We filter out very small polygons from the original data to remove
glitches. These glitches are caused by trying to subtract the water from
a polygon that includes some land and some water, but using two
different definitions or resolutions of mean high water line.
If we don’t do this then we end up with a bunch of very small polygons
which lie far outside the understood area of a place, causing large
overspill.
We need to increase the threshold for this process because we’re still
seeing this problem around Bristol and Norwich.
This does mean we lose a few very small polygons in places like Shetland
and the Scilly Isles, but not in such a way that we would avoid
broadcasting to them (because they’d still be caught by the
simplification and overspill).
Since the key relies on visual association between the shapes on the
maps and the styling of the key, it won’t work for non-visual users.
An alternative way of giving them the same information is by providing
the size of the area numerically.
We have a bunch of stuff for doing lat/long transformation in the
`BroadcastMessage` class. This is not a good separation of concerns, now
that we have a separate class for dealing with polygons and coordinates.
This commit does two things:
- uses our new polygon-simplifying library to process the polygons
before storing them, rather than processing them in real time
- stores only the polygons in the database, rather than the whole
GeoJSON feature, because we don’t need any of the other information
about the feature
Simplifying polygons means reducing the number of points used to render
them. This commit implements simplification such that, for any given
input polygons, the combined point count of the simplified polygons is
less than 100.
When simplifying the polygons we are trying to get the smallest number
of points while meeting these two rules:
1. No part of the area the user has chosen can be cut off
2. The area of the simplified polygon should be as small as possible
This commit introduces two techniques we weren’t using before:
1. Dilating and eroding the area to fill in concave details of the
shape, like inlets and harbours[1]
2. Making the simplification threshold proportionate to the perimeter of
all polygons, so bigger and crinklier polygons get more
simplification applied
It also shows the estimated bleed as a separate polygon. This lets us
make it bigger (so it’s more closer the the approximate bleed) without
having to send a bigger area to the CBC and compounding the amount of
actual bleed.
1. Inspired by this blog post about ‘removing the crinkley bits’ from
Vancouver Island:
http://blog.cleverelephant.ca/2010/11/removing-complexities.html
