This document will show how to set up your design files with either Imperial (English) or Metric working units, as well as how to convert between the two systems. Take care when changing working unit values, as this changes the size of any elements in the design file. The examples below will help in your conversion calculations.
Distances in design files are expressed in the form MU:SU:PU (Master Units:Sub Units:Positional Units). These are your Working Units. The Master Unit is divided by Sub Units, which in turn is divided by Positional Units. The Positional Units are the base which make up the Sub Units and Master Units. Since the design plane has a set number of Positional Units, the more 'Sub Units per Master Unit' and 'Positional Units per Sub Unit' you have, the smaller the 'Working Area in Master Units' will be.
When setting your Master Units, it is advised to use common symbols for the units of measurement. For example, you can use ft or ' for feet, and in or " for inches.
Setting Up a Design File in Imperial Working Units With a Relationship to Metric Units
Set your Master Units for feet, and your Sub Units for inches. Now set your Sub Units per Master Unit to be 12. This tells MicroStation that you want your Master Units (feet) to contain 12 Sub Units (inches). Now you can set up your Positional Units per Sub Unit. In this example, we are going to set up our Positional Units in Metric. This gives us a relationship between the two measurement systems, and makes it a simple exercise to convert the file to Metric measurements if needed. Set your Positional Units per Sub Unit to be 254. Since there are 25.4 millimeters per inch, this makes the Positional Unit in the file equal to one-tenth of a millimeter.
This setup gives you a working area of 1409110 feet squared. If you wish to increase the amount of Positional Units per Sub Unit, you simply increase the number. A value of 2540 will increase your Positional Units to a hundredth of a millimeter, and give you a working area of 140911 feet squared.
Setting Up a Design File in Metric Units
Setting up Metric units in your design file is much easier, because Metric units are all based on the number 10. Once again, use common symbols or abbreviations--m for meter, cm for centimeter, mm for millimeter, and so on. Set your Master Units for meters and your Sub Units for centimeters. Set your Sub Units per Master Unit to be 100, because you want 100 centimeters in your meters. Now set your Positional Units per Sub Unit to 100. Since we are dividing the centimeters by 100, this also sets our Positional Units to be one-tenth of a millimeter. You now have a working area of 429496 meters squared. Increasing the Positional Units per Sub Unit to 1000 will set our Positional Units to be a hundredth of a millimeter, and give you a working area of 42949 meters squared. Changing this file to Imperial measurements is a simple matter of changing the Positional Units to be a ratio of the 1 inch = 2.54 centimeters rule.
Converting a File With Imperial Measurements to Metric
Converting our Imperial example above will be an easy exercise. Since the file is already based in tenths of millimeters, changing the working units involves a minimum of calculation. You need to be careful that you use values that are proportionate to the Positional Units value. Don't turn mm into cm, or ft to yards. Since the Positional Units are known (tenth of a mm), you can calculate up from there. Here are a few possible MU:SU combinations that do not change our tenth of a millimeter base, nor the size of the elements.
- m:cm, or meters and centimeters, having a value of 100 cm per m. You also have to change the Positional Units per Sub Units to 100. This keeps the tenth of a millimeter base that was originally created, since there are 100 cm in a meter, and 100 tenths of a millimeter in a centimeter.
- m:mm, or meters and millimeters, having a value of 1000 mm per m. Change the Positional Units per Sub Units to 10, for the tenth of a millimeter base. There are 1000 mm in a meter, and 10 tenths of a mm in a millimeter.
- cm:mm, or centimeters and millimeters, having a value of 10 mm per cm. Change the Positional Units per Sub Units to 10 to keep the tenths of a millimeter base. There are 10 mm in a centimeter, and 10 tenths of a mm in a millimeter.
If you remember to keep the correct ratio between MU:SU:PU when converting, there won't be a problem.
Converting a File With Metric Measurements to Imperial
Since the Metric System is on a base of 10, it is a simple matter to convert to Imperial. In the Metric example file above, the working units are set to 100 centimeters per meter, and 100 positional units per centimeter. Change the Positional Units to 254, because you know there are 254 tenths of a millimeter in an inch. Change your Sub Units per Master Unit to 12. Change the name of your Master and Sub Units to feet and inches.
If you want to change your Master Units to yards, and your Sub Units to feet, you need to do a calculation. Multiply the value of the Positional Units (254) by 12, to get 3048. The value of the Positional Units must stay the same for each Sub Unit. The value of the Sub Unit is increasing by 12, from 1 inch to 12 inches (1 foot). If you increase the Positional Units by the same amount, then you are keeping the tenths of a millimeter ratio that we set up earlier. This means there are 3048 tenths of a millimeter in 12 inches (a foot). Change the Sub Units to Master Units to be 3 (3 feet in a yard). This changes the measurement units, but not the size of the elements in the design file.
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