Understanding the AutoCAD Coordinate System

Before you get too far into your use of AutoCAD, you'll want to know a little about its coordinate system. Coordinates are important because they allow you to specify exact locations in your drawing. You'll use coordinates frequently to set the size and location of objects, to move and copy objects, and to align objects relative to each other.

AutoCAD uses a standard XY Cartesian coordinate notation with a horizontal x-axis and a vertical y-axis, as shown in Figure 2.1.

Figure 2.1: AutoCAD's coordinate system

In a new drawing, AutoCAD shows the positive X and Y range of coordinates with the 0,0 coordinate, or origin, placed in the lower-left corner of the drawing area. The origin is an important coordinate location because you can use it as a reference for multiple drawings. For example, you can use the origin in a set of house plans to coordinate locations among a site plan drawing, a landscape drawing, and a floor plan.

Many AutoCAD commands ask you to choose a location by entering a point. For example, if you start the Line command, you'll see the following prompt:

 Specify first point: 

You can specify exact coordinates by entering the X and Y coordinates separated by a comma. You give the X coordinate first and then the Y coordinate. For example, you enter a location that is 5 units in the X direction and 4 units in the Y direction as 5,4 (see Figure 2.1). If you want to include the Z coordinate for a 3D location, you include it at the end of the list with a comma, as in 5,4,2; in this case, 2 is the Z coordinate. When you enter coordinates in this way, you are said to be using absolute coordinates, since these are exact locations in the overall coordinate system.

But for the most part, you won't be entering absolute coordinates to specify locations in a drawing. Usually, you'll want to specify locations that are relative to a point you select in the drawing area. You can specify a distance and direction from a selected point using relative coordinates.

You specify relative coordinates in the same way you specify absolute coordinates, with one major exception-you must precede the coordinate with the @ sign. The @ sign means "from the last point selected." For example, suppose you want to draw a horizontal line 3 units long in a random location in the drawing. You can start the Line command and then at this prompt

 Specify first point: 

you can select a point anywhere on the screen by clicking the drawing area. The next prompt asks for the next point of the line:

 Specify next point or [Undo]: 

At this point, you enter @3,0. This tells AutoCAD to draw a line that is 3 units in the X coordinate and 0 units in the Y coordinate from the last point selected.

Another way to specify distance and direction is to use polar coordinates. With polar coordinates, you specify a distance and an angle. For example, if you want to specify a point that is 40 units from another point and at a 45° angle, you enter @40<45. Here, the @ sign tells AutoCAD that you want to select a point relative to the last point selected, the 40 is the distance, and the <45 is the angle, as shown in Figure 2.2.

Figure 2.2: Drawing a line at a 45° angle

You won't have to use coordinates for everything you draw. You can accurately select locations on objects using object snaps, or osnaps for short. Osnaps are a way of selecting exact locations on existing objects in a drawing, such as the endpoint or midpoint of a line or the intersection of a line and a circle. See "Setting Up Osnap Locations" later in this chapter for more about osnaps.

Experimenting with Coordinates Using Lines

The preceding explanation of AutoCAD's coordinate system, though brief, can serve as a reference as you work with AutoCAD. If you need some additional practice, try the following to see firsthand how to use the coordinate methods. You'll use the Line command as a practice tool. These exercises will also help you become more familiar with the way AutoCAD works in general; so if you are totally new to AutoCAD, it is a good idea to do the exercises in this section.

Throughout the rest of the book, I'll be asking you to select tools from the control panels of the Dashboard. Instead of showing you the entire control panel, I'll show the tool as a margin icon next to the step that uses the tool, as shown in step 3 of the following exercise. If you need to refresh your memory of the locations of the control panels, see Figure 1.7 in the previous chapter.

To begin a line, follow these steps:

1. Start AutoCAD, choose File è New, and select the acad.dwt template. Make sure you are in the 2D Drafting & Annotation workspace.

2. Make sure the Dynamic Input option is off by clicking the DYN button in the status bar. It should appear in the off, or "up," position. You'll get a chance to use this feature later.

   

3. Click the Line tool on the 2D Draw control panel, or enter L↲ to start the Line command. AutoCAD responds in two ways. First, in the command line, you'll see this message asking you to select a point to begin your line:

    Specify first point: 

   The cursor has also changed its appearance; it no longer has a square in the crosshairs. This is a clue telling you to select a point to start a line (see Figure 2.3).

4. Click to select a point on the screen a little to the left of center. As you select the point, AutoCAD changes the prompt to the following:

    Specify next point or [Undo]: 

   Now as you move the cursor, notice a line with one end fixed on the point you just selected and the other end following the cursor (see the top image in Figure 2.3). This action is called rubber-banding.

   If you move the cursor to a location directly to the left or right of the point you clicked, you'll see a dotted horizontal line, along with a message at the cursor. This action also occurs when you point directly up or down. In fact, your cursor will seem to jump to a horizontal or a vertical position.

   

   This feature is called polar tracking. It helps restrict your line to an exact horizontal or vertical direction as a T square and triangle would. You can turn polar tracking on or off by clicking the Polar button in the status bar. If you don't see it, it's likely that it's turned off.

   Now continue with the Line command.

5. Click a point below and to the right of the center of the drawing area. The first rubber-banding line is now fixed between the two points you selected, and a second rubber-banding line appears. (See the bottom image in Figure 2.3.)

6. If the line you drew isn't the exact length you want, you can back up during the Line command and change it. To do this, click Undo (the left-pointing curved arrow) in the Standard toolbar, or enter U↲.

Figure 2.3: A rubber-banding line

The Undo tool in the Standard toolbar contains an Undo drop-down list from which you can undo to a specific command. See Chapter 1 for more information.

Now the line you drew previously will rubber-band as if you hadn't selected the second point to fix its length. You've just drawn, and then undrawn, a line of an arbitrary length. The Line command is still active. Two items tell you that you are in the middle of a command. If you don't see the word Command at the bottom line of the command window, a command is still active. Also, the cursor will be the plain crosshairs without the box at its intersection.

From now on, I will refer to the crosshair cursor without the small box as the Point Selection mode of the cursor. If you look ahead to the sidebar titled "Interpreting the Cursor Modes and Understanding Prompts," you'll see all the modes of the drawing cursor.

Specifying Exact Distances

Next, you will draw a simple rectangle 10 units wide by 2 units high. By drawing a rectangle to a specific dimension, you'll get a chance to specify exact distances. You can use either relative polar coordinates or Cartesian coordinates to draw in AutoCAD. You'll start by using relative polar coordinates, which is the most common method for distance input.

Specifying Polar Coordinates

To enter the exact distance of 10 units to the right of the last point you selected using polar coordinates, do the following:

1. Enter @10<0. As you type, the characters appear at the command prompt.

2. Press ↲. A line appears, starting from the first point you selected and ending 10 units to the right of it (see Figure 2.4). You have just entered a relative polar coordinate.

Figure 2.4: Notice that the rubber-banding line now starts from the last point selected. This tells you that you can continue to add more line segments.

Remember, you specify polar coordinates by starting with the @ sign followed by the distance, a less-than symbol (<), and finally the angle.

If you are accustomed to a different method for describing directions, you can set AutoCAD to use a vertical direction or downward direction by pressing 0↲. See Chapter 3 for details.

Angles are given based on the system shown in Figure 2.5, in which 0↲ is a horizontal direction from left to right, 90° is straight up, 180° is horizontal from right to left, and so on. You can specify degrees, minutes, and seconds of arc if you want to be that exact. I'll discuss angle formats in more detail in Chapter 3.

Figure 2.5: AutoCAD's default system for specifying angles

Specifying Relative Cartesian Coordinates

For the second line segment, let's try the Cartesian method for specifying exact distances. Follow these steps:

1. Enter @0,2↲. A line appears above the endpoint of the last line.

   As in the polar coordinate method, @ tells AutoCAD that the distance you specify is from the last point selected. But, in this step, you're giving the distance in X and Y values. You give the X distance, 0, first, followed by a comma, and then the Y distance, 2. This is how to specify distances in relative Cartesian coordinates.

2. Enter @-10,0↲.The result is a drawing that looks like Figure 2.6.

   The distance you entered in this step was also in X,Y values, but here you used a negative value to specify the X distance. Positive values in the Cartesian coordinate system are from left to right and from bottom to top (see Figure 2.7). (You might remember this from your high-school geometry class!) If you want to draw a line from right to left, you must designate a negative value.

3. Enter C↲. This C stands for the Close option of the Line command. It closes a sequence of line segments. AutoCAD draws a line connecting the first and last points of a sequence of lines (see Figure 2.8), and the Line command terminates.

Figure 2.6: These three sides of the rectangle were drawn using the Line tool. You can specify points using either relative Cartesian or polar coordinates.

Figure 2.7: Positive and negative Cartesian coordinate directions

Figure 2.8: The finished rectangle

To finish drawing a series of lines without closing them, you can press Esc, ↲, or the spacebar.

You can also use these methods when you need to specify distances when moving or copying objects or any time you need to specify a distance and direction.

Using the Direct Distance Method for Quick Relative Coordinates

A third way to enter distances is to simply point in a direction with a rubber-banding line and then enter the distance using the keyboard. For example, to draw a line 5 units long from left to right, click the Line tool on the 2D Draw control panel, click a start point, and then move the cursor so that the rubber-banding line points to the right at some arbitrary distance. While holding the cursor in the direction you want, enter 5↲. The rubber-banding line becomes a fixed line 5 units long.

Using this method, called the direct distance method, along with polar tracking, can be a fast way to draw objects of specific lengths. Use the standard Cartesian or polar coordinate methods when you need to enter exact distances at angles other than those that are exactly horizontal or vertical.

A slight variation on the direct distance method is to first enter an angle, such as <30↲, and then point in the direction you want and enter the distance.

Using the Dynamic Input Option

Many users find it bothersome to have to look at the command window while drawing and entering data. AutoCAD provides the Dynamic Input option that offers a kind of "heads-up" display for the command window. Dynamic Input displays the command window prompts at the cursor so you don't have to look down to see them. It also offers yet another method for entering distances and directions. Try the following exercise to see how Dynamic Input works:

1. Click the DYN button in the status bar to turn on Dynamic Input. The button should look like it is in the on, or "down" position. Also turn off the Polar option.

   

2. Click the Line tool in the 2D Draw control panel. Now as you move the cursor, you'll see the Specify first point: prompt at the cursor. You'll also see the coordinate readout.

   

3. Click a point below the left corner of the rectangle you just drew. Now as you move the cursor, you'll see the actual dimension of the rubber-banding line as well as its angle near the cursor. You'll also see the Specify next point or prompt at the cursor, and the length dimension is highlighted.

   

4. The length dimension being highlighted tells you that you can enter a value for that dimension. Enter 10, and then press the Tab key. As you enter 10, it appears in the horizontal dimension. Once you press the Tab key, the line remains fixed at a length of 10 units while the angle continues to follow the cursor.

   

5. Notice that the angle value is now highlighted. Enter 0↲ to fix the angle at 0°. The line is now fixed in place, and the next line segment is ready to be entered.

For the next line segment, try entering the angle first:

1. Press the Tab key. Notice that the angle value is now highlighted without affecting the length dimension.

2. Make sure the cursor is somewhere above the previous line, then enter 90, and press the Tab key. The angle is fixed at 90°, but the length is still variable. After you press the Tab key, the length dimension is highlighted.

   

3. Enter 2↲ for the length. Now you have a vertical line segment 2 units long. Now try entering a Cartesian coordinate.

4. The length value is now highlighted again ready for another line segment. Enter -10,0↲. A line segment is drawn from right to left. Notice that you didn't have to include the @ sign. With Dynamic Input turned on, AutoCAD automatically assumes you want to specify the object's dimension rather than a coordinate in the overall coordinate system. This applies only to points you enter after the initial point you select to start the object. If you prefer, for consistency, you can still use the @ sign while entering relative coordinates with the Dynamic Input display.

5. Enter C↲ to close the set of lines.

6. Turn off the Dynamic Input option by clicking the DYN button in the status bar.

As you worked through the exercise, you might have noticed a little downward-pointing arrow in the command line at the cursor. This tells you that you can press the down arrow key to access command options. The options are the same as those inside the square brackets in the command line.

The Dynamic Input option offers yet another method for drawing and editing objects. As you can see from this example, you can enter a value for an object's dimension and then use the Tab key to move to the next value displayed by the cursor. If you prefer, you can use the Tab key to move to a different value from the current highlighted value without entering anything, or you can just enter distances in the usual way, using Cartesian or polar coordinates.

Other commands, such as Rectangle and Circle, offer different dimension values that are more appropriate to the object being drawn, but the process is still the same: enter a value for the highlighted dimension by the cursor, and then use the Tab key to move to the next dimension value. If you want to enter relative Cartesian or polar coordinates, you don't have to include the @ sign, though you can if you want to for consistency. You can also use the direct distance method mentioned earlier, with the added benefit of using the angle readout of the Dynamic Input display. Use the Dynamic Input display's angle readout to point the rubber-banding line at the desired angle, and then enter the length you want followed by ↲. AutoCAD will draw the line at the length you specify and at the indicated angle.

If you want to enter an absolute coordinate from the Dynamic Input display, use a number sign (#) in place of the @ sign.

Now that you have some general practice, you're ready to set up an actual drawing.

For the rest of the book, unless I'm specifically discussing the Dynamic Input option, I will assume that it is turned off. Although it is a great tool, it can create some confusion for new users and can create some visual clutter in some of the exercises and examples. You might also be receiving help from others who are not familiar with the Dynamic Input option, which would create further confusion for you as a new user. As you become more accustomed to using AutoCAD, you might want to turn it on and experiment with it on your own. I'll also explain other features of the Dynamic Input option in later chapters.