View Full Version : creating a spiral
2004-06-25, 07:56 PM
I work with spiral-curve-spiral transitions along alignments between tangents. LDT creates the spiral by making a spline. The spline is actually made up of ten circular arcs, all with differing radii. Is there a way to change the number of arcs that make up the spiral?
2004-06-25, 08:05 PM
A spiral is a curve comprised of short segments that have differing rates of
curvature or radius. You must use the Autodesk Land Desktop commands to
create spirals. All Autodesk Land Desktop commands expect spirals to meet
certain design criteria. If the spirals do not meet these criteria, then the
commands do not process the spirals properly. Even though a spiral returns
polyline information when listed using the LIST command, you cannot create
them with the PLINE command. If the horizontal alignment includes spirals,
you must define the alignment using the Define from Objects command. If you
use the PEDIT command to join a spiral with other objects, the spiral
definition will be lost.
Use the following guidelines when drawing spirals:
The spiral commands prompt for either a length or spiral A parameter. The A
parameter describes the flatness of the spiral, and is a commonly used
metric parameter that equals the square root of the product of the length
and the radius. To use the A value, type A at the First spiral length (or A)
and/or Second length (or A) prompt, and then enter the "A" value.
When attaching spirals or curves to an object, entering a positive radius
draws the spiral or curve to the right (clockwise), and entering a negative
radius draws the spiral or curve to the left (counter-clockwise).
All the options in the Attach Spiral command prompt you for a radius or
degree of curvature. The degree of curvature is not generally used as spiral
parameter, but if you attach a spiral in or out of a curve and know the
degree of curvature but not the radius, then the options calculate the
radius of the spiral from the degree of curvature.
To select the spiral type
Do one of the following to display the Alignment Values dialog box:
From the Lines/Curves menu, choose Create Spirals Spiral Type.
From the Projects menu, choose Edit Drawing Settings to display the Drawing
Settings dialog box. Under Program, select Land Desktop. From the Settings
list, select Spiral Type and then click the Edit Settings button.
Select a spiral type:
Clothoid: For more information, see Clothoid Spirals.
Sinusoid: For more information, see Quadratic, Cosinusoidal, and Sinusoidal
Cosinusoid: For more information, see Quadratic, Cosinusoidal, and
Quadratic: For more information, see Quadratic, Cosinusoidal, and Sinusoidal
2004-07-01, 03:45 PM
We have applications for the sprial that are outside the norm. It may prove useful to us to modify how the spiral is created.
The 10-segment thing is an AutoCAD implementation detail, used to approximate something that really has infinite segments, and should only be important to AutoCAD. If its lack of precision is what you're trying to avoid, then all I can do is wish you luck, and wonder what application you have that needs greater precision in a spiral. It must not be a road.
Otherwise, if your application truly needs a spiral, then you should be able to use AutoCAD's spiral commands to create the required spiral, and the 10-segment thing shouldn't matter. You should only have to adjust the spiral type and flatness. If you need to adjust anything else, then that would tend to indicate you don't actually have a spiral in your application, and are doing something else. In that case, a spiral isn't actually what you need, and at best is only an approximation, and that fact should be taken into account in your work. Maybe this is what you're trying to do, but then essentially what you're trying to do is hack into AutoCADs implementation of one feature to accomplish some other task. If that's the case, then you might find something that works, but you're on your own, and it might not work in other or future versions of AutoCAD.
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