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Implement a correct euler spiral in the allignment

Implement a correct euler spiral in the allignment

description of the issue:


I have research further and can confirm that it is expected results.
To calculate exact points at any point on a spiral integral or regression series math algorithms must be used, C3D (and most surveyors) use a simplified formula to set stakeout points. They use 2 trigonometric formulas, based on the delta angle, to calculate the distance along the spiral tangent and then perpendicular to that point. Surveyors know that this method is not exact but that the actual differences are less than 0.01ft or .003 m.

In case you have designed blocks the exact size and try to fit them in it will create little overlaps. Should they be AutoCAD blocks, I would redesign them 1cm smaller for example and overwrite the block definition.

There is no way you can improve the spiral precision, sorry for that.

I will however report this issue to our development team, as we may want to re-think about how C3D calculate the spiral. Changing the calculation method would slightly impact speed but will give more accurate results... Let's see what product management and dev. will think.
For now this is the best I can do.


For rail alignments this correct euler spiral stationing is neccesary.





1. An existing surface

2. An existing alignment 

3. A txt file containing arbitrary alignment stations. For eg- 1+112.49, 1+345.52....


Required output- 

XYZ report of the arbitrary station points. 



Importing a txt file from point creation tools through 'alignment-import from file' option. Select the correct format, and the alignment and the points will be placed on the alignment.



The points are evenly placed on top of the alignment and the curve, but there's a slight shift of points w.r.t the spiral curve. Please look into the matter whomsoever concerned.






I've combined these two ideas because they are related, and here's why: When we calculate a internally we use the exact mathematical formula to make the calculation. This formula is used to calculate points on the transition either for adding points based on station offset, for corridor stations/


We also use the formula to calculate the tessellation points we use when we draw an alignment in AutoCAD. We calculate a series of points and connect them with a arc segments to approximate the transition. If you layout points using the station offset command, they may be slightly off the AutoCAD graphics because of this approximation.




Peter Funk

Autodesk, Inc.

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