Last updated July 28, 2026
SPER draws a line perpendicular to a spline. That sounds simple until you try it: AutoCAD’s perpendicular object snap works against a spline, but getting a line that is genuinely normal to the curve at a chosen point — and knowing where along the curve you are — is fiddly.
The job comes up constantly in road and drainage work, where the alignment is a spline and you need cross-sections, offsets or tick marks square to it.
Full AutoCAD only
Like the EPU dimension routine, this one uses ActiveX — the interface that lets AutoLISP treat drawing objects as programmable objects, which is how you get at a spline’s curve mathematics without doing the geometry yourself.
ActiveX is not available on AutoCAD LT at any release, so this cannot run there, even on LT 2024 and later where AutoLISP itself is supported. See what runs on AutoCAD LT. These two are the exceptions in this library — nearly everything else here is plain AutoLISP with a fair chance on LT.
Written with proper housekeeping
Worth pointing out because so many routines in this collection do not bother. SPER saves your command echo and object snap settings before it starts, switches them to what it needs, and puts both back when it finishes.
More to the point, it installs an error handler that does the same restoring. Cancel it halfway through and your settings still come back — which is exactly the failure that leaves other routines here with your command echo switched off and no explanation. Compare it with the zero-radius fillet routine, which puts your fillet radius back only if you let it finish.
Object snap being switched off during the work is deliberate: the routine computes exact points on the curve, and running snaps would drag them onto nearby geometry.
Command reference
The file defines a single command, SPER — spline perpendicular. It announces the selection step with a bracketed prompt, “Select Spline”, so you know what it is waiting for.
How to load it
Unzip Sper.zip and load sper.lsp with APPLOAD, or add it to the Startup Suite if you do alignment work. No support files. Our AutoLISP guide covers loading.
Step-by-step usage
- Set the layer you want the perpendicular lines on.
- Type SPER.
- At the “Select Spline” prompt, pick the spline.
- Follow the prompts for where along the curve and how long the perpendicular should be.
If nothing happens when you pick, check you actually selected a spline rather than a polyline — the two look identical on screen once drawn, and this routine wants the real thing.
Compatibility notes
No date in the file, but the ActiveX approach places it well after the mid-1990s — that interface arrived with AutoCAD R14 and Visual LISP. On full AutoCAD it should be sound; on LT it will not run, as covered above.
It only adds lines and restores everything it changes, so there is very little risk in trying it. That is more than can be said for most of what is on this site.
Download
| Name: | SPER |
| Description: | Draws perpendicular lines to a spline. |
| Type: | AutoCAD AutoLISP Routine |
| Author: | Anon |
| File Size: | 1 Kb |
| Cost: | Free |
| Worked on: | AutoCAD |
| Download File: | Sper.zip |
Related routines
- EPU — the other ActiveX routine here, with the same LT limitation.
- PDIST — chainage along a polyline alignment.
- COGO bearing and distance label — survey annotation for the same drawings.