Metric Units in AutoCAD

AutoCAD is essentially a dimensionless drawing tool. A drawing unit in AutoCAD can represent any kind of dimension—meter, foot, inch, angstrom, mile and so forth. AutoCAD does not automatically assume a system of units for its drawing unit. Thus, a user can draw a line 100 units long, and those 100 units can represent a foot, a meter, a chain or anything else. This gives AutoCAD great flexibility, especially considering that its zoom ratio is about 10 trillion to one. AutoCAD will, however, assume some defaults for the user to simplify the set up of drawing dimensioning and plotting.

AutoCAD Automates Conversion

AutoCAD can automate conversion between metric units and English units as engineering units (feet, inches and decimal inches 0' to 00.00"), architectural units (feet and fractional feet 0' to 0 1/4") and others. It does not have linear units, although it offers angular units. This article demonstrates how a user can annotate a metric drawing to display English units (feet and hundredths of a foot [0.00']).

Assuming the user received a metric drawing whose units are millimeters and that has already been dimensioned in millimeters (see Figure 1), the user should update the existing dimensions to display surveyors' units. This building must be laid out and the user has not yet purchased metric tapes.

To force AutoCAD to display the dimensions in millimeters and feet, as shown in Figure 2, the user should enable alternate units in the dimensioning style. Because the current units are millimeters, the alternate units will be feet. The user should type the AutoCAD command ddim to call up the dimensioning dialog box (Figure 3).

The AutoCAD dimension style dialog box shows that the current dimension style is a metric style, ISO-5. The user could create a new dimension style, but this example will merely modify the current style. The user should leave everything the same on this screen and press the annotation button to get to the next screen (Figure 4), which controls the annotation for the current dimension style only.

Labeling Dimensions

There are separate controls for Primary units and Alternate units. Under Primary units the user should be sure that the suffix is set to "mm," so that all dimensions will have a unit label attached to them. It could be costly if someone presumed that "100" meant "100ft" instead of "100mm."

Under Alternate units the user should be sure to check the box Enable Units to tell AutoCAD to put two numbers on every dimension—the primary units and the alternate units. The user also should specify that the alternate units suffix is "ft." The other important thing to do is tell AutoCAD the correct scaling factor for the surveyor's units. The user should select the units button under "Alternate units" to call up the next dialog box (Figure 5). Under "units" the user should select Decimal and 0.00 for precision. For scale a little calculation is necessary to convert millimeters to feet and hundredths of a foot because AutoCAD defaults to converting the millimeters to inches. The conversion of millimeters to feet is as follows: 25.4mm/in x 12in/ft = 304.8mm/ft, so to convert from millimeters to feet the user needs the inverse or 0.00328084, which is the scale factor for millimeters to feet. To convert meters to feet the user should move the decimal place to the right three places. This scale factor should look familiar.

After typing this scale factor into the "Linear" edit box, the user should select OK to exit the "Annotation Units" dialog box, OK to exit the "Annotation" dialog box and OK once more to exit the "Dimension Styles" dialog box.

Using Two-Unit Dimensions

The current dimension style has now been modified. AutoCAD will then display two sets of units on any new dimensions created. If the drawing has already been dimensioned, the existing dimensions will not display the alternate units unless the user explicitly tells AutoCAD to do that.

A user who wants to have both the original, unchanged dimensions and the new ones should use AutoCAD's dimension commands and start creating the new (two-unit) dimensions.

Updating Dimensions

To tell AutoCAD to update the existing dimensions with the new alternate units, the user should use the command dimstyle, select the option Apply, then select the objects (dimensions) to apply this new style (using the ALL option when selecting objects will select everything in the drawing). When the user has finished selecting objects and presses return, AutoCAD applies the new dimension style to all the dimensions selected.

Thus the user has modified the dimension style to show the alternate units of any dimension. Rather than scaling or modifying any geometry in the drawing, the user merely directed AutoCAD to provide more information about the existing geometry.

A user who needed to fit this drawing to another drawing in feet would need to scale one or the other. For example, to insert a bridge plan designed in meters into a right-of-way plat in decimal feet, a user would have to scale the bridge design down to feet (this could be done by using the AutoCAD INSERT command and using X and Y scale factors of 3.28084). Incidentally, when AutoCAD prompts for a scale factor the user can respond with the AutoLISP expression (cvunit VALUE "FROMUNITS" "TOUNITS"), where VALUE is the number representing how much to convert, the FROMUNITS would be the units to convert from, and the TOUNITS the units to convert to. The user should be sure to put parentheses around the entire statement and quotation marks around the units. For example, to convert one meter to feet the user would type (cvunit 1 "m" "ft"). The file ACAD.UNT in the acad support directory can be used for additional unit conversions.

Adding Line Work

A user who needed to add more line work to the original metric drawing would need to convert each distance before typing in the information because AutoCAD has no mechanism for converting coordinates on the fly like using a scale factor. However, a user could (as I have) write an AutoLISP routine to perform those conversions and draw the line automatically.

The next challenge when dealing with a metric drawing is to plot it out at the correct scale. This will be dealt with in the next article, "Metric Plotting."

R. J. Zimmer is a consultant based in Helena, Montana, providing AutoCAD support as well as customizing, programming and GIS services, and is a Contributing Editor for the magazine.

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