If you’re adding another area to your broadcast it’s likely to be close
to one of the areas you’ve already added.
But we make you start by choosing a library, then you have to find the
local authority again from the long list. This is clunky, and it
interrupts the task the user is trying to complete.
We thought about redirecting you somewhere deep into the hierarchy,
perhaps by sending you to either:
- the parent of the last area you’d chosen
- the common ancestor of all the areas you’d chosen
This approach would however mean you’d need a way to navigate back up
the hierarchy if we’d dropped you in the wrong place. And we don’t have
a pattern for that at the moment.
So instead this commit adds some ‘shortcuts’ to the chose library page,
giving you a choice of all the parents of the areas you’ve currently
selected. In most cases this will be one (unitary authority) or two
(county and district) choices, but it will scale to adding areas from
multiple different authorities.
It does mean an extra click compared to the redirect approach, but this
is still fewer, easier clicks compared to now.
This meant a couple of under-the-hood changes:
- making `BroadcastArea`s hashable so it’s possible to do
`set([BroadcastArea(…), BroadcastArea(…), BroadcastArea(…)])`
- making `BroadcastArea`s aware of which library they live in, so we can
link to the correct _Choose area_ page
At the moment there are some areas which have:
- a `count_of_phones` value of `None`
- no sub-areas
This is wrong, but until we fix the data the phone counting code needs
to handle this.
This commit:
- adds the `or 0` in the right place (where it will catch these areas
with missing data)
- adds a test which checks these areas, and compares them to other kinds
of areas
This is a better name for the module because it’s:
- not just constants, there’s a method in here now
- only stuff to do with populations, not other kinds of constants
We need to give people a better feel for the consequences of
broadcasting an alert. We’ve seen in research that some users will
assume it is subscription based, or opt-in, rather than going to every
phone in the area.
I reckon that the most effective way to communicate this is to put some
numbers next to the areas, to give people an idea of how many people
will get alerted.
We can estimate how many phones are in an area by:
- taking the population of all electoral wards in that area
- multiplying it by the percentage of people who own an internet
connected phone[1]
The Office for National Statistics publish both these datasets.
The number of people who own an intenet connected phone varies a lot by
age. Since the population data for each ward is broken down by age we
can factor this in. Simplified, the calculation looks like this:
- take the _Abbey_ ward of _Barking and Dagenham_
- in this ward there are 26 people aged 80
- 40% of people over 65 have an internet-connected phone
- therefore 10 of these 80-year-olds would be likely to receive a
broadcast
- (repeat for all other ages)
These numbers won’t be exact, but should be enough to give people a feel
for the severity of what they’re about to do. We can see if they acheive
this aim in user research.
1. This is a proxy for the number of people who are likely to have a 4G
capable phone, because only 4G capable phones will be receiving
broadcasts to begin with
What was previously ward -> local authority is now a ward -> local
authority -> county. County only covers rural counties and not
metropolitan boroughs and other unitary authorities. Previously, there
was a page full of local authorities (unitary authorities and
districts), and each one of those would have a list of electoral wards.
However, now there are counties that contain a list of districts - so
this needs a new page - a checkbox for "select the county" and then a
list of links to district pages.
If you want to select multiple districts, you'll need to go into each
one of those sub-sections in turn and click select all.
Needed to tweak the query to retrieve the list of areas in a list for a
library. Previously, it just returned anything at top level (ie: didn't
have a parent). However, rural districts now have parents (the rural
counties themselves). So the query now returns "everything that isn't a
leaf node", or in more specific terms, everything that has at least
other row referring to it as a parent. So no electoral wards, since
they dont have any children, but yes to districts and counties.
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
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.
Now that the data needed to create a `BroadcastArea` is pretty
lightweight because it doesn’t include the GeoJSON we can go back to
putting it in memory when we start up the app, to make the pages load
really fast.
Rough estimate for the size of this dataset:
> 10,000 areas
> Average length of area name = 20 characters
> Average length of area id = 20 characters
> Size of one area in bytes = 20 + 20 = 40
> Size of dataset = 40 * 10,000 = 400,000 bytes = 400kb
Rather than querying all the features whenever we look up area(s) let’s
only get them when we need them.
The features are really big blobs of data to pass around, so there’s a
significant performance gain to be had from doing this.
