Map Context without Clutter

by Adam Ratcliffe

When using a map in tableau it often cleans up the view to washout the map layer entirely to just have the relevant areas you are looking at. The USA is a classic example as it has a widely recognised shape.

Nice and simple, easy to see its USA no need for Canada and Mexico to be getting in the way of this All-American chart.

But sometimes you don't have fields like State or County etc. Sometimes you have very specific points you make from a longitude and latitude. So in this case washing out the map layer does this.

Where do you think that is?

Okay it's London and maybe you guessed that but then tell me this, where is this point?

There's no way you know where that is other than "In London" and we're not here for vague guesses. Let's give people a proper idea of where things are. We could try just a normal map layer underneath.

I guess this is better but all the place names are obscured and the points are becoming harder to see and if they were any bigger we'd lose even more place names.

So the solution I propose to get a result more similar to our USA map at the beginning is to join our data with some spatial file that outlines London. Spatial files of most places will exist on the internet so this applies to anywhere.

For this we'll use London boroughs, get the shape file here.

To join this to our current data we'll have to do a particular join calculation.

We join the two files on the basis that the points we make intersect with the polygons of the boroughs in the spatial file. Which of course they do as this is all London data.

That middle part where it says 'Intersects' will usually be '=' so make sure to change that once you have the calculation written out. This is all for data with Latitude and Longitude to make these points in a view.

Once you have this join done open a new sheet and double click your Geometry field from the London borough file.

It should then look like this. Go to map layers and make washout 100%. Change the colour in the marks card to 0% opacity. Then add your points in as a new marks layer.

A few final touches like putting the name of the borough (or whatever polygon) onto details so there is one mark for each.

Now we have the unmistakable River Thames running through our London map. If we wanted to know where that point from before it we can put name labels on our boroughs or in our tooltips.

Just a nice way to bring context without cluttering up a map.

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Adam Ratcliffe

Fri 04 Jun 2021

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