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...
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Tim Daneliuk     tundra at tundraware.com
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