Where does the River Thames end and the North Sea begin? One of the things I love about undertaking categorisation work is that it throws up questions you never really thought about deeply before. But in continuing my tiling of postcode sectors along the coast I have to work out where to stop. And the answers can often reveal something you didn't know before.

What is the coast, anyway?

So we have the sea, and we have the land, and that point where the two meet is the coast. An easy enough concept to explain to any 5 year old, and by extension any chief executive. That should make my job easy - if a postcode is bordered by the sea then it is a coastal sector and should fall under that category. Otherwise it is in-land and is allocated to a non-coastal category.

Easy.

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But easy definitions are often the most troublesome.

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Postcode sectors cover different shapes and sizes of land and are not organised in the way that suits my categorical definitions. Some sectors have quite a thin footprint against the sea and go quite far inland, whilst others do the opposite. So if we just use adjacency to the sea as our guide then we get an inconsistency where some houses are judged to be in a coastal category when they are much further from the sea than another house who is in a sector which doesn't.

So we have to update our definition - but to what? How far inland can a sector be before it is no longer coastal?

No, really, what is the coast?

One of our guiding principles from part 1 was that a sector should be categorised by what a random person in that sector would say was the defining feature of where they lived. We can use this to help create our definition as being not just geographical but also people-centric. The area should be set up to, in some way use (or cope with) the sea.

That could mean beachfront properties with a good view. It could be a bustling seaside with tourist attractions and amenities. It could be a fishing port. How it is used doesn't really matter, but the presence of the sea should dictate how the land adjacent to the sea is used. And we can go in-land as far as we like until that stops.

I really couldn't think of any better way to define it than this: a coastal sector is one that 'points' at the sea.

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It sort of makes sense, and it sort of doesn't. But maybe that is part of the power of what AI can do? Unlike humans it doesn't need to be able to put into words what it sees, and it doesn't care about looking silly with a definition that doesn't sound concrete. It knows what it means in the same way I do, and then just gets on with it. So that's what I'm going to do - if it 'points' at the sea it is coastal.

Where does the Thames end?

Growing up in deepest darkest Somerset it was impossible to not learn what a tidal river was because most years they flooded the fields as far as the eye could see. Here's Glastonbury in 2014.

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I lived in a small town called Somerton that is about 4 miles from an even smaller town called Langport. It's an Anglo-Saxon town who's name means the long port. It is suggested as the site for the Battle of Llongborth in the Welsh medieval poem Geraint son of Erbin. And Longphort is an Irish word that means a Viking ship enclosure.

One small issue - Langport is 16 miles from the sea.

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The reason for this is because Somerset is very flat, so it didn't take the sea level falling by very much to uncover a lot of land. We see this nearly every year where if you are on one of the few hills looking over the farmland you can see flooded fields to the horizon, to the point that you would want sunglasses to drive to avoid the glare of the sun reflecting off the water. Heavy rainfall naturally runs into the rivers, but the rivers in Somerset do not quickly flow into the sea because they too are very flat. Typically this is about a 20cm drop per mile, or an angle of 0.007 degrees. For context your house building regulations allows for a maximum drop of 4mm / meter, or 0.23 degrees.

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These rivers are therefore so flat that when the sea level rises with the tide so does the river height. This combined with a lot of rain causes mass flooding. That's often manageable in Somerset because most of the towns are quite old (Anglo-Saxon or earlier) so were built according to the old coastline and therefore not in the places where it floods a lot. Of course not everyone is so lucky.

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But do you know where else has a tidal river, and this time with a large urban population? Yes, London! From Teddington in west London all the way to the sea the Thames height is affected by the tide. To protect from flooding there is the Thames Barrier, which can close if the tides of the North Sea get too high.

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Get to the point, Dan!

So, here we are. I've completed the sectors around Kent and am heading towards London and reach the Isle of Sheppey, on the south side of the Thames opposite Southend-On-Sea. Technically, geographically, we are now on a riverbank and not the coast. But everything still 'feels' coastal.

If I keep going I will get to the Thames Barrier. And whilst we are now deep upstream into a river it is a river that rises and falls with the tide, and is just one catastrophe away from being the new coastline. And there are plenty of companies that are using the area to dock boats, with the Thames Barrier being level with the Royal Albert docks (and downstream of Grenwich and Canary Wharf). Is this the right place to stop?

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Ultimately there is no single definitive answer, and I once again have to fall back to the mantra of saying that my categorisation is based on what the average person there would categorise the area as being.

The Thames Barrier feels just a bit too far. Whilst there are some coast-like features (a big dock) the river feels more like a barrier than a feature. Nothing here is 'pointed' towards the water, but rather it is backed up against it as far as it can go. Here the river might as well be any barrier - a forest, a cliff, even a motorway.

Similarly the technical start of the river feels too soon. We have beaches and coastal walks far upstream of that point.

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So the right stopping point for me is somewhere in between - Gravesend. There is a quite sudden change in the landscape, where everything to the west is industrial, and everything to the east rural. How the land interacts with the water changes quite suddenly.

Conclusion

That's probably way more effort than I should have put in to answering the question of what is the coast, but it is important to create that consistent framework. I now know that I should only classify a postcode sector that is near the sea as coastal if that sector is making use of being on the coast - or in more abstract terms is 'pointing' towards it.

And that's the power of this sort of categorisation work. It is about taking a large number of factors, which are not easy to define or structure into a dataset, and trying to condense it down to a smaller number of groups which have a likeness that you would not otherwise expect. Hopefully it might even predict something.

Next stop - Skegness.

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