Where were COVID-19 cases most active?

In this week’s #WOW2022 instalment, Luke set this map-based challenge to visualise COVID-19 cases across the continental US states on 30 December 2021.

Each ‘pyramid’ on the map represents a county within a state, with the height and colour of the pyramid indicating the volume of cases recorded.

Luke provided a workbook with some pre-modelled data as a starting point, so I used that.

To familiarise myself with the data I built out a basic table, focusing on the State of Idaho (ID) only.

Clat10 and Clon10 are the latitude and longitude positions of the County and metrics.caseDensity is the key measure used to indicate the volume of cases.

To draw the pyramid, we need to plot 3 marks for each County, 1 for each point in the triangle. This means we need to densify the data so we have multiple rows for each County. If we add Point as a discrete dimension (blue pill) to the table, we can see we can generate up to 20 rows per County.

This is obviously excessive – we only need Points 1-3, so for simplicity, we can add Point to Filter and limit to just 1, 2 and 3. I added the filter by duplicating the Point pill from Rows (hold ctrl and then click and drag the pill from Rows to Filter – this added the filter as a discrete dimension and I could just select options 1, 2, 3).

With only 3 points, we can now build the lat and long coordinates for each point. I am assuming point 1 is the bottom left point, 2 is the top point, 3 is the bottom right.

Triangle Long

CASE [Point]
WHEN 1 THEN [Clon10] – 0.1
WHEN 2 THEN [Clon10]
WHEN 3 THEN [Clon10] + 0.1

As the requirements stated the base of pyramid was 0.2 longitude points wide, then the bottom left and bottom right points need to be 0.1 points to the left or right of the County longitude, while the peak is aligned centrally.

Triangle Lat

CASE [Point]
WHEN 1 THEN [Clat10]
WHEN 2 THEN [Clat10] + ([metrics.caseDensity]/60)
WHEN 3 THEN [Clat10]

The latitude of the left and right bottom points are aligned with the County latitude, while the height is an additional 1/60 of the metrics.caseDensity value above the County longitude.

Change both the Triangle Lat and Triangle Long fields to be of the Geographic role type of Latitude or Longitude accordingly (right click field -> Geographic Role -> Latitude/Longitude).

Adding these into the table, we can see how the coordinates are forming

So with this, we can now build the map

Add State to Filter and exclude AK, HI, PR (Alaska, Hawaii, Puerto Rico).

Double click Triangle Lat then double click Triangle Long to add them to the sheet – a map should automatically be presented.

Add County to Detail, and a mark should appear for every County.

Add Point to Detail and change to a dimension – there should now be 3 marks per County forming triangles. Zoom in to really see this.

Change the mark type to line and the points all join up to form the pyramid.

Add metrics.caseDensity to Colour, then edit the colour. Choose the red-gold colour palette, then click the Advanced button, and first change the centre to 75, then set the start & end value to 0 and 200.

Modify the Tooltip and reduce the Size of the line. Zoom back out

Finally amend the map background ( Map menu -> Background Layers). Uncheck base and land cover, check terrain and coastline. Uncheck Country/Region Names and State/Province Names.

Hide the null indicator and then place on a dashboard. Done!

My published viz is here.

Happy vizzin’!



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