How to Change Drawing Units in Autocad 2008

When you first open AutoCAD, you are presented with a default drawing called Drawing1. You can start drawing right away in Drawing1 without having to set up a drawing area or determine the drawing units you want to use. And even if you do set up a drawing for a particular area and type of drawing unit, you can always change them at any time. So as you learn how to set up a drawing, keep in mind that you can alter, expand, and modify the setup at any time.

Selecting the Drawing Units

Though not absolutely essential, it is helpful to set up the drawing units in AutoCAD before you start your drawing. And eventually, you'll want to set up the drawing units, so you might as well make a habit of setting them up first.

You'll most likely use either imperial or metric measurement systems, although you are not limited to those two systems. You can regard the base AutoCAD unit as anything you want as long as you are consistent in your use of that unit.

The most commonly used system in the United States is the imperial system of inches and feet. Since the imperial system has some special requirements, AutoCAD provides additional options when you're using it.

To select a drawing unit, you use the Drawing Units dialog box.

If you want to just start drawing right away without setting up the drawing units, you can assume that the basic unit is the inch. Metric users can decide to use millimeters, centimeters, decameters, or meters, but whichever you choose to be the basic unit, make sure you stick with it throughout the drawing. You can tell AutoCAD what you want that unit to represent later by using the Drawing Units dialog box.

1. Choose Format è Units to open the Drawing Units dialog box (see Figure 2.9).

2. In the Length group, click the Type drop-down list, and then select the option that best represents the unit of length you will be using in your drawing. (See Table 2.1 for a description of these options.) If you are creating an architectural drawing using imperial units, select the Architectural option. This will let you specify distances in feet and inches.

   As you select an option in the Type drop-down list, you'll see a sample of the type in the Sample Output box at the bottom of the dialog box.

3. In the Angle group, click the Type drop-down list, and then select the option that best represents the type of angle you plan to use. (See Table 2.2 for a description of these options.) The Decimal Degrees option is the most commonly used angle option for most drawings, but if you are drawing a site plan, you can select Surveyor's Units to use a "metes and bounds" style of distance measurement.

4. Click OK.

   Once you've set up the drawing units, you can use that unit as you specify distances in AutoCAD. For example, if you select Architectural, you can specify distances in feet and inches. The Engineering option lets you specify distance in feet and decimal feet. If you are using a metric system, stick with the default Decimal option.

Figure 2.9: The Drawing Units dialog box

Table 2.1: The Length Options
Open table as spreadsheet

TYPE	DESCRIPTION
Architectural	Feet, inches, and fractional inches, as in 12'6½". You would enter this as 12'6 1/2" with a space between the 6 and the 1/2.
Engineering	Feet and decimal inches, as in 12.5' or 12'6.5". You would enter this as 12'6.5".
Decimal	Whole and decimal units that can be anything (metric distances, decimal inches, decimal feet, or decimal miles, for example).
Fractional	Whole and fractional units that can be anything (fractional metric distances, fractional inches, feet, or miles, for example).
Scientific	Scientific notation for distances. Units can be anything (meters, angstroms, miles, astronomic units, parsecs, or light years, for example).

Table 2.2: The Angle Options
Open table as spreadsheet

TYPE	DESCRIPTION
Decimal Degrees	Whole and decimal degrees of angle.
Deg/Min/Sec	Degrees, minutes, and seconds of angle.
Grads	Angles specified in grads. For example, you would specify 45° of angle by entering 50g.
Radians	Angles specified in radians. For example, you would specify 45° by entering 0.785r.
Surveyor's Units	Angles specified in degrees from north or south to east or west, as in N45dE.

Using Other Drawing Unit Options

You might want to know about a few other options in the Drawing Units dialog box. The following options are not as important as the type of length and angle you want to use, but they might play an important role in your work at some point:

• Precision The Precision option lets you control how AutoCAD reports length and angle values. This option does not actually affect the precision of the objects in the drawing. For example, if you select Decimal as the length type and 0.0 for the precision, AutoCAD displays a distance of 1.2 for a distance of 1.167. If you choose 0.000 for the Precision option, you will see the full distance of 1.167 whenever AutoCAD displays a distance.

• Direction By default, AutoCAD assumes that 0° is a horizontal direction from left to right. For example, 90° is directly vertical, and 180° is a horizontal direction from right to left. Angle values increase from 0° in a counterclockwise fashion. Figure 2.5 (earlier in this chapter) shows the cardinal directions and their degree values in a default AutoCAD setup.

  If your drawing requires a different direction for 0°, you can use the Direction option to choose an angle. When you click the Direction button, the Direction Control dialog box opens. Click one of four radio buttons to select the 0° direction from the four cardinal directions. If your desired 0° direction does not conform to the cardinal directions, you can specify an angle numerically or graphically by selecting the Other option.

• Clockwise By default, angle values increase counterclockwise, but you can change this to clockwise by turning on the Clockwise option in the Angle group of the Drawing Units dialog box.

• Insertion Scale AutoCAD can automatically scale a drawn object using the DesignCenter feature (see Chapter 8 for more about the DesignCenter).

  If you draw an object in inches but you later need to import that object into a drawing created using millimeters, AutoCAD can automatically scale your object so it is the correct size in the metric drawing. To take advantage of this feature, you must specify the type of unit you are using for the current drawing in the Insertion Scale drop-down list. For example, if the current drawing uses feet and inches, select inches from the list.

• Lighting When creating 3D models, you can add light sources and control their intensity. This option determines the standard used for measuring light intensity.

Determining the Drawing Area

One important concept you need to know is that AutoCAD's drawing area is virtually limitless. Although your view of a new drawing might show you an area that is only 60 units by 30 units, you are not confined to that area. If you like, you can include the entire Western hemisphere of the world in your drawing area, even if you are only drawing a plan of your backyard.

With such a limitless area to work with, you need to set boundaries. To set up your drawing area, think of a reasonable area for the drawing you are about to start. You don't have to be too precise because you can always change it.

Drawing a Reference Rectangle

The first step is to determine what the drawing unit represents and then think of the area you want in real-world terms. You can go about this in a number of ways. This example uses a rectangle to help you see the drawing area more easily.

Suppose you are starting a drawing of a house plan. You know that the lot size is 100' by 50'. First, set up your drawing units to be Architectural, and then do the following:

1. Choose Format è Units to open the Drawing Units dialog box, select Architectural from the Type drop-down list, and click OK.

2. Choose Rectangle from the 2D Draw control panel. In the command window, you'll see the following prompt:

                 Specify first corner point or [Chamfer/Elevation/Fillet/Thickness/Width]:            

3. Enter 0,0. This tells AutoCAD you want the first corner of the rectangle at the origin of the drawing, which is the lower-left corner of a new drawing. If you remember your high-school geometry, 0,0 is the coordinate for the origin of an XY graph. You'll see the next prompt:

                 Specify other corner point or [Area/Dimensions/Rotation]:            

4. Enter 100', 50' to draw a 100' by 50' rectangle. The value you entered, 100', 50' is an absolute coordinate that is 100' in the X direction and 50' in the Y direction.

5. The rectangle is larger than the display of a new drawing, so to view the entire rectangle, choose View è Zoom è Extents.

You've now set up your view to include the entire area enclosed by the rectangle. This is a quick way to set up your drawing area, and it gives you the visual reference of the 100' by 50' rectangle.

Using Limits to Set Up the Drawing Area

In the previous section, you saw how to use a rectangle to set up a drawing area. Another tool for setting up the area is the Limits command. Unlike drawing a rectangle, the Limits command just defines an area. No visual clues show you what that area looks like, but you can choose View è Zoom è All to display the area set by Limits.

Here's how to set up the limits of a drawing:

1. If you did the previous exercise, delete the rectangle from the drawing. To do this, select the rectangle, and press the Delete key or click the Erase tool in the 2D Draw control panel.

2. Choose Format è Drawing Limits. You'll see the following prompt:

                 Reset Model space limits: Specify lower left corner or [ON/OFF] <0'-0",0'-0">:            

3. Press ↲ to accept the default location for the lower-left corner, which is the origin of the drawing. You'll then see the following prompt:

                 Specify upper right corner <1'-0",0'-9">:            

4. Enter a coordinate value representing what you want to use for your drawing. It should be the actual area at full scale. To use the example of the site plan from the previous section, enter 100', 50'. Once you've entered a value, it will appear as though nothing has happened.

5. Choose View è Zoom è All. Again, nothing apparently has happened. However, now as you move your cursor to the upper-right corner of the drawing area, you'll see that the coordinate readout in the lower left of the AutoCAD window shows a coordinate of about 100' by 50'.

The drawback to the Limits command is that it does not give you any visual feedback. So why use it? It provides the following features that can be quite useful:

• You can choose View è Zoom è All to quickly display the area set by the Limits command.

• You can use the On/Off option of the Limits command to force your drawing to stay within the boundary set by the Limits command.

• The Grid feature, which displays a grid of dots, displays only within the boundary set by the Limits command. (This is important if you plan to use the Grid feature.)

If you find that you use the same drawing setup repeatedly, you can create template files that are already set up to your own, customized way of working. I discuss templates in Chapter 5.

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