Create Associative Center Marks and Centerlines

Have you ever needed to mark the center of an arc or circle? How about needing to draw a line at the apparent midpoint between two lines or polylines. I'm sure you have and there are two commands, CENTERMARK and CENTERLINE, that can make performing these tasks easier.

For an arc or circle, you might have used the DIMCENTER command, but it can be limiting. Or maybe you created a block for center marks and scaled the block as needed which also has its limits and can be time consuming.

Using the CENTERMARK and CENTERLINE commands can be much more efficient. With a few clicks you, can place a center mark at the center of a circle, arc, or polygonal arc. Or by selecting two lines you can draw a centerline at the apparent midpoint between two line segments or polylines. And by using center mark and centerline settings, layers and linetypes can be managed automatically.

Note: The video doesn't contain audio or closed captions.

Note: Steps, images, and videos may differ slightly from your version of the product.

Let's take a look at the various parts that make up a center mark.

When creating a center mark, three main parts are drawn.

Center mark - Cross that identifies the center of an arc or circle.
Centerline - Line that extends from the center mark cross to the edge of the selected geometry. A centerline is also the line drawn at the apparent midpoint between two lines or polylines.
Centerline Extension - Distance the centerline extends past the selected geometry.

Center marks and centerlines are associative by default. If you modify or move the associated object, the center marks and centerlines associated with the objects adjust accordingly.

In this example, I'll use grips to increase the radius of the selected circle. See how the centerlines extend past the edge of the resized circle the same distance they did when the circle was smaller?

Here, I'll select the circle and move it to the right. See how the center mark moves with the circle.

Note: Creating a copy of the circle will not copy the center mark associated with the circle.

Center marks and centerlines can also be disassociated and reassociated to their objects.

In the previous example, when the circle was moved, the center mark also moved because it's associated to the circle.

But if the center mark is moved away from the circle, the center mark becomes disassociated. When a center mark is not associated with an object, a yellow badge is displayed that indicates it is disassociated.

If the yellow badge is not displayed, check to see that the annotation monitor is active.

For information on how to use the annotation monitor, see the Annotation Monitor section of Have You Tried: Associative Dimensions.

I'll cover how to reassociate and disassociate center marks and centerlines later in this article Reassociate and Disassociate Center Marks and Centerlines.

Creating a Center Mark

Let's start by creating a center mark using the default settings.

Open a new drawing and create a few circles and arcs.
Click Annotate tab > Centerlines panel > Center Mark. Find
You can also enter CENTERMARK at the Command prompt.
Select a circle or arc.
Continue selecting circles or arcs to create additional center marks.
Press Enter to end the command.

A center mark is a single object that can be modified using grips.

I'll cover how to use grips to modify center marks and centerlines later in this article Grip Editing Center Marks and Centerlines.

Creating a Centerline

A centerline can be created between the apparent midpoint of the start and endpoints of selected line segments.

Line segments can be parallel or at an angle. They can also cross each other and even be perpendicular creating a centerline 45 degrees from the perpendicular intersection.

Parallel
Angle
Crossing
Perpendicular
Polylines

Note: Polylines are treated as separate line segments when creating a centerline.

For lines that intersect, the centerline bisects the angles of the two intersecting lines. In the following example, the location of the pick points defines the direction of the centerline.

Let's create a few centerlines using the default settings.

Open a new drawing and create a few line and polyline objects.
Click Annotate tab > Centerlines panel > Centerline. Find
You can also enter CENTERLINE at the Command prompt.
Select the first line.
Select the second line.

When creating centerlines between two lines of different lengths, you might need to use grips to modify the centerline to achieve the desired results.

In the following example, line 1 and line 2 are of different lengths.

I'll use the CENTERLINE command and select line 1 and line 2 to create a centerline. Notice the centerline did not extend past the bottom of the design? That's because the lines I selected for the centerline are not of the same length.

When lines are of different lengths, an apparent intersection is calculated between the endpoints of the two selected lines.

The centerline extends past the apparent intersection by the value specified in the CENTEREXE system variable, in this case 1 unit.

To modify this, I'll select the centerline and use the length grip to drag the centerline past the edge of the geometry.

Now the centerline extends past the edge of the geometry.

That worked out great. So far, we created several center marks and centerlines based on different types of geometry.

Creating a Center Mark for a Polygonal Arc

Center marks can also be created for a polygonal arc.

Note: When creating a center mark for a polygonal arc, each arc is treated as a separate arc segment.

Let's start by creating a polyline with arcs like the one shown in the following image.

Open a new drawing.
At the Command prompt, enter PLINE and specify a start point.
Enter a (for Arc).
Continue specifying points to create a polyline with arcs like the one shown below.
This polyline contains three arc segments.

See the PLINE Command topic for more information.
Click Annotate tab > Centerlines panel > Center Mark. Find
Select an arc segment along the polyline.
Continue selecting arc segments to add center marks.
Each arc segment should now have its own center mark.
Press Enter to end the command.

Changing the Properties of Center Marks and Centerlines

Properties can be changed using the Quick Properties and Properties palettes.

Common center mark property changes can include:

Turning extension lines on or off
Changing the color
Setting the extension length
Setting the cross size and gap

Select a center mark.
Note: If you select a center mark and the Quick Properties palette does not display, check to see that the QPMODE system variable is set to 1.
From the Quick Properties palette, Show Extension field, choose No.

Properties can also be changed using the Properties palette. With the Properties palette displayed, select a center mark or centerline to view, and change properties such as Color, Cross size, Cross gap, Extension length, and so on.

Setting a Default Layer for Centerlines and Center Marks

Let's set the layer for center marks and centerlines using the CENTERLAYER system variable. Until now, all the centerlines and center marks have been created using the current layer and properties, in this case layer 0.

CENTERLAYER is a layer override that only applies to center marks and centerlines created with the CENTERMARK and CENTERLINE commands.

Create a new layer and specify a name, such as centerlines, and a color so it's easily identified. It does not need to be set to the current layer.
At the Command prompt, enter CENTERLAYER.
Enter centerlines for the new value.

When a centerline or center mark is created it will be placed on the centerlines layer regardless of the current layer.

Note: If the layer name entered does not exist, a new layer is created with the specified name based on the properties of layer 0. Values other than the "." (use current) value override the current layer.

Setting a Default Linetype for Center Marks and Centerlines

Let's set the default linetype for center marks and centerlines using the CENTERLTYPE system variable. By default, the linetype used for center marks and centerlines is CENTER2.

At the Command prompt, enter CENTERLTYPE.
Enter the name of an available linetype you want to use for center marks and centerlines.

The CENTERLTYPE system variable applies only to the linetype used on the center marks and centerlines created with the CENTERMARK and CENTERLINE commands. This system variable accepts these values; "." (use current), ByLayer, or any valid linetype name.

See About Linetypes for more information.

Note: The CENTERLTYPEFILE system variable applies only to center marks and centerlines created with CENTERMARK and CENTERLINE commands. This system variable determines the .lin file that contains the linetype assigned to the CENTERLTYPE system variable.

Reassociate and Disassociate Center Marks and Centerlines

Center marks and centerlines have some of the same associative properties as dimensions. They can become disassociated if the geometry they measure is deleted. Like a dimension, when a center mark or centerline becomes disassociated and if the annotation monitor is active, you will see a yellow badge next to the center mark or centerline.

For information on how to use the annotation monitor, see the Annotation Monitor section of Have You Tried: Associative Dimensions.

If a center mark or centerline becomes disassociated from its geometry, you have a couple of options. You can delete the center mark or centerline if it is no longer needed, or you can reassociate it to geometry in the drawing.

To reassociate a center mark or centerline

At the Command prompt, enter CENTERREASSOCIATE.
Select the center mark or centerline to reassociate.
Select the geometry to associate with the center mark or centerline.

The disassociated center mark has been reassociated to the circle.

Associativity can also be removed from center marks or centerlines as needed.

At the Command prompt, enter CENTERDISASSOCIATE.
Select one or more center marks or centerlines to disassociate.

When the annotation monitor is active, a yellow badge will be displayed indicating the center mark or centerline is disassociated from the geometry. In this example, the center mark was disassociated without moving the center mark or deleting the geometry.

Grip Editing Center Marks and Centerlines

Center marks and centerlines can also be edited using grips. Hovering over a grip displays the dynamic dimensions for the extension line.

Hovering over the multi-functional center grip of a center mark displays a menu. Here you can stretch to move the center mark and uniformly change the length of all four centerline extensions.

Grips allow for the direct manipulation of center marks and centerlines. In this example, the centerlines were individually modified. When you drag a Length grip, it adjusts the length of the centerline and the centerline extension moves along with it. Dragging the Overshoot grip only changes the centerline extension.

When grips are used to modify centerlines, overrides are applied, and the modified centerlines become a fixed length. Any changes to the size of the circle will not affect the modified centerlines. Only the centerlines that have not been modified change lengths when the circle is modified. However, when you move the circle, the center mark (including the modified centerlines) moves with the circle.

See Have You Tried: Just Grip It! for more information on grip editing.

Reset Centerline Extension Length

The CENTERRESET command can be used to reset the lengths of centerline extensions modified using the Overshoot grips. If needed, the centerline extension can be reset to the value specified in the CENTEREXE system variable.

Note: CENTERRESET has no effect on changes made using the Length grip.

To restore the centerline extensions to the value stored in the CENTEREXE system variable.

At the Command prompt, enter CENTERRESET.
As an alternative, you can select one or more center marks and centerlines, and right-click > choose Reset Extension Length.
Select one or more center marks or centerlines.
Press Enter to end the command.

Customizing Center Marks and Centerlines

You can use system variables to control the appearance and behavior of a new center mark and centerline. There are four system variables that control the main aspects of a center mark and centerline:

CENTERCROSSSIZE - Determines the size of the associative center mark.
CENTERLTYPE - Specifies the linetype used by center marks and centerlines.
CENTEREXE - Controls the length of centerline extensions.
CENTERCROSSGAP - Determines the gap between the center mark and its centerlines.

Note: Changing these system variable settings does not affect existing center marks and centerlines.

Summary

I hope this helped to give you a better understanding of the CENTERMARK and CENTERLINE commands. With the ease of creation and flexibility offered by these commands, I'm sure you will be able to put center marks and centerlines to work for you.

Center Mark and Centerline Related Commands and System Variables

Here are some frequently used commands and system variables related to working with center marks and centerlines.

Command	Description
CENTERDISASSOCIATE	Removes associativity of center marks or centerlines from objects they define.
CENTERLINE	Creates centerline geometry associated with selected lines and linear polyline segments.
CENTERMARK	Creates an associative, cross-shaped mark at the center of a selected circle or arc.
CENTERREASSOCIATE	Associates or reassociates a center mark or centerline object to selected objects.
CENTERRESET	Resets the centerlines to the current value specified in the CENTEREXE system variable.

System Variable	Description	Default Value	Saved in
CENTERCROSSGAP	Determines the gap between the center mark and its centerlines.	"0.05x"	Drawing
CENTERCROSSSIZE	Determines the size of the associative center mark.	"0.1x"	Drawing
CENTEREXE	Controls the length of the centerline extensions.	0.1200 (imperial) or 3.5000 (metric)	Drawing
CENTERLAYER	Specifies a default layer for new center marks or centerlines.	"use current"	Drawing
CENTERLTSCALE	Sets the linetype scale used by center marks and centerlines.	1.000	Drawing
CENTERLTYPE	Specifies the linetype used by center marks and centerlines.	"CENTER2"	Drawing
CENTERLTYPEFILE	Specifies the loaded linetype library file used to create center marks and centerlines.	"acad.lin"	Drawing
CENTERMARKEXE	Determines whether centerlines extend automatically from new center marks.	1	Drawing