Choosing your units

AutoCAD is extremely flexible about drawing units; it lets you have them your way. Usually, you choose the type of units that you normally use to talk about whatever you’re drawing: feet and inches for a building in the United States, millimeters for almost everything in almost all the rest of the world, and so on.

Speaking of millimeters, here’s another choice you have to make even before you choose your units of measure — and that’s your system of measure.

Most of the world abandoned local systems of measure generations ago. Even widely adopted ones, like the Imperial system, have mostly fallen by the wayside (just like their driving force, the British Empire) — except, of course, in the United States, where feet, inches, pounds, gallons, and degrees-Fahrenheit still rule.

Make sure everyone agrees on the units being used. NASA once crashed a very expensive space probe onto Mars because of a mix-up between imperial versus metric systems.

During drawing setup, you choose settings for length units (for measuring linear objects and distances) and angle units (for measuring angles between nonparallel objects or points on arcs or circles) in the Drawing Units dialog box, as shown in Figure 4-1. (I show you how to specify these settings in the section “Setting your units,” later in this chapter.) AutoCAD’s length unit types are as follows:

• Architectural units are in feet and inches and use fractions to represent partial inches: for example, 12′3 1/2″. The base unit is the inch unless otherwise specified, so if you enter a number like 147.5, then AutoCAD will understand it to be 12′3 1/2″.
• Decimal units are unitless — that is, they’re not based on any particular real-world unit. With decimal units, each unit in the drawing could represent an inch, a millimeter, a parsec, a furlong, a fathom, a cubit (if you’re into building arks in case that rainy day should come), or any other unit of measure you deem suitable, from Danish alens to the Swiss zoll. An example would be 15.5.
• Engineering units are in feet and inches and use decimals to represent partial inches: for example, 12′3.5″.
• Fractional units, like decimal units, are unitless and show values as fractions rather than decimal numbers: for example 15 1/2.
• Scientific units, also unitless and show values as exponents, are used for drawing really tiny or really large things. If you design molecules or galaxies, this is the unit type for you. Examples would be 15.5E+06 (which is 15,500,000) and 15.5E–06 (which is 0.0000155).

AutoCAD’s angle unit types are as follows:

• Decimal Degrees show angles as decimal numbers and are by far the easiest to work with, if your type of work allows it.
• Deg/Min/Sec is based on the old style of dividing a degree into minutes and minutes into seconds. Seconds aren’t fine enough to display AutoCAD’s precision capabilities, though, so seconds can be further divided into decimals. There is no degree symbol on a standard keyboard, so AutoCAD uses the lowercase letter d. An example would be 45d30′10.7249″. One nautical mile (6,076 feet) is approximately 1 minute of arc of longitude on the equator. David Letterman once said that the equator is so long that it would reach once around the world.
• Grads and Radians are mathematically beautiful (so we’re told) but are not widely used in drafting. Apparently, the French artillery uses grads but as long as we’re friends with them we shouldn’t have to worry. There are 400 grads, and 2*pi (6.2831…) radians, in a circle.
• Surveyor’s Units type is similar to Deg/Min/Sec but uses quadrants (quarter circles) rather than a whole circle. An angle in Deg/Min/Sec might measure 300d0′.00″, while the same angle in Surveyor’s Units would be represented as S 30d0′0.00″ E.

The unit types you’ll most likely use are Decimal, Architectural, and Decimal Degrees. You’ll know or be told if you need to use one of the other types.

AutoCAD always works internally to something like 16-digit accuracy. Changing the type of units changes only how values are displayed, suitably rounded off. For example, a line that’s 15.472563 decimal units long would display as 12″3 1/2″ long when units are changed to Architectural. The actual length — and any calculations based on it — remains at 15.472563 and does not change to 15.5″.

When you use dash-dot linetypes (Chapter 11) and hatching (Chapter 15) in a drawing, it matters to AutoCAD whether the drawing uses an Imperial (inches, feet, miles, and so on) or metric (millimeters, meters, kilometers, and so on) system of measure. The MEASUREINIT and MEASUREMENT system variables control whether the linetype and hatch patterns that AutoCAD lists for you to choose from are scaled with inches or millimeters in mind as the plotting units. For both variables, a value of 0 (zero) means inches (that is, an Imperial-units drawing), whereas a value of 1 means millimeters (that is, a metric-units drawing). If you start from an appropriate template drawing (as described in the section “A Template for Success,” later in this chapter), the system variable values will be set correctly, and you won’t ever have to think about it. For an explanation of system variables and how to set them, see Chapter 26.

So why are there two variables? Simple. MEASUREINIT (short for measurement initial) sets the default value for all new drawings, and MEASUREMENT changes the value for the current drawing.

AutoCAD automatically sets the measurement system according to your country when it’s being installed, but it can be changed.

Weighing up your scales

Somewhat surprisingly, you don’t need to consider scale when setting up a new drawing. In fact, rule number 1 in AutoCAD is to always draw everything full size. Hmmm, that makes quite a few rule number 1s, doesn’t it?

“Wait a minute! I want to draw a map of the known universe! If I draw full size, where am I going to find a sheet of paper big enough to print it and who will help me fold it?”

Trust me. All will become clear shortly.

“Okay,” you’re saying, “I understand that I need to print my drawings at a scale acceptable to the discipline I work in. But if I’m drawing stuff full size, when do I need to worry about the scale factor?” Grab yourself a nice mug of cocoa and settle down ’round the fire because I’m going to tell you. By now you know (because I’ve told you so) that you draw real things full size, but drawings contain other things that are not real, such as text, dimensions, hatch patterns, title blocks, dash-dot linetypes, and so forth. And those nonreal things need to be legible on your plotted drawing.

Say, for example, you draw a plan of your garage, and now you want to plot it on an 11 x 17″ sheet of paper. No problem; just tell the PLOT command to scale everything down by a scale factor of 1:24, which architects would commonly represent as 1/2″=1′0″.

Oops, problem. Text annotations are typically about 3/32″ or 1/8″ high. Now, if you draw your 6-inch–wide wall full size, put a 1/8″-high title beside it, and then print the drawing at a scale of 1:24, the wall itself will measure 1/4″ on the sheet, and the note will be an illegible little speck beside it. You fix it by making the text 24 times larger, or 3 inches tall, so that it scales down to the correct size when plotted. See, sometimes two wrongs do make a right. Unusually, it takes three, although three rights make a left and two Wrights made an airplane.

The “Drawing scale versus drawing scale factor” sidebar explains how you arrive at the scale factor, but this isn’t an issue until it comes time to annotate your drawing. I cover noncontinuous linetype scaling in Chapter 9, text sizing in Chapter 13, dimensions in Chapter 14, and crosshatching in Chapter 15. The good news is that AutoCAD can now take care of all this automatically, but rumor has it that there may still be a few drawings out there that did it the old way so I cover both methods in the appropriate chapters.

You shouldn’t just invent some arbitrary scale based on what looks okay on whatever size paper you happen to have handy. Most industries work with a small set of approved drawing scales that are related to one another by factors of 2 or 10. If you use other scales, at best, you’ll be branded a clueless newbie. At worst, you’ll have to redo all your drawings at an accepted scale.

The SCale command covered in Chapter 11 has nothing to do with ladders, fish, or setting drawing scales or scale factors!

Table 4-1 lists some common architectural drawing scales, using both Imperial and metric systems of measure. The table also lists the drawing scale factor corresponding to each drawing scale and the common uses for each scale. If you work in industries other than those listed here, ask drafters or coworkers what the common drawing scales are and for what kinds of drawings they’re used.

After you choose a drawing scale, engrave the corresponding drawing scale factor on your desk, write it on your hand (don’t reverse those two, okay?), and put it on a sticky note on your monitor. You eventually need to know the drawing scale factor for many drawing tasks, as well as for some plotting. You should be able to recite the drawing scale factor of any drawing you’re working on in AutoCAD without even thinking about it.

Even if you’re going to use the Plot dialog box’s Fit to Paper option (rather than a specific scale factor) to plot the drawing, you still need to choose a scale to make the nonreal things (such as text, dash-dot linetypes, hatch patterns, and so on) appear at a useful size. I cover plotting in Chapter 16.

Thinking about paper

You don’t normally need to worry about the size of the paper that you want to use for plotting your drawing until much later in the drawing process. And that’s the beauty of CAD: You can easily move views around to suit after you get the basic object drawn, and you don’t need to worry about scale factors until you’re ready to add annotations. I cover this in excruciatingly more (just kidding, it’s actually quite simple) detail in Chapters 12–16. Here again, most industries use a small range of standard sheet sizes.

Here are the two ways of laying out a drawing so it’s ready to be plotted:

• In model space: In this process, everything was drawn in model space, which was the only known universe. The drawing was created full size, while text and dimension component sizes, hatch pattern scaling, title blocks, and borders were all created at the inverse of the final plotting scale. This was the only way for many years, so you’ll encounter many drawings that were made this way.
• In paper space: Finally, Autodesk programmers figured out how to tunnel through into a parallel universe called paper space, which revolutionized drawing production. Current preferred practice is to draw the object in model space, cut a viewport in paper space so you can look through to the model space, and then apply documentation in paper space.

I cover model space versus paper space in Chapter 16.

Defending your border

The next decision to make is what kind of border your drawing needs. The options include a full-blown title block, a simple rectangle, or nothing at all around your drawing. If you need a title block, do you have one, can you borrow an existing one, or will you need to draw one from scratch? Although you can draw title block geometry in an individual drawing, you’ll save time by reusing the same title block for multiple drawings. Your company or client should already have a standard title-block drawing ready to use, or someone else who’s working on your project may have created one for the project.

The most efficient way of creating a title block is as a separate DWG file, drawn at its normal plotted size (for example, 36″ x 24″ for an architectural D-size title block, or 841mm x 594mm for an ISO A1-size version). You then insert (see Chapter 17) or xref (see Chapter 18) the title block drawing into each drawing sheet.

Setting your units

Follow these steps to set the linear and angular units that you want to use in your new drawing:

1. Click the Application button and then choose Units from the Drawing Utilities group.

   The Drawing Units dialog box appears, as shown in Figure 4-3.

2. Choose a linear unit type from the Length Type drop-down list.

   Choose the type of unit representation that’s appropriate for your work. Engineering and Architectural units are displayed in feet and inches; the other types of units aren’t tied to any particular unit of measurement. You decide whether each unit represents a millimeter, centimeter, meter, inch, foot, or something else. Your choice is much simpler if you’re working in metric: Choose Decimal units.

   AutoCAD can think in inches! If you’re using Engineering or Architectural units (feet and inches), AutoCAD interprets any distance or coordinate you enter as that many inches. You must use the ′ (apostrophe) character on your keyboard to indicate a number in feet instead of inches. Using the ″ symbol to indicate inches isn’t required but is acceptable.

3. From the Length Precision drop-down list, choose the level of precision you want when AutoCAD displays coordinates and linear measurements.

   The Length Precision setting controls how precisely AutoCAD displays coordinates, distances, and prompts in some dialog boxes. For example, the Coordinates section of the status bar displays the current coordinates of the cursor, using the current precision.

   The linear and angular precision settings affect only AutoCAD’s display of coordinates, distances, and angles on the status bar, in dialog boxes, and in the command line and Dynamic Input tooltip areas. For drawings stored as DWG files, AutoCAD always uses maximum precision to store the locations and sizes of all objects that you draw, regardless of how many decimal places you choose to display in the Drawing Units dialog box. In addition, AutoCAD provides separate settings for controlling the precision of dimension text. (See Chapter 14 for details.)

4. Choose an angular unit type from the Angle Type drop-down list.

   Decimal Degrees and Deg/Min/Sec are the most common choices.

   The Clockwise check box and the Direction button provide additional angle measurement options, but you’ll rarely need to change the default settings: Unless you’re a land surveyor, measure angles counterclockwise and use east as the 0-degree direction.

5. From the Angle Precision drop-down list, choose the degree of precision you want when AutoCAD displays angular measurements.

6. In the Insertion Scale area, choose the units of measurement for this drawing.

   Choose your base unit for this drawing — that is, the real-world distance represented by one AutoCAD unit. I discuss the significance of this in Chapter 17.

   The AutoCAD (but not the AutoCAD LT) Drawing Units dialog box includes a Lighting area where you specify the unit type to be used to measure the intensity of photometric lights. I introduce lighting as part of rendering 3D models in Chapter 23.

7. Click OK to exit the dialog box and save your settings.

Making the drawing area snap-py (and grid-dy)

For the first three decades, AutoCAD’s grid consisted of a set of evenly spaced dots that served as a visual distance reference. You can still configure a dot grid, but starting with AutoCAD 2011, the default is a snazzy graph–paper–like grid made up of a network of lines.

AutoCAD’s snap feature creates a set of evenly spaced, invisible hot spots, which make the cursor move in nice, even increments as you specify points in the drawing. Both Grid mode and Snap mode are like the intersection points of the lines on a piece of grid paper, but the grid is simply a visual reference — it never prints — whereas Snap mode constrains the points that you can pick with the mouse. You can set grid and snap spacing to different values.

You don’t have to toggle Snap mode on and off. It’s on only when you’re using a command that asks you to specify a point. For example, when Snap is on, you can move your crosshairs freely around the screen, but when you start the Line command, Snap mode kicks in, and your cursor jumps to the closest snap point.

Set the grid and the snap intervals in the Drafting Settings dialog box by following these steps:

1. Right-click the Snap Mode or Grid Display button on the status bar and choose Settings from the menu that appears.

   The Drafting Settings dialog box appears with the Snap and Grid tab selected, as shown in Figure 4-4.

   The Snap and Grid tab has six sections, but the Snap Spacing and Grid Spacing areas within that tab are all you need to worry about for most 2D drafting work.

2. Select the Snap On check box to turn on Snap mode.

   This action enables default snaps half a unit apart (ten units apart if you’re working with the default metric template).

   AutoCAD usually has several ways of doing things. You can also click the Snap Mode button on the status bar to toggle snap on and off; the same goes for the Grid Display button and the grid setting. Or you could press the function keys: F7 toggles the grid off and on, and F9 does the same for Snap mode.

3. Enter the snap interval you want in the Snap X Spacing text box.

   Use the information in the sections preceding these steps to decide on a reasonable snap spacing.

   If the Equal X and Y Spacing check box is selected, the Y spacing changes automatically to equal the X spacing, which is almost always what you want. Deselect the check box if you want to specify unequal snap spacing.

4. Select the Grid On check box to turn on the grid.

5. Enter the desired grid spacing in the Grid X Spacing text box.

   Use the information in the sections preceding these steps to decide on a reasonable grid spacing.

   As with snap spacing, if the Equal X and Y Spacing check box is selected, the Y spacing automatically changes to equal the X spacing. Again, you usually want to leave it that way.

   X measures horizontal distance; Y measures vertical distance. The AutoCAD drawing area normally shows an X and Y icon in case you forget.

   If you’re an old AutoCAD hand and find the graph paper grid too obtrusive, select the Display Dotted Grid in 2D Model Space check box in the Grid Style area to switch to the old-style rows and columns of dots. Interestingly, it seems most beginners like the grid, but most experienced users turn it off.

6. Specify additional grid display options in the Grid Behavior area:
   • Adaptive Grid: AutoCAD changes the density or spacing of the grid lines or dots as you zoom in and out.
   • Allow Subdivision Below Grid Spacing: Select this check box in conjunction with the Adaptive Grid check box. The spacing can go lower than what you’ve set, and it may go higher if you’re zoomed a long way out of your drawing. (If it didn’t, you couldn’t see your drawing for the grid!)
   • Display Grid Beyond Limits: This allows the grid to display over the entire drawing area, no matter how far you’re zoomed out. Clearing this check box makes AutoCAD behave the way it always behaved in earlier releases — that is, the grid is displayed only in the area defined by the drawing limits.

   • Follow Dynamic UCS: This option (not available in AutoCAD LT) is a 3D-specific feature that changes your drawing plane as you mouse over 3D objects. I cover this feature in Chapter 21.

7. Click OK to close the Drafting Settings dialog box.

Setting linetype, text, and dimension scales

Even if you’ve engraved the drawing scale factor on your desk and written it on your hand — not vice versa — AutoCAD doesn’t know the drawing scale until you enter it. Keeping AutoCAD in the dark is fine as long as you’re just drawing continuous lines and curves representing real-world geometry because you draw these objects at their real-world size, without worrying about plot scale.

However, as soon as you start using text, dimensions, and noncontinuous dash-dot linetypes (line patterns that contain gaps in them), you need to tell AutoCAD how to scale the gaps in the linetypes based on the plot scale. If you forget this, the dash-dot linetype patterns can look waaaay too big or too small. Figure 4-5 shows what I mean.

The process of setting the three relevant scale factors was messy in earlier releases but is now trivial. The Annotation Scale button near the right-hand end of the status bar displays the current drawing scale, which by default is 1:1/100%. Click it to display a list of 25 standard drawing scales.

You can always change the current drawing scale later if the need arises.

At some point in your career, you may encounter a drawing that was created in an earlier release. In the appropriate chapters, I explain how to deal with scale factors in these older drawings.

Entering drawing properties

I recommend one last bit of housekeeping before you’re finished with model space drawing setup: Enter summary information in the Drawing Properties dialog box, as shown in Figure 4-6. Click the Application button; in the Drawing Utilities section, choose Drawing Properties to open the Drawing Properties dialog box; then click the Summary tab. Enter the drawing scale and the drawing scale factor you’re using in the Comments area, plus any other information you think useful.

Don’t confuse drawing properties (which are really file properties) with your drawing’s object properties — they’re different things. The properties you enter here can help you or others you love when they open your drawing and wonder how you set it up. Object properties are a big enough topic to merit their own chapter. See Chapter 9.

Want to find a specific file hidden on your hard drive? You can search for values you've entered in the Drawing Properties dialog box by using Windows Explorer’s search functions.