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You're right, creating a map like that is working with watersheds, and yes, it could be done in Manifold. Not only that but Manifold would be an especially good tool to create such maps. The continental divides are the ridges which are the boundaries of the largest scale watersheds in North America. If you have terrain elevation data for North America you can create watershed areas and then convert the watershed areas into the boundary lines of those areas. Do some clean-up and some editing to get the display seen in the map above. Like many such maps it's a simplification. Topics: Watersheds, Example: Create Watershed Areas, Watershed Areas, Sinks, Upstream Areas and Lines. If you look at the red lines indicating the Great Divide, rain that falls to the west of the line flows west into the ocean/seas, and rain that falls to the east of the line flows east to the ocean/seas. The brown line indicating the Great Basin marks a sink, where water that falls into that sink stays in the sink and does not flow to the ocean. I suppose the closed red loop lines in the Great Divide also indicate sinks. There are many ways of creating a map like this from original data but they all start with creating watersheds from terrain elevation data. Create watersheds that accumulate very large amounts of water to get big watersheds (a map like this doesn't care about small watersheds). From there you can pick out the mouths of major rivers and use Upstream Areas to find all the watersheds that flow into that river. For example, I suppose the Rio Grande and the Mississippi river mouths would pick out many of the watershed areas that abut the Great, Laurentian, St. Lawrence and Eastern Divides around the central part of the US. I think the essential thing to understand about the map is that somebody interpolated it by eye, not by some universal, perfect algorithm, so I wouldn't spend a lot of effort trying to figure out some way to completely automate it. If you know southern Florida, for example, you know that's not a realistic proposition given the flatness of the Everglades terrain to the south of Lake Okeechobee, where drawing a neat yellow line like shown in the map is more a matter of opinion than perfect physical reality. There's also the reality that wildly varying resolutions of terrain elevation data across a region as big as North America aren't going to result in a single, neat, algorithmic solution. I think the way the map was drawn was to search the literature on traditional continental and regional divides drawn over the years by geographers who were familiar with each region and to then create a reasonable composite of their maps. It probably wasn't somebody sitting down with high resolution terrain elevation data to compute watersheds. But if you wanted to create a map from first principles you could do that, and Manifold would certainly be a good choice because it has the horsepower (make sure to have a GPU in your system) to do the watershed computations over lots of terrain elevation data. Or, if you wanted to do it the more traditional way, take the image you posted and georeference it as a layer. Next, use your favorite AI or a traditional search engine to find online images of maps of continental and regional divides in various regions. Georeference those, stack them up in a map together with the image you posted, and then draw your own lines that reconcile/interpolate to your taste the various proposed divides you found online.
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