[madisfun]

Finding ocean is easy: water and gravity have formed the surface to have few local minima. It would be much more interesting finding peaks: there are many local maxima.

And the most important maxima are often surrounded by lots of local maxima. They may also have some steep slopes so gradient methods can easily overshoot. Sometimes the surface function is not even continuous (overhanging walls).

[iainmerrick]

water and gravity have formed the surface to have few local minima

From the article: "It turns out that the earth is filled with local minima"

(I suspect the deal is that you're right that water always flows downhill to the ocean; but that it often happens at scales too small to capture in the GIS data, or underground.)

[chrisfosterelli]

Yeah you got it! Water moves in ways that are not captured by the resolution of the elevation dataset. Also, water moves into/through the ground.