offset alignment with clothoide

[Anonymous]

hi everybody!

creating an offset of an alignment which consists of lines and curves i get a polyline with lines an curves

but creating an offset of an alignment whith lines, curves and clothoides i dont get an offset of the curves

see attached screen capture

is this usual?? Message was edited by: cadascadcan

[Anonymous]

Hi,

Offsetting a clothoid spiral has the theoretical difficulty that if you
offset the spiral such that the distance from the spiral perpendicular to
the spiral remains constant, the resultant curve is NOT a clothoid spiral.
Hence the AutoCAD offset command which is designed for tangents and arcs
which maintain the same mathematical formula when offset) is not a competent
tool to offset a spiral. Nor is it easy to create a mathematical formula to
compute the location of points on a curve offset from a spiral.

I gave up analysing what the formula might be (I haven't studied maths since
the early 1960's) and took the approach that a polyline drawn though
computed points at a reasonable spacing along the spiral would be an
adequate approximation for Civil Engineering purposes. The results of this
work are freely available in the Passing Lane program at
www.civil3DTools.com

You could use the program to create a 'Passing Lane, with very short inlet
distance and outlet distance from the start to the end of an alignment to
get an offset alignment inclusive of the "Parallel to Clothoid" components.

While I was writing the program (in Civil 3D 2006) I used the Alignment
Offset command as a check on my computations and found that the command
simply created a polyline correctly offset from the spiral at one place and
with varying offset along the rest of the spiral.

If you would like some more understanding of this, picture a spiral
continued from its starting point with infinite radius to a point where some
loops have occurred and the radius has become trivial. Picture also an
offset curve to the outside where the offset value is 10 times the small
radius. This curve will never be capable of reducing it's radius below the
offset value, and hence cannot be a clothoid.

Picture also the offset curve to the inside and this curve must cross itself
as the clothoid radius reduces.

--

Laurie Comerford
CADApps
www.cadapps.com.au

. wrote in message news:5331648@discussion.autodesk.com...
hi everybody!

creating an offset of an alignment which consists of lines and curves i get
a polyline with lines an curves

but creating an offset of an alignment whith lines, curves and clothoides i
dont get an offset of the curves

see attached screen capture

is this usual??

Message was edited by: cadascadcan

[Anonymous]

I have what looks like the same issue here.
Did you successfully resolve this?
The reply I have seen does not, I believe, quite get to the bottom of it - it is not the spiral that is not offsetting correctly, but the circular arc.
On top of this, the line drawn is not from the ends of the arc, but from some other 2 points along the arc.
I have exploded the offset polyline and this line is just that, a 'line', not an 'arc' as it should be.
To add to the confusion, a machine with identical specs does not have this problem on the same file.
Also, once an alignment is created on this second machine, the problem is resolved for this alignment on the first.
It seems to be about how these separate machines are creating the alignments.

Any ideas???

[Anonymous]

Hi,

There are theoretical difficulties with offsetting a clothoid. The curve
created by offsetting a clothoid is NOT another clothoid. (If there is an
algorithm for the curve, I have not been able to find it and lack the
interest to try and derive it myself) Hence when you offset, Civil3D draws
a polyline which matches the offset value somewhere along the clothoid, but
not entirely. For practical purposes this does not matter and I would
expect that computations are done with correct data.

--
Regards


Laurie Comerford
www.cadapps.com.au
www.civil3dtools.com
wrote in message news:5382113@discussion.autodesk.com...
I have what looks like the same issue here.
Did you successfully resolve this?
The reply I have seen does not, I believe, quite get to the bottom of it -
it is not the spiral that is not offsetting correctly, but the circular arc.
On top of this, the line drawn is not from the ends of the arc, but from
some other 2 points along the arc.
I have exploded the offset polyline and this line is just that, a 'line',
not an 'arc' as it should be.
To add to the confusion, a machine with identical specs does not have this
problem on the same file.
Also, once an alignment is created on this second machine, the problem is
resolved for this alignment on the first.
It seems to be about how these separate machines are creating the
alignments.

Any ideas???

[Anonymous]

Hi,
This is not a theoretical problem but practical guano AUTODESK has created in Service Packs. Original Civil 3D 2006 and Civil 3D 2007 works properly.Do you have SP installed in both machines?

[Anonymous]

Yes, all service packs are installed. Are you suggesting I uninstall the service packs to get the offsets to work properly?

[Anonymous]

Yes.
I've done this and OFFSET command works OK.

[Anonymous]

I'm Having the same problem!!!!
The service pack seemed to make things worse. Is there a "fix" for this? Its so frustrating when is used to work fine. Un installing a service pack seems counter productive.

T