# Horizontal Curves - PT



## Blu1913 (Oct 2, 2006)

We all know Sta PC = Sta PI - T BUT:

Can anyone tell me why Sta PI + T does not equal Sta PT???

I know that I doesnt, but in my mind i cant understand why that is.


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## Road Guy (Oct 2, 2006)

It seems odd , but since the stations are measured along the curve, not the tangent, it will be off if you compute the PT by adding the "T" to the PI.

especially odd since most of the time you can get the PC by subtracting the T from the PI Station.


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## GTScott (Oct 2, 2006)

This is an odd thing for us non-transpo folks. In horiz. curves, you subtract T from the PI to get the PC however adding T does not get you to the PT. As Road_Guy points out, horiz curves are stationed ALONG the curve. Now, change all of this to vertical curves and the length of curve has no relevance when it comes to stationing. Who came up with this??

On this topic, I saw a sample question where you were given two vertical curves and all of the station about the curves including the VPI stations. The question asked for the distance between the two VPIs. I looked at the question for 5 minutes thinking there HAD to be something else to it. Nope...a simple subtraction problem. Then again, even subtraction takes time with this crappy calculator 

-GT


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## Blu1913 (Oct 2, 2006)

Well, I guess the best answer is: It is odd, and dont do it.

Forget the theoretical stuff. Awesome. :whatever:


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## 3gorgesdam (Oct 2, 2006)

The curve is not always symmetric, therefore you would have two different tangents. The only time that Sta PT = Sta PI + T is when the curve is perfect symmetric. That is what the text book presented, the symmetric case. Use curve length L to calculate Sta PT is exact no matter the curve is symmetric or not.


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## Blu1913 (Oct 2, 2006)

But why then is the PC always PI - T??? Maybe this is getting to theoretical? I understand the unsymetrical curves........but it just wont click when the PC is ALWAYS PI - T.


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## 3gorgesdam (Oct 2, 2006)

> But why then is the PC always PI - T??? Maybe this is getting to theoretical? I understand the unsymetrical curves........but it just wont click when the PC is ALWAYS PI - T.


The tangent T by definition is between PC and PI. The tangent between PI and PT is not definted as term T. This is Just their convention. That doesn't mean sta PT can't be found using sta PI and the tangent between the two points. The math part still works, just they don't call this tangent T but some distance. That is all.


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## Blu1913 (Oct 2, 2006)

CLICK! Awesome, I got it beat into my head now. Thanks everyone for the beating...to much: estudiar en la bibliotecka. Mi LOCO....(&lt;----6 years of spanish and thats all i remember) :brick:


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