![population density map us 2018 population density map us 2018](https://i.pinimg.com/originals/f4/45/54/f44554d374b7e933d3132ed741b83c68.gif)
It reveals the structure of the US population surface. It leaves areas where no-one lives devoid of data. The result is a map that pushes the data into areas where people actually live. Then I got the dot density renderer in ArcGIS Pro to draw the dots, one for each vote resulting in a map with nearly 130 million dots. The urban get 35% of the data and the rural polygons get 15% of the data. There's some weighting involved so the dense urban polygons get (in total) 50% of the data. I then did some data wrangling in ArcGIS Pro (more of that in a different blog) to reapportion the Democrat and Republican total votes at county level into the new polygons. I used the impervious surface categories and created a polygon dataset with three classes, broadly dense urban, urban, and rural. I used the National Land Cover Database to extract urban areas. The point of the map is to show where people live and vote rather than simply painting an entire county with a colour which creates a map that often misleads. Those different areas are, broadly, urban. It uses a technique developed by the late Waldo Tobler called pycnophylactic reallocation modelling. I'll be doing a proper write-up on the ArcGIS blog in due course but, in summary, a dasymetric map takes data held at one spatial unit (in this case counties) and reapportions it to different (usually smaller) areas.
![population density map us 2018 population density map us 2018](https://www.esri.com/arcgis-blog/wp-content/uploads/2019/03/us-6400.png)
So how to make the map? Well, it's a product of a number of decisions, each one of which propagates into the map. Part of what I do at work to push the software to see what it is capable of, to test it and to show others what capabilities it affords. So, for me, the map is a technical challenge. Now, using ArcGIS Pro I can use the dot density renderer and let the software take the strain and if I were going all out then why not try and make a map where 1 dot = 1 vote.
![population density map us 2018 population density map us 2018](https://www.vividmaps.com/wp-content/uploads/2017/12/tW7YsWm.png)
For 2012 I had to generate up to 12 million points and position them. It took 3 months to cajole the largest scale map onto the web!!! I wanted to update the map and the four years that have intervened have brought new software capabilities. At the largest, 1 dot = 10 votes and if you printed the map out it would be as large as a football field. At the smallest scale 1 dot = 1,000 votes. Made using ArcMap (full disclosure for those who don't know I work for Esri - who make ArcGIS). It was a product of the web mapping technology of the time. In 2012 I made a similar mapĀ for the Obama/Romney election.