## The Buffer Function in Tableau

by Harriet Owen

In the 2020.1 release of Tableau a new function, Buffer, was added which allows you to create a radius (or boundary) around a specific spatial point. This could be used to either see where boundaries overlap or see what points are situated within a radius. Within this example I am going to delve into the latter point to see what bus stop locations exist within a 1 mile radius of various potential housing locations within London.

But first, let's take a look at what is required within the Buffer function.

1. Geometry field: this is a spatial point which may already exist within your dataset. If not you will need to use the MakePoint function using your latitude and longitude fields.
2. Number: this is the size of the radius you want to create.
3. Units: decide the unit measure of the number (miles, km...).

To create the visual, two datasets are required. The first is a small data of mocked up locations and their latitude and longitude points like so:

The second is the different bus stop names and again, their latitude and longitude values:

Therefore, because both datasets don't have a singular field containing spatial point data, both will require the use of the MakePoint function.

Step 1: Joining the two datasets together.

• Connect to both datasets in Tableau and create a join between them
• The type of join you want to create is an 'intersects'
• The intersect join will need to be made on calculations from both datasets
• The calculation of the housing location datasets is the following:
• The calculation of the bus stops dataset is the following:
• This type of join is therefore saying, return bus stop locations that intersect within the housing location buffers and should look like the following:

Step 2: Create House Catchment and Bus Locations calculations

• We need to take the same calculations written in the join clauses above and create them as calculated fields within our new dataset. This will allow us to create the desired spatial points needed for analysis
• Therefore, the same calculation in the 'Housing Locations Dataset Join Calculation' image above must be created as a calculated field named 'House Catchment'
• Secondly, the same calculation in the 'Bus Locations Dataset Join Calculation' image above must be created as a calculated field named 'Bus Locations'

Step 3: Create visual showing bus locations within a mile of potential housing location

• Double click the House Catchment field just created and Tableau will automatically populate a map view of different circular boundaries as well as a generated latitude and longitude on rows and columns
• Next, drag the Bus Locations field also just created onto the map view and an 'Add a Marks Layer' will appear. Drag it on top to add it as a layer and your spatial points within the 1 mile buffer (boundary/radius) will populate

As identified earlier, there are different ways to use and manipulate the buffer function that can and should definitely be explored. Nonetheless, the function definitely adds a more advanced level of spatial analysis!

Harriet Owen

Wed 02 Jun 2021

Thu 13 May 2021

Wed 05 May 2021