You need the area of a site boundary that is already drawn. You click it and get four loose lines and no number at all. That boundary has to be one of the polylines in AutoCAD before the program will report an area.
This article covers how to merge those four lines, including the awkward case where a gap is left between two of them, on AutoCAD 2027. For the whole discipline of accurate drafting, see the AutoCAD for Construction course.
Where to start: three commands and their entry points
Three commands appear in this article. Their entry points are read straight out of the 2027 configuration files.
| Job | Full command | Alias | Ribbon | Classic menu | Toolbar |
|---|---|---|---|---|---|
| Draw a new one | PLINE | PL | Home → Draw → Polyline | Draw → Polyline | Draw → Polyline |
| Merge and edit | PEDIT | PE | Home → Modify → Edit Polyline | Modify → Object → Polyline | Modify II → Edit Polyline |
| Join collinear lines | JOIN | J | Home → Modify → Join | Modify → Join | Modify → Join |
PL and PE are the two aliases worth learning by heart. The ribbon button for PEDIT sits inside the Modify panel's drop-down, behind the little arrow, so typing the alias stays quicker.
The prompts, step by step
Drawing a new polyline is easy; merging a set of loose lines is the job people actually struggle with. Below is one complete run of PE, with the middle column copied verbatim from the 2027 release.
| Step | What the program asks | What you do |
|---|---|---|
| 1 | Select polyline or [Multiple]: | Type M and press Enter |
| 2 | Select objects: | Window the whole set, then press Enter |
| 3 | Convert Lines, Arcs and Splines to polylines [Yes/No]? <Y> | Press Enter to accept Y |
| 4 | Enter an option [Close/Open/Join/Width/Fit/Spline/Decurve/Ltype gen/Reverse/Undo]: | Type J and press Enter |
| 5 | Enter fuzz distance or [Jointype] <0.0>: | Press Enter if the lines touch |
| 6 | Enter an option [Close/Open/Join/...]: | Press Enter to leave the command |
The three places people give up are all in that table.
Step 1 requires the M. Skip it and the command edits one object only, which is exactly why people conclude it cannot merge a set.
Step 3 must be answered Y. A line is not yet a polyline, and without the conversion there is nothing to join.
Step 5 is the deciding one when gaps are left between the lines, and we measure it separately below.
Polylines in AutoCAD are one object, not several lines
Drawing a three-point path with the pline command and read back the object it produced. The alias is PL, two keystrokes, which is why it is worth making the habit.
| Measurement | Result |
|---|---|
| Object type | LWPOLYLINE |
| Vertices | 3 |
| Width | 0 |
PLINETYPE | 2 |
Three points, two segments, and only one object. That is the whole difference from the ordinary line command, which creates a separate object for every segment. How that command behaves is covered in our article on drawing straight lines.
The prompt, copied verbatim from the 2027 release, shows at once what this command can do that the line command cannot.
Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]:
Arc switches to drawing a tangent arc inside the same object, Close closes the shape, and Width changes the polyline width. That arc option is the one worth learning first: a run of straights and curves that would otherwise be five separate objects stays a single one, and it keeps its area and perimeter throughout.
When the line comes out thick: the culprit is PLINEWID
A very common complaint, and the measurement is short.
PLINEWID holds the default width of the polyline about to be drawn. It reads 0, a thin line. Set it to 30, draw again, and the new object carries a width of exactly 30.
PLINEWID | Width recorded in the object |
|---|---|
| 0 | 0 |
| 30 | 30 |
So a suddenly fat line is nothing broken. Somebody, or the Width option during an earlier run, set that variable away from zero. Type PLINEWID, enter 0, and normal service resumes.
A line already drawn is fixed with the polyline edit command, choosing Width and entering 0.
What the join command accepts and what it refuses
Plenty of tutorials say this command only works on objects drawn with the polyline command. Three situations say otherwise.
| Situation | What the program answered | Result |
|---|---|---|
| Two collinear lines, touching | 1 line joined to source | one line left |
| Two collinear lines, 50 apart | 1 line joined to source | one line left |
| Three lines meeting at right angles | 0 lines joined to source, 2 objects discarded from the operation | still three lines |
The first two rows show the command works perfectly well on ordinary lines, and even bridges a gap with nothing extra to declare. The third row is the real limit: the pieces must lie on the same straight line. Turn a corner and it refuses, and the program says plainly that two objects were discarded from the operation.
One more detail worth remembering: what you get back is still a line, not a polyline. For a polyline you need the command in the next section.
The pedit command converts a whole set at once
This is the command that solves the problem this article opened with. Drawing a square 2000 by 1500 from four loose lines, merged them, and measured.
| Measurement | Before | After |
|---|---|---|
| Objects | 4 | 1 |
| Object type | LINE | LWPOLYLINE |
| Area reported | none | 3000000 |
The 3000000 checks immediately: 2000 times 1500. The program also printed 3 segments added to polyline, meaning three lines were absorbed into the first.
The sequence is five beats. Type PE, choose Multiple, window the whole set, press Enter, then answer Y to the conversion question and choose Join.
The Multiple option is the part most tutorials leave out, and without it the command edits one object at a time, which is why people conclude it does not work on a set.
Two prompts are worth copying verbatim, because they are where people give up halfway.
Convert Lines, Arcs and Splines to polylines [Yes/No]? <Y>
Enter fuzz distance or [Jointype] <0.0>:
The first asks whether to convert lines into polylines; answer Y. The second asks for the tolerated gap, and that is the subject of the next section.
A small gap: raising the fuzz distance is only half the answer
The familiar advice is "just raise the fuzz distance until it joins". That advice is missing half of itself.
Two collinear lines with a gap of exactly 50 between them. Fuzz set to 100, comfortably more than the gap. Then each of the three join methods the command offers.
| Join method | What the program answered | Result |
|---|---|---|
Extend, the default | 0 segments added to polyline | still two objects |
Add | 2 segments added to polyline | one object |
Both | 2 segments added to polyline | one object |
That first row is where people give up: the fuzz is plenty and still nothing joins. The reason sits in Autodesk's own description on the PEDIT page. Extend is "Joins the selected polylines by extending or trimming the segments to the nearest endpoints."
Two segments lying on the same straight line have no intersection to extend to. We confirmed that with a control: two segments at different angles, with the same kind of gap, joined immediately under Extend, and the program printed 1 segments added to polyline.
So the rule shortens to this. Segments that turn a corner need nothing but the default; collinear segments with a gap need Add or Both, which Autodesk describes as "Joins the selected polylines by adding a straight segment between the nearest endpoints."
What you gain once the lines are merged into polylines in AutoCAD
Merging is not tidiness for its own sake. Four gains, each either measured or immediately visible.
An area and a perimeter you can read. This is the biggest one on a construction drawing, and it is the 3000000 in the table above.
One click selects the whole boundary. A set of loose lines has to be windowed, and it is easy to miss a segment without noticing.
Filleting and chamfering the whole run at once. Both are covered in our article on the fillet command.
A width that belongs to the object, instead of changing the lineweight of a whole layer.
A fifth gain is worth naming for anyone producing a drawing set rather than a single sheet. Hatching, area schedules and quantity takeoffs all want a closed boundary, and a set of loose lines will either be refused or, worse, accepted with a boundary the program guessed at. Merging first removes a whole class of quiet errors that surface only at the end.
Against that, editing one segment on its own becomes fiddlier, and housekeeping on a drawing with very many objects is a separate subject, covered in our article on slimming down a CAD drawing.
There is always a way back. Drawing a closed four-sided polyline, exploded it, and counted exactly 4 lines, with the polyline itself gone from the drawing. Merging and exploding are a two-way street, but the area goes when you explode, because four loose lines carry no such information.
Other names people use for joining lines
The same job goes by several names: joining lines, merging segments, or converting a line to a pline. pline is simply the PL alias of the polyline command, so converting a line to a pline and converting it to a polyline are one and the same.
Which command to use depends on what you have.
| What you have | What you want | Use |
|---|---|---|
| two collinear segments | one straight line | JOIN |
| a set of segments meeting at corners | one polyline | PE, option Multiple then Join |
| a single arc | a closed circle | JOIN then the cLose option |
The second prompt of JOIN changes with the type of the first object you pick, and that is what confuses people. Copied verbatim from the 2027 release:
| Source object | What it asks next |
|---|---|
| a line | Select lines to join to source: |
| an arc | Select arcs to join to source or [cLose]: |
Pick a line as the source and it will only take more lines; pick an arc and it will only take more arcs. Mix the two and nothing joins, with no error message at all.
JOIN bridges a gap rather than ignoring it. Two 1500-long collinear lines with a 500 gap between them: type JOIN, the program answers 1 line joined to source, and measuring what is left gives exactly 3500.
The cLose option is the one few people know. Pick an arc as the source and type L; AutoCAD prints Arc converted to a circle. and the arc becomes a closed circle, which is quicker than drawing a new circle and deleting the arc.
An ordinary polyline has one elevation for the whole line. A route whose level changes at every vertex needs 3DPOLY; see 3D polylines in AutoCAD.
Three things often repeated that are not true
| Often repeated | What the 2027 release actually does |
|---|---|
| The join command only works on polylines | It joined two ordinary lines, even with a gap of 50 |
| The join command will merge lines that turn a corner | The program answered 0 lines joined to source and discarded two objects |
| With a gap, just raise the fuzz distance | The default Extend still answered 0 segments added; you must switch to Add |
Frequently asked questions
Why does selecting a boundary give me no area?
Because it is a set of loose lines rather than one polyline. Merge them with PE, the Multiple option and then Join, and the area becomes readable.
How do I fix a line that comes out thick?
Type PLINEWID and enter 0. The width recorded in the object equals the value of that variable at the moment of drawing.
How do the join command and the pedit command differ?
JOIN merges only pieces lying on the same straight line and hands back a line. PE merges a whole set that turns corners and hands back a polyline.
There is a tiny gap and nothing joins.
Go into the Jointype option and choose Add or Both before entering the tolerated gap. The default method cannot join two collinear segments.
Can I split a merged polyline again?
Yes, with EXPLODE. We exploded a four-sided polyline and counted exactly 4 lines. The area goes with it.