2D and 3D Polylines

Did you know that there are a few differences between polylines drawn in 2D and polylines drawn in 3D? To start with the command to draw each type of polyline is different.

The command POLYLINE will allow you to draw a 2D polyline where the command POLY3D will draw a 3D polyline.

A 2D polyline can be made up of line and arc segments, each with optional widths assigned to the segments, and is treated as one object from this point forward.  It also holds the elevation of the first point picked for it's entire length.

A 3D polyline is made up of straight line segments only - no arcs and no widths assigned to the segments, and is treated as one object from this point forward.

A 2D polyline can be converted to a B-Spline Polyline (which is a smoothed polyline but does not hold the vertices) by the command SPLINE or to a Curved Polyline (which is a smoothed polyline and does hold the vertices) by the command PCURVE. Smoothing a 2D polyline (with either method) will remove any widths assigned to the segments.

A 3D polyline can be converted to a 3D B-Spline Polyline (which is a smoothed polyline but does not hold the vertices) by the command SPLINE. A 3D polyline can not be converted to a Curved Polyline.

You can decurve a polyline by using the same command you used to smooth it but you must use the Decurve option of the command (SPLINE DECURVE or PCURCE DECURVE). This will make the polyline revert back to its original 2D or 3D polyline.

There are other editing tools for Polylines available in the program but will not be covered in this technical note.

Drawing Breaklines for Modelling: Make sure you are using the POLY3D command when drawing the breaklines. If you do not then the breaklines will not represent the true elevations at each vertice when you pick each point in the drawing. Also make sure that you are snapping to the actual points when drawing the breaklines. If you use the 2D POLYINE command then the breaklines will be drawn all at one elevation, the elevation of the first point chosen.

On your screen you can only have a width on a Polyline when it is in 2D and not smoothed by either option. If you have a 3D or smoothed polyline then you can not have a width assigned to the polyline.  Linetypes will not be honoured on polylines that have been smoothed and will only appear on 2D polylines, on each segment, if it is long enough to show the linetype.

Do you need a wide polyline on your plotted plans? If so, even though a width is not assigned to the polylines, you can plot your linework with different sized pens to get different widths on the linework.

The trick to plotting any linework with a wide pen is to draw all the linework, which needs to be the same width, using the same color. Then in the Print/Plot command you configure the pens with colors and widths. When you plot the drawing any polylines drawn in green (for example), on the screen would be plotted with the pen width assigned to the green pen. Same process for all other colors and linework. So you can assign a different pen width for each color you have drawn linework with and the plotter will plot the linework using the width assigned. This will work for Lines, Arcs, Polylines (2D or 3D, smoothed or not), text and any other linework in your drawing. This allows more flexibility than using polylines with widths, in most cases.

Your plotted plans can now be completed with as many or more different line widths as you have ever done before.

Glen W. Cameron, C.E.T.

Technical Support Manager