Stationing by Chord Definition - Is it there a better solution?

[Anonymous]

In C3D 2010 and earlier it was necessary to fudge in equation stations after every circular curve to get proper stationing on those alignments that are stationed by chord definition (as many railroad alignments are defined). As many of you are aware the equation station solution isn't very clean (and the problem gets bigger as the quantity of curves increases).

Is the equation station workaround still the only method to deal with chord definition stationed alignments in 2011 (let's not rehash the merits of circular vs chorded stationing as this is client driven)?

If there is no valid C3D approach, does anyone know of a third party solution (other than the standard workaround - use Microstation)?

Thanks,

JK

[BrianHailey]

Honestly, I've never heard of the chord defined stationing so I may be way out of my league here but I'll give it a shot. Perhaps you need two alignments, one with the lines and the chords and a second with the curves. I suppose it depends a lot on what you will be using these for.

Brian Hailey
http://www.cad-1.com
http://www.AtYourDeskTraining.com
http://Civil3DPlus.wordpress.com

[Anonymous]

JKinAK wrote:
> In C3D 2010 and earlier it was necessary to fudge in equation stations
> after every circular curve to get proper stationing on those
> alignments that are stationed by chord definition (as many railroad
> alignments are defined). As many of you are aware the equation station
> solution isn't very clean (and the problem gets bigger as the quantity
> of curves increases).
>
> Is the equation station workaround still the only method to deal with
> chord definition stationed alignments in 2011 (let's not rehash the
> merits of circular vs chorded stationing as this is client driven)?
>
> If there is no valid C3D approach, does anyone know of a third party
> solution (other than the standard workaround - use Microstation)?
>
> Thanks,

I have seen some railroad design software that instead of circle puts
chords in curves. I have never heard for chord stationing before and
thought that this was software fault because stationing from one
railroad design software to another varies even when elements are the
same (same spirals) and the possible reason for that was the segmented
definition for circles.
Is that really demanded in some countries or what? Whats the point? I
have never taken those differences seriously (like they really should be
there) because i have seen some other bizzarre things in those
softwares. For example, one of them doesnt display tangent intersection
correct.

[Anonymous]

Sorry, I can not help with a better solution, but the arc and chord definitions are taught in surveying 101. In this area roads and property boundarys use the arc definition and railroads use the chord definition.

Gary E.

[Anonymous]

garyelswick5627 pretended :
> Sorry, I can not help with a better solution, but the arc and chord
> definitions are taught in surveying 101. In this area roads and property
> boundarys use the arc definition and railroads use the chord definition.
>
> Gary E.

so let me see if a got this right: railroad alignment in curves is
stationed through segments?

[Anonymous]

They are both a way of defining the geometry of a curve. Google arc and/or chord definition curves and you will get many results. RR curves are not stationed by segments.

[Anonymous]

2011 help under the creating curves concept tab has illustrations for both arc and chord definitions.

[Anonymous]

After serious thinking garyelswick5627 wrote :
> 2011 help under the creating curves concept tab has illustrations for both
> arc and chord definitions.

Thanks. I understand now the difference beetwen those two values but is
it the only thing that it affects - deegree of curve value? Does it
affect geometry? How does it affect stationing?

[Anonymous]

Yes chord definition stationing/curve definition is still used - typically on railroads. It's usually a railroad company standard. If a railroad has historically had it's curves and stationing by chord definition: why change? It's a valid workable option (except in C3D). Many railroads in the US still use chord definition and there would be confusion if they changed - (essentially two sets of stationing for existing lines). It makes more sense to stick with one method.

Domagoj X asks "so let me see if a got this right: railroad alignment in curves is stationed through segments?"

That is correct - and it works fine. The problem comes when stationing is calculated on the arc when it's defined by the chord" stationing gets a little bit further out with each curve.

I take it from the posts that I will soon be owning a copy of Microstation.

[Sinc]

Can you explain further how this works?

I'm used to stationing around curves being on the curve itself, which means it is irrelevant whether or not you use chord definition or arc definition - all that really affects is the degree of the curve. It wouldn't affect stationing.

So I'm having a hard time picturing how this "chord stationing" works...

-- Sinc
http://www.ejsurveying.com
http://www.quuxsoft.com

[AllenJessup]

It affects stationing by X per 100' of cord definition. X being the difference between the 100' chord and the arch length. I've never had to design using the chord definition but I would think that if that's the railroads standard then everyone would have to adhere to that.
It makes a difference to me in Surveying because the RR ROW is defined by station and offset. So I need to have the alignment follow the chords to determine the ROW.
Allen Edited by: AllenJessup on Jun 1, 2010 1:53 PM

[AllenJessup]

I've always taken Chord Stationing to mean exactly what it says. The stationing follows the 100' chords that define the degree of curve. The RR Engineers & Surveyors seem to have developed this method as an easy way to stake out the long curves used by railroads. Set up on the PC, BS, turn the deflection, measuer out 100' with a steel tape, set a point, move ahead, BS, deflect, etc. That way your measuring and moveing an even 100' of stationing per chord. Any intermediate sta-off points are calculated perpendicular to the chord.
I've found this method used in some of the RR in the NE. Between this and the face that the Erie changed its Guage and move a rail after the valuatin maps were done it makes for very interseting Surveying along the railroads.
Allen

[Anonymous]

Here is a curve element list for a chord definition curve from Microstation:


SC ( ) 168+88.3972 603203.2170 106903.8263
PI ( ) 173+31.3120 603236.5046 106462.1641
CC ( ) 605107.8925 107047.3796
CS ( ) 177+58.7438 603460.7716 106080.2245
Radius: 1910.0775
Delta: 26^06'37.4417" Right
Degree of Curvature(Chord): 3^00'00.0000"

Length: 870.4461
Length(Chorded): 870.3467

Tangent: 442.9149
Chord: 862.9336
Middle Ordinate: 49.3700
External: 50.6799
Tangent Direction: 274^18'36.6080"
Radial Direction: 4^18'36.6080"
Chord Direction: 287^21'55.3288"
Radial Direction: 30^25'14.0497"
Tangent Direction: 300^25'14.0497"


Note that the curve has two lengths 870.4461 and 870.3467 (which is chorded along 100' chord segments). The difference in this curve is .0994


Note that the PC (or SC here) is at 168+88.3974 and the PT (or CS here) is at 177+58.7438. This is a difference of 870.346 (the chorded length).


The difference is slight for one or two curves and folks may not realize what is happening. The problem comes as the number of curves increase and as the curve radii decrease. With curves similar to the one above, you get about a foot of stationing error for every 10 curves if you ignore the difference in stationing methods.

[Anonymous]

AllenJessup wrote:
> I've always taken Chord Stationing to mean exactly what it says. The
> stationing follows the 100' chords that define the degree of curve.
> The RR Engineers & Surveyors seem to have developed this method as an
> easy way to stake out the long curves used by railroads. Set up on the
> PC, BS, turn the deflection, measuer out 100' with a steel tape, set a
> point, move ahead, BS, deflect, etc. That way your measuring and
> moveing an even 100' of stationing per chord. Any intermediate sta-off
> points are calculated perpendicular to the chord.
> I've found this method used in some of the RR in the NE. Between
> this and the face that the Erie changed its Guage and move a rail
> after the valuatin maps were done it makes for very interseting
> Surveying along the railroads.
> Allen

I am pretty sure that here in Croatia they dont use chord stationing. At
least, during design we dont use it. But maybe it is the reason why
there are differences in some software compared to civil 3d.

[Sinc]

So then does the stationing follow 100' curves, with the remainder (if anything) just thrown on the final chord? In other words, if I have a curve with 4 100-foot chords and a final chord of 50' to get to the end of the curve, does the curve have 450' of stationing?

-- Sinc
http://www.ejsurveying.com
http://www.quuxsoft.com

[Anonymous]

Robert Steltman has a third-party tool that works with Railway stationing...
http://www.rsteltman.com/software.html

Matthew Anderson, PE


On 6/1/2010 1:08 PM, JKinAK wrote:
>
> Here is a curve element list for a chord definition curve from
> Microstation:
>
>
> SC ( ) 168+88.3972 603203.2170 106903.8263
>
> PI ( ) 173+31.3120 603236.5046 106462.1641
>
> CC ( ) 605107.8925 107047.3796
>
> CS ( ) 177+58.7438 603460.7716 106080.2245
>
> Radius: 1910.0775
>
> Delta: 26^06'37.4417" Right
>
> Degree of Curvature(Chord): 3^00'00.0000"
>
> Length: 870.4461
>
> Length(Chorded): 870.3467
>
> Tangent: 442.9149
>
> Chord: 862.9336
>
> Middle Ordinate: 49.3700
>
> External: 50.6799
>
> Tangent Direction: 274^18'36.6080"
>
> Radial Direction: 4^18'36.6080"
>
> Chord Direction: 287^21'55.3288"
>
> Radial Direction: 30^25'14.0497"
>
> Tangent Direction: 300^25'14.0497"
>
>
> Note that the curve has two lengths 870.4461 and 870.3467 (which is
> chorded along 100' chord segments). The difference in this curve is .0994
>
>
> Note that the PC (or SC here) is at 168+88.3974 and the PT (or CS
> here) is at 177+58.7438. This is a difference of 870.346 (the chorded
> length).
>
>
> The difference is slight for one or two curves and folks may not
> realize what is happening. The problem comes as the number of curves
> increase and as the curve radii decrease. With curves similar to the
> one above, you get about a foot of stationing error for every 10
> curves if you ignore the difference in stationing methods.
>

[stewarteng]

Sinc

Here's how Route Location and Design explains it

"The length of curve L (arc definition) is the distance around the curve from A (the PC) to B (the PT). Since any two arcs of the same circle are proportional to the opposite central angles, we have L/100=Delta/D; from which L = 100(Delta/D)

...(this equation) is also used for the chord definition, but it implies the total length as if measured along the 100 foot chords of an inscribed polygon. If the curve begins or ends with a subchord, true subchord lengths (not nominal) should be used"


Brian

[Sinc]

> {quote:title=stewarteng@mfx.net wrote:}{quote}
> "The length of curve L (arc definition) is the distance around the curve from A (the PC) to B (the PT). Since any two arcs of the same circle are proportional to the opposite central angles, we have L/100=Delta/D; from which L = 100(Delta/D)
>
> ...(this equation) is also used for the chord definition, but it implies the total length as if measured along the 100 foot chords of an inscribed polygon. If the curve begins or ends with a subchord, true subchord lengths (not nominal) should be used"


That seems to define "arc definition" and "chord definition", but my question was actually about the railway stationing...

From my understanding, this stationing is along the chords, and not along the curve itself, as is normal in other applications. Furthermore, these chords are all 100' long, for as many chords as needed, and then the final chord is the remainder... This means that if we have a 4 1/2 degree curve, it will have (roughly) 450' of stationing... At least, if I understand it correctly.

-- Sinc
http://www.ejsurveying.com
http://www.quuxsoft.com

[Sinc]

Actually, I think I said that wrong...

Maybe the attached image will be clearer. If I understand it correctly, then in the attached image, the end of the curve should be at Sta 4+48.20, as shown. However, if I try to create an alignment that follows the curve, Civil 3D will label the end of the curve as Sta 4+50.

Does that sound right?

-- Sinc
http://www.ejsurveying.com
http://www.quuxsoft.com

[Anonymous]

That's how it works.

I'm dealing with it using equation stations. I haven't dealt with equation stations in C3D so I'm not sure what the impact of placiing an equation station on the end of each curve will be (other than making alignments a pain to update).
JK