Comment by OscarCunningham | Hacker News Reader

Comment by OscarCunningham | Hacker News Reader

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OscarCunningham

They use height data rather than land/water data, and assume that there's just a cutoff at sea level. I think this gives them the correct answer for the water path, but it gets things wrong for the land path because there are a lot of lakes and rivers above sea level and also some dry land below sea level.

If you don't allow the path to cross any lakes or rivers at all then I think the land path has to be much shorter, since rivers makes it impossible to make any progress at all in most places. If you trace their path it definitely goes through several lakes. The optimal path is probably across a desert, my guess would be Antarctica.

If you do allow the path to cross lakes and rivers then I think there's a longer path than the one they give, starting in Liberia and ending near Fuzhou, China. They probably didn't spot this one because it passes too close to the Dead Sea, which is below sea level (and crosses the Suez canal, which is at sea level).

> They probably didn't spot this one because it passes too close to the Dead Sea, which is below sea level

This is adressed in the paper. The relevant part of the problem statement: "the longest distance one could drive for on the earth without encountering a major body of water"

and about the Dead Sea: "Guy Bruneau of IT/GIS Consulting services calculated [5] a path from Eastern China to Western Liberia as being the longest distance you can travel between two points in straight line without crossing any ocean or any major water bodies. However, the path crosses through the Dead Sea (which can be considered to be a major water body), and hence does not satisfy the constraints originally set out."

Oh, I didn't spot that they had already considered that path.

However I don't accept their defence.

1) Depending on environmental conditions, my path can cross the Lisan Peninsula. https://en.wikipedia.org/wiki/Lisan_Peninsula (EDIT: In fact there's enough clearance to just go completely south of the Dead Sea)

2) Their path crosses the Volga River, which is much larger in total surface area than the Dead Sea. And at the point at which they cross it, the Volga is just as wide as the Dead Sea.

But rivers don't fit in the conventional definition of "major body of water" even if their surface area is large.

It's odd to say you "don't accept their defense" when you're really just operating from a different set of assumptions in the first place.

>rivers don't fit in the conventional definition of "major body of water"

I suppose that's true. But it raises the question of what a river is. Lots of things that people would describe as lakes in fact have water flowing into them, through them, and out of them. So it's hard to distinguish them from a broad river.

Anyway, the distinction is moot since you can in fact squeeze just south of the Dead Sea and avoid hitting it at all.

Without looking up a formal definition, how I believe the common person would differentiate between a river and lake are the following:

    * Is the body of fluid, within a set of bounds, mostly still (and not flowing)?
    * Is it closer to a:
    * > bowl (hemisphere with cut side facing up)
    * > sliced cylinder (again flat slice facing up)
    * > sheet* (a large thin expanse)
    * Is the fluid at the bottom shifting location, how quickly?

Rivers tend to have high flow to volume ratios, faster moving flows at depth, are usually more shallow, and are defined more like a squashed tube in natural states.

Lakes tend to be deeper, more stable (slower flow, if discernible at all), and are generally placid. Lakes /usually/ have a large dimension in at least two surface directions while rivers usually have that in only one.

Exceptions to the above occur with canals/channels* (though that's an ocean thing) which might be closer to the fuzzy boundary.

River deltas also occur in high sediment deposit areas, such as the ends of rivers where they transition in to lakes / oceans; the extreme end of a river delta being bogs and other swamp like areas with shallow slow moving water. (I argue that such areas are neither river nor lake, but a third category.)

> It's odd to say you "don't accept their defense" when you're really just operating from a different set of assumptions in the first place.

Why is that odd? Their set of assumptions is exactly what he is criticizing.

I think of criticizing the defense as criticizing the argument or reasoning offered.

Put another way, if the problem is with the assumptions, the criticism goes far deeper than just that defense.

Is that a straight line? Maybe they allow crossing rivers but not seas/lakes.

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