OT: Re: Looking For Geodetic Python Software

Rocco Moretti roccomoretti at hotpop.com
Thu Jun 23 21:31:28 CEST 2005


Tim Daneliuk wrote:
> Diez B. Roggisch wrote:
> 
>> Tim Daneliuk wrote:
>>
>>> Casey Hawthorne wrote:
>>>
>>>>
>>>> Do your planes fly over the earth's surface or through the ground?
>>>
>>>
>>>
>>>
>>> Why do you presume this has anything to do with airplanes?
>>>
>>
>> That was supposed to be a funny remark regarding that  your 
>> "straight-line-distance" makes no sense at all - because that would 
>> mean that you'd have to go underground. So it has no 
>> real-world-application - unless you actually have underground-planes ;)
>>
>> Diez
> 
> 
> 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?

If you re-read what you wrote you'll see the phrase "straight line 
flying distance.":

 > 1) Given the latitude/longitude of two locations, compute the distance
 >    between them.  "Distance" in this case would be either the
 > straight-line
 >    flying distance, or the actual over-ground distance that accounts
 > for the earth's curvature.

Casey was pointing out that, due to the convex curvature of the Earth, a 
"straight line" between, say, Hong Kong and New York would happen to 
pass several miles below the surface of California. For an extreme 
example, a Euclidean straight line from the North pole to the south pole 
would pass through the center of the earth. Note that you've attached 
"Flying distance" to the phrase "Straight line" - Hollywood not 
withstanding, there isn't a machine able to "fly" through the center of 
the earth. The fact that it might be an unintentional error only adds to 
the humor. (c.f Freudian Slips)

Given the relative thinness of the atmosphere (~10-20 km) in comparison 
with the radius of the earth (~6,400 km), any plane flight of a 
considerable distance will be curved in the Euclidean sense, no matter 
how they changed their altitude inbetween.



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