What Do You Mean the Global Origin Is Wrong?

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What's all the fuss?

By Rick Dewitt

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A full understanding of Global Origin is a good place to start.

In the many years I have worked with CAD professionals, one area that seems to be consistently misunderstood is "global origin". What exactly is it? What happens if you change it? How do you change it? How does it affect reference files? Does Axiom have any products that can help with this?

So, let's clear the air in this article by starting with exactly what the global origin is and give some explanation of how it is used.

The "global origin" of a MicroStation design file (V7) or model (V8) is a setting which defines where, relative to the MicroStation design plane (2D) or cube (3D), the point where X=0, Y=0 and, for 3D files, Z=0 is located.

It helps when visualizing this concept to think of the MicroStation 2D design plane as a big piece of paper and the global origin as a point on this big piece of paper that we refer to as X=0 and Y=0 in the coordinate system. Although you could put the global origin outside of the piece of paper, we'll assume, for this example, it's on the paper. You could set the global origin point, as one example, to be the lower left-hand corner of our piece of paper.

(See Figure 1.) If we decided to set our MicroStation design plane with this global origin setting then all the elements we draw on the design would have to have positive X and Y coordinate values because we are not allowed to draw elements outside the piece of paper.

If we set our global origin to be in the middle of our piece of paper then we would have the ability to draw using both the positive and negative X and Y axis directions.

So, what's all the fuss about global origins?

Why not just set the global origin in the center of the design plane or cube for every drawing and get to work?

Well, for example, one of the main reasons for difficulties with global origins stems from the fact that, in MicroStation versions prior to V8, the design plane wasn't big enough for many users' needs.

To solve this problem, many companies and government entities developed CAD standards that demanded the use of a global origin that was located far away from the center of the design plane and still insist on this location today, in order to remain compatible with older (pre-V8) designs. If you will indulge me, I shall elaborate on this so you will have a complete understanding of what I just wrote.

Figure 1 - If you set the global origin to the lower left-hand corner of a 2D design plane, then all of your drawing coordinates will be positive numbers.
Figure 2 - Figure 2. If the center of your design plane was, for instance, the center of a city, then setting the global origin in the center of the design plane would allow you to draw or place elements to the left (a negative "X" coordinate) or below the center of the city (a negative "Y" coordinate).

Prior to V8, the design plane was limited to 4,294,967,296 (2³²) accessible points on each axis. The accessible points in a design plane were called "positional units".

Depending on the degree of accuracy required, these accessible points were divided by the number of positional units assigned to each master unit (such as feet or meters) to determine the overall "real world" size of the design plane (how big it is in feet or meters).

A common usage would be a master unit of "feet" with a resolution of 1,000 positional units for each foot. So, if we divide our total number of accessible points by 1,000, we get the total length of each axis in feet. In this case 4,294,967.296 feet.

Now let's say we put the global origin in the middle of our pre-V8 design plane. This would then give us 2,147,483.648 feet in both the positive and negative direction for each axis (in MicroStation). For many MicroStation users, this size was sufficient.

However, for civil designs that use real-world coordinates, the number of feet required can easily exceed 4,000,000 on either axis.

For these users, setting the global origin outside the design plane was the only way they could place design elements on the design plane when drawing to locations that required large coordinate values.

Example: A road to be built in Oregon has to be drawn using exact locations based on a recognized geographic coordinate system (a standard).

In the United States, one common standard requires that geographic locations be represented by the number of feet that a location is to the east and to the north of a specific point (which point is also defined in the standard).

In parts of Oregon, the X coordinate (based on this geographic standard) can be in the neighborhood of 4,298,000 feet from that fixed point.

Therefore, when attempting to draw a road at this location in MicroStation V7, users had to have the global origin well off the design plane in order to draw their road at the correct "real-world" geographic coordinate point. (See Figure 3.)

So, what's all the fuss about global origins? Why not just set the global origin in the center of the design plane or cube for every drawing and get to work?

Figure 3 - Figure 3. Prior to MicroStation V8, some users such as civil engineers would sometimes have to set the global origin off the 2D design plane as the design plane simply wasn't big enough to accommodate elements placed in design files with very large coordinate values.

Fortunately, as of MicroStation V8, the size of the design plane is now approximately 2 million times larger on each axis, which allows for a "real world" design plane that is much larger than available in previous versions of MicroStation.

This fact alone frequently causes confusion when other companies (that don't still have this old requirement) create design files that are not based on this CAD standard.

Hopefully this description gives you a better understanding of "global origin" and of one of the main reasons global origins can cause difficulties.

Unfortunately, the Editor informs me that I have already exceeded the column inches available and must wait until next issue to finish my explanation and answer the other questions posed at the beginning of this article.

UPDATE: While MicroStation V8 remains the beginning point for the expanded coordinate plane, it must be mentioned that the above data is also valid for MicroStation V8i and MicroStation CONNECT.

See the next part of this article in an upcoming issue of MicroStation Today.

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