Ann Jackson’s husband Josh (@VizJosh) set the challenge this week, to build an ‘application’ to help visualise the scale of wildfires; that is when a fire is said to be 5000 acres, you can use the app to view how that compares to an area of the world you may know, so you can really appreciate just how large (or small) the fire is.

I have to hold my hand up here, and say that after reading the requirements several times, I was absolutely stumped as to where to start. We were provided with some ‘data’ to copy which consisted of 5 rows, which I duly copied and pasted into Desktop, but I then like ‘what now….?’ I knew I needed something geographic to build a map, but couldn’t understand the relevance of the 5 rows… I’ve said before I don’t use maps that often, so was unsure whether there was something I needed to do with this data. After staring at the screen for what seemed like an age, I ended up looking at the solution.

The data is just ‘dummy’ data and is just something to allow you to ‘connect’ Tableau to. You can’t build anything in Tableau without a data source. It could just have been 1 row with a column headed ‘Dummy’ and a value of 0. If it had been that, it might have been more obvious to me 🙂

• Defining the parameters
• Building the map
• Apply button
• Dashboard Actions

Defining the parameters

Ultimately the ‘data’ being used to build the viz is driven by parameters – the Location selector and the Latitude & Longitude inputs.

pLocation

An integer list parameter that stores values, but displays worded locations – wherever you choose. I opted for my hometown of Didcot in the UK alongside locations Josh had used, mainly so I could validate how the rest of the ‘app’ would work when I came to build it.

pLongitude

Float input, defaulted to the longitude of location 1 (ie Didcot) above.

I just googled Didcot Latitude and Longitude to find the relevant values

Note – Longitude W means an input of -1 * value. Similarly for Latitude S needs to be a negative input.

Then I created

pLatitude

Since we’re talking about parameters, there’s a couple more required, so lets create them now

Acres

Integer parameter defaulted to 5000

pZoom

Integer, list parameter with the values below, defaulted to 2.

Building the map

Now we have some lat & long values (in the pLatitude and pLongitude parameters), we can create some geographic data needed to build a map.

Location

MAKEPOINT([pLatitude], [pLongtitude])

This gives us the centre point which we want to build the ‘fire size’ buffer around. For this we need the calculation JOsh kindly provided :

Acres to Feet

SQRT(([Acres]*43560)/PI())

and then we can create the buffer

Fire Size

BUFFER([Location],[Acres to Feet],’ft’)

Double click on this field and it should automatically create you a ‘map’

Adjust the map ‘format’ via the Map > Map Layers menu option. I chose to set it to the dark style at 20% washout, then ticked various selections to give the details I needed (I added and removed options as I was testing against Josh’s version). I also set the colour of the mark via the Colour shelf to be pale red.

Also, as per the requirement, turn off the map options via Map > Map Options menu, and unchecking all the selections.

So this is the basic map, and you can input different lats & longs into the parameters to test things out.

Now we need to deal with the zoom requirement.

I wasn’t entirely sure about this, so had a bit of a search and found Jeffrey Shaffer’s blog post How to create a map zoom with buffer calculation in Tableau – bingo!

The zoom had to be x times the size of the circle on the map, so achieved by

Zoom

BUFFER([Location],[pZoom] * [Acres to Feet],’ft’)

Add this a map layer (drag field onto the map and drop onto the Add a Marks Layer section that displays)

This has generated a 2nd circle and consequently caused the background map to zoom out. We don’t want this circle to show, nor to be selected, so on the Colour shelf, set the Opacity to 0%, and the Border and Halo to None. To prevent the circle from showing when you hover your mouse on the map, you need to Disable Selection of the Zoom marks card

Apply Button

On a separate sheet, double click into the space below the Marks card, and type ‘Apply’ into the resulting ‘text pill’ that displays, and then press return.

This will create a blue pill, which you can then add to the Label/Text shelf. Align the text to be middle centre

This view is essentially going to act as your ‘Apply’ button on the dashboard. When it is clicked on, we want it to take the Lat & Long values associated to the place listed in the pLocation parameter, and update the pLatitude & pLongitude parameter values.

For this, we need a couple of extra calculated fields

Location Lat

CASE [pLocation]
WHEN 1 THEN 51.6080
WHEN 2 THEN 40.7812
WHEN 3 THEN 51.5007
WHEN 4 THEN 48.8584
END

Note – as before, all these values were worked out via Google as shown above.

Location Long

CASE [pLocation]
WHEN 1 THEN -1.2448
WHEN 2 THEN -73.9665
WHEN 3 THEN 0.1246
WHEN 4 THEN 2.2945
END

Add both these fields to the Detail shelf of the Apply sheet.

Dashboard Actions

When you add the 2 sheets to the dashboard, you then need to add parameter actions to set the values of the pLongitude & pLatitude parameters on click of the Apply button

Set Lat

A parameter action that runs on Select of the Apply sheet, setting the pLatitude parameter with the value from the Location Lat field.

You need another action Set Long which does a similar thing by passing the Location Long field into the pLongitude variable.

Finally, you don’t want the ‘Apply’ button to look ‘selected’ (ie highlighted pale blue) once clicked. Create calculated fields True = True and False = False and add both of these to the Detail shelf on your Apply button sheet.

Then add a dashboard filter action that uses Selected Fields and maps True to False

Hopefully, this should provide you with all the core features to get the functionality working as required. Ultimately once I got out of the starting blocks, it wasn’t too bad…

My published viz is here.

Happy vizzin’! Stay Safe!

Donna