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.