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.
We’ve observed people using ‘national’ and ‘local’ during user research.
It has less tongue-twisting ambiguity than county vs country.
But we think that maybe just getting rid of ‘counties’ is enough to
disambiguate them. So this commit just takes the ‘local’ concept.
This commit also gives the libraries and areas new IDs, which means if
we want to rename them in the future it won’t be a breaking change.
Broadcasting is not a precise technology, because:
- cell towers are directional
- their range varies depending on whether they are 2, 3, 4, or 5G
(the higher the bandwidth the shorter the range)
- in urban areas the towers are more densely packed, so a phone is
likely to have a greater choice of tower to connect to, and will
favour a closer one (which has a stronger signal)
- topography and even weather can affect the range of a tower
So it’s good for us to visually indicate that the broadcast is not as
precise as the boundaries of the area, because it gives the person
sending the message an indication of how the technology works.
At the same time we have a restriction on the number of polygons we
think and area can have, so we’ve done some work to make versions of
polygons which are simplified and buffered (see
https://github.com/alphagov/notifications-utils/pull/769 for context).
Serendipitously, the simplified and buffered polygons are larger and
smoother than the detailed polygons we’ve got from the GeoJSON files. So
they naturally give the impression of covering an area which is wider
and less precise.
So this commit takes those simple polygons and uses them to render the
blue fill. This makes the blue fill extend outside the black stroke,
which is still using the detailed polygons direct from the GeoJSON.
