COMPUTER AIDED ARCHITECTURAL DESIGN
Working Units and Scale Systems in Microstation

Setting up working units and then using them to control points in your coordinate system is a sometimes difficult concept to grasp. This handout will explain some of the concepts that underly the settings of Master Unit, Sub Unit, Positional Unit, and the relationships between the three.

When working in MicroStation, you are working within a defined and finite "universe". This "universe", known as the MicroStation drawing plane (drawing cube in 3D) has a fixed size based upon the way that MicroStation stores the coordinates of lines, points, and other elements in the design.

MicroStation stores each coordinate point as a number, and that number is stored in the .dgn file. That number, however, can only be so big. In fact the number has an upper limit of 2 multiplied by itself 32 times, which is a little over 4 billion. This is a pretty big number, so we would think that the overall size of the design plane would be quite large, but this is not necessarily the case.

When working in CAD, we cannot just say I need something to be 2 feet long and that's it. We, and the computer, need to go a little bit deeper than that and calculate a value for how precise the system is. This is controlled by setting a smallest unit of accuracy that the computer will understand, and equating that to a unit of normal measure in the real world such as feet or meters. When we do this, we are beginning to set up what our units are going to be in the model or drawing, and how the computer determines the lengths of lines or other objects according to this.

The smallest unit of accuracy in the CAD "universe" is known in MicroStation as a Positional Unit. This unit is static in terms of its use in the drawing database, and from this static unit, we work upward in scope to determine the relationship of higher level unit structures to this base positional unit size. The Working Units settings in MicroStation are used to control these relationships, and they proceed as follows:

The Positional Unit is the smallest unit of measure that MicroStation can detect. It has no defined size in real world space until we establish a Sub-Unit and a Master-Unit to have a meaning.

The Sub Unit is a superset of positional accuracy, defined as a certain number of Positional Units in length. Let's say that there are 1000 positional units contained within 1 Sub Unit.

The Master Unit is then a superset of the Sub-Unit, defined again as being some number of Sub Units in length. Let's say that there are 100 Sub Units for every Master Unit. This stacking order then gives meaning to the lengths of objects in CAD

This may seem to be backwards, and it is in our normal way of thinking about measurement from the top down, but this is the way that units are defined in CAD. They must be done from the bottom up, since the CAD system is only setting up relationships among values, all working off of the smallest unit that it can understand. With working units set up as above, we would have the equivalent of a metric unit structure, where the Master Unit might be named meters (m) and the Sub Unit would therefore be centimeters (cm). There are then 1000 increments of a centimeter that this measurement system can differentiate.

Let's take another example. If there are 64 positional units per sub unit, and 12 sub units per master unit, we can name the master unit feet and the sub unit inches. We now have an english unit system accurate down to 1/64 of an inch. An object which is 1 foot long, would therefore be

1 foot
or
12 inches
or
768 positional units (12 * 64) long.

The CAD system only understand the last one, positional units, but we understand the first two. The Working Units settings gives us the ability to make the CAD system work in measurement systems that are known to us, rather than the purely numerical structure that it understands and stores its database in.