Dividing up the Continents

Friends, I wanted to share with you a project that I recently completed: Continental Divides, a series of six 42 × 51cm (16.5 × 20.1in) cyanotype posters.

We’ll dive into the details, below, but first: you can indeed buy copies of any (or all) of these if you’d like. Each one is hand-printed, so there will be some variations from print to print, and yours won’t look exactly like the ones above.

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Update: I’ve also made a series of smaller (23 × 19cm) versions as well!

Background

Roughly, continental divides are lines that separate the rivers that flow into one ocean/sea, from the rivers that flow into another. So, for North America, there are the lines demarcating which waters flow into the Pacific, the Gulf of Mexico, the Atlantic, etc. There are also various endorheic basins — places where the rivers do not reach the sea.

The choice of which divides to show on a map can be somewhat arbitrary. The water that flows into Gulf of Mexico eventually flows into the Atlantic Ocean, for example, so is it really fair to separate them? And, of course, all seas and oceans flow together in the end. The lines I’ve drawn are simply one interpretation, and not the only way it can be done.

While North America’s continental divides are depicted on many maps, I was surprised to find that this concept, as near as I can tell, isn’t often applied to other continents. Searches for the term “continental divide” mostly turns up results related to North America. Maybe that’s just because I’m performing my searches from an IP address in the United States. Or maybe people on other continents just don’t find this concept as interesting to map out.

Method

For some years, I’d had an idea of how I wanted to depict continental divides: as uniform ridges, sloping evenly down to the sea. That’s an abstraction; in reality, divides can be great mountains, or they can be barely-noticeable bumps, all depending on where you are.

It took a few intermittent failed experiments before I happened upon a way to make the process work, and it turned out to be simpler than I had expected. Here’s how it works:

First, I grabbed vector data for the divide lines — these came mostly from HydroSHEDS, though it took a fair bit of manual selection and adjustment to get the lines that I wanted. These are the high points for the final map. For the low points, I grabbed ocean vectors from Natural Earth, to which I added some hand-drawn lines that went roughly through the centers of the endorheic basins.

I rasterize those two layers, and for each, I create a proximity raster. So now I have two datasets: distance to the high areas (the divides) and distance to the low areas (the seas).

Then I divide the sea proximity raster by the sum of the two proximity rasters.

This gives our slopes running from the high point of the ridges to the low point of the seas (or endorheic basin centers). From there, I can use this as a DEM and generate a shaded relief in Blender, clipped to the shape of the land.

And finally, I apply some gratuitous halftones. They’re sized so that you can’t see them from a distance, but when you get in close, they’re large enough to be obvious. It’s an effect that I quite enjoy.

Each poster is then printed in the sun using the cyanotype process, on some nice kozo from Awagami Factory.

I think the end result is a fun and interesting way to think about the relationship between the land and the sea. Partly art, partly educational. And if you’d like one to take home, here is that button again.

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