Dimensioning in the Model Tab or a Layout Tab

The first big question new users ask regarding dimensioning is which tab to use for dimensioning. Just as with text, you can add dimensions in either the Model tab or a layout tab. This is an important choice because it affects the way you set up your dimensions. Here are some points to remember when deciding which tab to use:

You can add your dimensions in a layout tab, which has the advantage of letting you specify sizes of text, arrows, and other dimension features at their full size without having to add annotative scales . The drawback is that you cannot see or edit the dimensions while you work on your drawing in the Model tab. You have to be either in the layout tab space known as paper space or in the Model tab space, also known as floating model space . However, dimensions do have a feature called associative dimensioning that allows them to automatically adjust to changes in the objects they are dimensioning, no matter which tab you use to add dimensions.
You can add your dimensions directly in the Model tab. This has the advantage of allowing you to see and edit text and graphics at the same time without switching between model and paper space. The disadvantage is that you have to make sure you're using the annotative scale feature and that you have assigned the proper annotative scale values to your dimensions. This ensures that the arrows and text appear at the correct size when you plot your drawing. (See "The Scale for Dimension Features Group" in Table 10.6 later in the chapter for the Scale Factor setting.)

Even if you choose to do all your dimensioning in a layout tab, you'll want to know how to work with dimensions that have been drawn in the Model tab. Countless older AutoCAD drawings are dimensioned in the Model tab. If you work with AutoCAD professionally, one of those drawings will eventually cross your computer screen.

As you begin to use dimensions in AutoCAD and you're working in the Model tab, you might find that the arrows and text are way too small for your drawing. This usually happens when new users try to dimension a drawing in the Model tab.

If this happens, you'll need to change the way AutoCAD scales dimension components. Just as with text, dimension components such as arrows and text need to be scaled up to appear correctly in the Model tab (see Chapter 9). You can use the annotative scale feature of AutoCAD to set the scale for dimensions. The procedure for using annotative scales for dimensions is the same as it is for text. See "Understanding Text and Scale" in Chapter 9 to learn more about the annotative scale.

Drawing Linear Dimensions

The most common type of dimension you'll be using is the linear dimension , an orthogonal dimension measuring the width and length of an object. AutoCAD provides three dimensioning tools for this purpose: Linear, Continue, and Baseline. These tools are readily accessible from the Dimension drop-down list.

Placing Horizontal and Vertical Dimensions

Linear dimensions are those that are aligned either vertically or horizontally. They constitute the bulk of dimensions in most projects. The following steps describe how to apply linear dimensions.

Using osnaps is crucial in producing accurate results in all dimensioning tasks .

Choose Dimension Linear from the drop-down menu, or enter dli ↲ at the command prompt.
At the Specify first extension line origin or <select object>: prompt, Shift+right-click, and use an osnap to select the exact location on an object, such as an endpoint or an intersection of two lines on the drawing you're dimensioning (see Figure 10.2).

The prompt in step 2 gives you the option of pressing ↲ to select an object. If you do this, you're prompted to select the object you want to dimension, rather than the actual distance to be dimensioned.
At the Specify second extension line origin: prompt, use an osnap to select the other end of the object you're dimensioning, as shown in Figure 10.2.
In the next prompt, Specify dimension line location or [Mtext/Text/Angle/Horizontal/Vertical/Rotated]: , you'll see a temporary dimension at the cursor location. Position the temporary dimension where you want the dimension to appear, and then click.

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Figure 10.2: Using a linear dimension

If you prefer to be more precise about the dimension line location, in step 4 you can enter a relative distance from the last point selected.

Continuing a Dimension

You'll often want to enter a group of dimensions strung together in a line. You can use the Continue option in the Dimension menu to do this. This option assumes that you've already placed one linear or aligned dimension and are ready to continue with a string of dimensions from the last dimension (see "Dimensioning Nonorthogonal Objects" later in this chapter for information about aligned dimensions).

Choose Dimension Continue, or enter dco ↲ .
At the Specify a second extension line origin or [Undo/Select] <Select>: prompt, select the next location you want to dimension (see the left image in Figure 10.3). You can continue to add more dimensions until you press ↲ .

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Figure 10.3: A continued string of dimensions

If you find that you've selected the wrong location for a continued dimension, click the Undo tool, or enter u ↲ .

The Continue Dimension option continues from the last dimension you added to the drawing. The last drawn extension line is used as the first extension line for the continued dimension. If you need to continue a string of dimensions from a dimension other than the last one you placed in the drawing, press ↲ at the Specify a second extension line origin or[Undo/Select]: prompt in step 2 of the previous example. When you see the Select continued dimension: prompt, click the extension line from which you want to continue. You can then proceed to add dimensions.

Drawing Dimensions from a Common Base Extension Line

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Frequently, you need to dimension from a single datum point, as shown in Figure 10.4. This means you need several dimensions starting from the same location. To accommodate this, AutoCAD provides the Baseline option.

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Figure 10.4: A baseline dimension

As with the Continue option, Baseline assumes you have already placed at least one other linear or aligned dimension in the drawing.

Since you usually select exact locations on your drawing as you dimension, you might want to turn on running osnaps to avoid the extra step of selecting osnaps from the Osnap shortcut menu.

Choose Dimension Baseline or enter dba ↲ to start a baseline dimension.
At the Specify a second extension line origin or [Undo/Select] <Select>: prompt, use an osnap to select another feature you want to dimension in your drawing.
Press ↲ twice to exit the Baseline Dimension command.

The Baseline Dimension option works in a similar way as the Continue Dimension option does, except that Baseline Dimension allows you to use the first extension line of the previous dimension as the base for a second dimension.

The distance between the two parallel dimension lines is controlled by the Baseline Spacing setting in the Lines tab of the New Dimension Style and Modify Dimension Style dialog boxes.

Just as with the Continue Dimension option, the Baseline Dimension option continues from the last dimension you added to the drawing by default. If you need to add more baseline dimensions from a dimension other than the last one you placed in the drawing, press ↲ at the Specify a second extension line origin or [Undo/Select] <Select>: prompt in step 2 of the previous example. Then click the extension line from which you want to continue. You can then proceed to add dimensions.

Adding a String of Dimensions with a Single Operation

AutoCAD provides a method for creating a string of dimensions using a single operation while in the Model tab . The Qdim command lets you select a set of objects instead of having to select points. The following exercise demonstrates how the Qdim command works.

To use Qdim, choose Dimension Quick Dimension, or enter Qdim ↲ . Next, place a crossing selection window (start your window to the left of the selection area) around the area you want to dimension, and then press ↲ . You'll see a string of dimensions at the cursor. Click to place the dimension string in the drawing.

The Qdim command can be a time-saver when you want to quickly dimension a wall in an architectural drawing, for example, or any set of contiguous lines in a drawing. It might not work in all situations, but if the object you're dimensioning is fairly simple, it can be all you need.

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This example uses a simple crossing window to select the wall. For more complex shapes , try using a crossing polygon selection window (see "Selecting Objects" in Chapter 5 for more about the Crossing Polygons selection).