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July is Cal Poly Month here at Weekly Map. This map was made by an undergraduate student in Cal Poly’s data science program.

This map is a choropleth of the counties of California. (Do you need a refresher on what a choropleth is? Visit our “Basics” page for a quick primer.) On this map, each county is colored in accordance with a particular statistic. Your job is to figure out what statistic is represented on this choropleth.

Stumped? Check back Tuesday, Wednesday, Thursday, and Friday for hints about where to focus your investigation. The answer will be posted on Monday, July 10. Good luck!

Tuesday’s first explanation: One thing to note about this choropleth is that the statistic in question is an absolute number, rather than a percentage or ratio. We don’t normally use choropleths to map absolute numbers, but the rules are a bit different for Cal Poly Month. So you’re looking for something of which the most can be found in the darkest counties and the fewest in the lightest counties.

Tuesday’s second explanation: An earlier version of this text warned you that something was a bit wonky in the Channel Islands, which had been colored differently. Since that time, Prof. Sun has found the error in the student’s Python code and produced a corrected map, which now correctly lumps each of the islands in with the county to which it actually belongs.

Tuesday’s hint: Some of you have noticed that there is a clear correlation between counties that are more mountainous and counties that are darker on this choropleth. It’s definitely the case that many counties in the Sierra Nevadas are the darkest on the map. And some that contain parts of the Coast Ranges are a little bit lighter, while the Central Valley tends to be the lightest. But then you have to wonder what’s going on with the Channel Islands, which are quite low, yet still a little bit green. So the question you have to ask yourself isn’t “where are the mountains?” but rather “what happens in moutainous areas that also might happen on an island?”

Wednesday’s hint: If you explore some of the counties that appear very dark on this choropleth, you’ll discover that one thing many of them have in common is the presence of a national park. For example, Tulare County contains parts of Sequoia National Park and Kings Canyong National Park. San Bernardino County contains parts of Joshua Tree National Park and Death Valley National Park. All three counties that share Yosemite National Park are fairly dark on this map. And the connection with national parks even explains why there is some green in the Channel Islands and on the northern coast of California. What might you find in a county that contains some or all of a national park?

Thursday’s hint: Since this choropleth is about absolute numbers rather than percentages or ratios, it’s important not to overlook small counties. Yosemite National Park is divided among three counties (and borders a fourth). All four of those counties show up fairly dark on this map, even though they’re small. Imagine how dark they would be if those four counties were to merge! Yosemite, of course, is very popular—in fact, it’s the third most visited national park in the entire United States (after Great Smoky Mountains and the Grand Canyon). The darkest counties on this map are not just ones with lots of federally protected land, but also ones in which that land attracts a lot of visitors.

Friday’s hint: This map has to do with a particular activity. It’s one you can do in Marin County—for example, in the Port Reyes National Seashore or the Muir Woods National Monument—but not in the more urban environment of San Francisco on the other side of the Golden Gate.

Answer: Click here to see an explanation of the answer to this week’s map question.

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