OT: Re: Looking For Geodetic Python Software

Tim Daneliuk tundra at tundraware.com
Thu Jun 23 23:57:51 CEST 2005

Paul Rubin wrote:

> Tim Daneliuk <tundra at tundraware.com> writes:
>>Huh?   When traversing along the surface of the earth, it's curvature
>>is relevant in computing total distance.  An airplane flies more-or-less
>>in a straight line above that curvature.  For sufficiently long airplane
>>routes (where the ascent/descent distance is trivial compared to the
>>overall horizontal distance traversed), a straight line path shorter
>>than the over-earth path is possible.   That's why I specified the
>>desire to compute both path lengths.  Where's the humor?
> It's just not clear what you meant:
>   A) The shortest path between two points on a curved surface is
>      called a geodesic and is the most meaningful definition of
>      "straight line" on a curved surface.  The geodesic on a sphere is
>      sometimes called a "great circle".
>   B) By a straight line you could also mean the straight line through
>      the 3-dimensional Earth connecting the two points on the surface.
>      So the straight line from the US to China would go through the
>      center of the earth.
>   C) Some people seem to think "straight line" means the path you'd
>      follow if you took a paper map, drew a straight line on it with a
>      ruler, and followed that path.  But that path itself would depend
>      on the map projection and is generally not a geodesic, and neither
>      is it straight when you follow it in 3-space.

Yeah, after rereading my original question, I realize that it could
be read that way.  My Bad.  What I had in mind was this:

A                 ------------------------------

E                   ---------------------------
                    /                           \
                   /                             \

Where A was an airplane's line of flight between endponts and E was the
great circle (geodesic) distance over ground.  It seemed to me that if
the ascent/descent distance for A is very small compared to the length of A,
the flight distance would be shorter than the over-ground distance.  But,
as Rocco points out in another response, this is not so.

I stand (well, sit, actually) corrected!

Many thanks to all of you who took the time to unscramble my English and
lack of geometric understanding...
Tim Daneliuk     tundra at tundraware.com
PGP Key:         http://www.tundraware.com/PGP/

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