" but given the size of the earth and
the limits of double-precision floating point, a near-miss that triggers
the coastline is probably creating an insanely small "pinch" of water,
smaller than would be useful.
(The coastline generation process also reduces data density, which has
the potential to collapse such a small pinch.)
Currently self intersections do not create total map rendering failures.
The major map renderers appear to use a "toggle" policy toward
polygons, thus a figure 8 closed polygon is filled in its two enclosed
areas. Self-intersections render the notion of "one side is wet, the
other is dry" impossible of course.
There are self-intersection tests in some of the other quality-assurance
tools, but I think a lot of them are available only in Europe. (Perhaps
that is why the US coast has so much more purple than Europe?)
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