Arc Definition v. Chord Definition 

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Peter.Polito
Posts: 13

Joined: 1/4/2009
Topic  Arc Definition v. Chord Definition       Flag »  Reply »
i've been taught that the railroad always uses the chord definition of a curve and the rest of the world used the arc definition of a curve.  when i ask the instructor why the railroad is in its own world, the answer is "no sabe" or "i don't care"

is there anyone out there that can provide me a complete, detailed answer without making me hunt for the snipe?
  Sunday, January 04, 2009 at 12:05:11 PM
Isleno
Posts: 42
Location: Gonzales, La USA

Joined: 10/20/2008
Reply  Re: Arc Definition v. Chord Definition Flag »  Reply »
I was told that the chord definition is less vunerable to round-off error problems that can plague long radius arc defined curves.  That, and tradition.  I never have looked into it myself.
  Monday, January 05, 2009 at 11:54:33 AM
Timber
Posts: 3
Location: Bakersfield, U.S.A.

Joined: 11/10/2008
Reply  Re: Arc Definition v. Chord Definition Flag »  Reply »
In the early days of railroad layout an engineers chain was used (100.00 feet 100 links) and not the Gunters chain (66 feet 100 links) even though it could be used. This made it easy to maintain engineer stationing as you moved forward. Plan and Profiles were done in the field not in the office like in modern times. Build as you move forward. The surveyors used charts to figure the curve. This made it easy to layout the curve as you went along.  Railroad curves are expressed as a degree of curve. This equates to 100.00 of chord distances. Thus the chord definition. Watch out for early curves on roads and highways as railroad surveyors would also lay them out using the chord definition. Hope this helps.
  Thursday, January 08, 2009 at 10:13:00 AM

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