Big Sky Country Part 2: Geographic Boogaloo

July 6, 2019 — Stephen Hudak

I had leftover pasta yesterday. It was ok. It was neither bad nor anything to write home about.

As I was pondering the mediocrity of the pasta, I realized the previous day was also leftover pasta day, and I had very similar thoughts about my pasta.

I felt trapped in some sort of cosmic cycle I couldn’t break out of. Every day I get hungry. And every day I eat—obsequiously marching a beat of which I have no control.

“Today will be different.” I thought, “I will take control and do something I keep putting off for another day.” And with that, I decided to write “Part 2” to my blog post Is Montana’s “Big Sky” Bigger Than Your Sky?.

In the first post I proposed a two-part explanation for why the sky seems bigger in Montana (thus the nickname “Big Sky Country”):

  1. With lots of local relief (elevation differential) you can see over obstacles
  2. Humans use visual cues to judge scale at distance.

This seemed to make sense to everyone who gave me feedback on my first post. But as I stated at the end of the post, I would eventually like to quantify this explanation to give it a little more weight.

According to my logic in Part 1, the area a person can see from a random point in Montana should be greater than the area a person can see from a random point in Illinois. Therefore, the GIS-testable hypothesis is:

The aggregate viewshed area of a random sample of 100 points in Montana will have a greater area than the same sample in Illinois.

Not that difficult to accomplish. I pulled some state data and cut out Montana and Illinois as separate feature classes. From there I used the Create Random Points tool to generate 100 observer points for each state via a notebook.

With the data available it was just a matter of running the Viewshed analysis tool which by default uses 90m DEM data hosted by Esri.

An example of a Montana viewshed polygon the tool generated:

And an example of an Illinois viewshed polygon the tool generated:

The sum of viewshed areas for Montana is 1,228 square miles. The sum of viewshed areas for Illinois is 380 square miles, which confirms my preconceived notion and therefore I accept it uncritically.

Thinking about the results a bit, it even appears the odds are stacked in Illinois’ favor making this the only time that phrase has been written for decades.

The viewshed analysis doesn’t account for trees or buildings, meaning the true viewshed is probably much smaller than this analysis gave for the Illinois samples.

I originally looked for my own DEM data with a high resolution but gave up and opted to use Esri’s service. During this search I read about a project with some government agency to have a national lidar dataset available by 2023. With that, I am ready to announce, here and now, the 2023 release of a Part 3 to account for trees and buildings.

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