
Mysterious Detour While Driving? It Could Be Due to the Curvature of the Earth - jackgavigan
http://www.travelandleisure.com/articles/gerco-de-ruijter-grid-corrections-highways-driving-wichita
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danpat
I work on vehicle routing. I've seen mysterious detours that ended up being
cause by using an imprecise segment length equation.

Because the earth isn't a sphere, the common length-on-a-sphere great circle
distance equation isn't perfect for the earth. However, it's a lot faster to
calculate than the haversine length, which takes into account the squashed
sphere that we live on.

We were calculating driving routes at high latitudes. The difference in the
length calculation meant that east-west road lengths were being calculated as
slightly longer than they really were.

Amortized across a long-distance route, this led to the mysterious tendency of
the shortest-path algorithm to prefer north-south oriented roads a lot more
often than you'd expect.

While we knew we were using the less precise great-circle distance, the
deviation is not all that big, so we didn't really expect the difference to
manifest in the output.

 _Those_ were some mysterious detours.

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edge17
wow, thanks for sharing that story.

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imglorp
I bet there's a number of grid corrections due to historical subdivision, not
cartography.

The first land developers like George Calvert and William Penn and more going
westward had large tracts of land to subdivide into smaller and smaller units
but they didn't all make the same choices. Eventually roads within needed to
meet and corrections made.

In the East, almost every town has a "County Line Road" and more times than
not, it's crooked and met with a number of dogleg corrections.

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pierrec
It should be easy to identify those that are due to curvature, though. My
understanding is that states parceled according to the PLSS have fairly
consistent township and section divisions, made using a system that takes the
earth's curvature into account in a systematic way. This method introduces
"corrections" like those in the article at regular, predictable intervals.

This is described in the second and third paragraphs of "Survey Design" here:
[https://en.wikipedia.org/wiki/Public_Land_Survey_System#Surv...](https://en.wikipedia.org/wiki/Public_Land_Survey_System#Survey_design)

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mabbo
Growing up in rural Ontario, I've seen plenty of these, but I was always given
a different explanation for it: chains stretching.

The idea was that these concessions were laid out via teams with two sets of
horses pulling a chain directly west, then when the chain was fully stretched,
putting down a post to mark the point at which the concession ended, tying the
chain to it, and pulling the other end ahead until it ran out again.

This worked, but if team A has the old chain, and team B has the new chain,
team A will keep being a few inches and then feet ahead of team B, who are
working a few miles to the south, because the chain started to stretch over
time.

In hindsight, I realize now that I have no idea if any of this is even
remotely true, but it was a plausible explanation that I'd heard from a couple
of sources.

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padobson
Isn't using the right projection for a certain geography a solved problem in
cartography? This article seems like it's saying: "Here's a common problem for
mapmakers that you didn't know!"

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SeanDav
It is a known problem, but not a solved problem, in that there does not appear
to be 1 best way to map a 3d surface onto a 2d map and vice-versa. There are
numerous approximations, but which one to use is an art and not a science.

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ghshephard
Couldn't this also be solved by simply making the plots of land without right
angled corners? I'm guessing there is a simple tiling solution that would
solve the problem without requiring detours.

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dmurray
Not while satisfying the requirement that each piece of land be six miles
square, with the boundaries running north-south and east-west. In fact that's
impossible to do for a single piece of land that does not straddle the
equator.

One solution would have been to make each parcel of land 36 square miles, with
the parcels getting taller and narrower further north.

~~~
ghshephard
If the boundaries are running magnetic north-south, then there is no way to
make the property square. I'm sure there is a good reason to survey the lands
as squares, rather than as trapezoids with the north/south borders running
along the lines of longitude - just not sure what it is right now.

The farmer wouldn't care - they would simply survey their property based on
the property stakes - and, at 6 miles, their property corners would be close
enough to right-angles they probably wouldn't notice, and certainly wouldn't
care.

And roads could then run all the way to the north pole, or from coast to
coast, without having to detour (mountains, lakes, cities, rivers and other
obstacles excepted).

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stblack
Mysteriously missing from this: discussion of influential land owners, party
hacks in some cases, who succeeded in steering routes around their plots.

