Many of the designers using CAD today don't fully realize the magnitude of accuracy provided by AutoCAD. Often in the past I could hear polemics, where Intergraph IGDS and Microstation users were "proving" that "their" software is better in all respects, one of them being accuracy. This was never the truth, as even in its infancy, in the early eighties, AutoCAD was using double precision floating numbers to store and process DWG data, where IGDS was using 32 bit integer cube to define space positioning of its elements.

Read on, and once you are through, download SOLAR.DWG and go on your interplanetary trip to witness AutoCAD accuracy :)

How much more precise is a double precision number from a 32 bit integer?
A double precision number uses 64 bits (8 bytes) to represent a single floating point number - twice as much as a 32 bit integer. Of these 64 bits, one is used for sign, 11 are used for exponent and 52 are used to define mantissa.

Graphical representation of such a 64 bit number would be:

S EEEEEEEEEEE FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF

where:

S - sign bit, 1 means negative, 0 positive
E - each E means one bit of exponent. 11 bit number can represent values from -1022 to 1023
F - each F represents one bit of a mantissa. 52 bit mantissa can represent 4503599627370496 fractions.

32 bit integer cube can represent  4294967296 discrete points in each of the three directions.

Below is a translation of what these numbers will mean to the processing and storage of CAD entities.

If you will try to draw a drawing that extends 4 km in each direction, Microstation's 32 bit format will limit your accuracy to 0.001mm. The same size extents in an AutoCAD drawing can be drawn with an accuracy of 0.000000001mm - that is a million times more accurate.

One could say "one accuracy is high enough and another is overkill, so what?". But then, he doesn't realize that with almost EVERY geometrical operation performed on an entity, the accuracy is reduced. When entities are moved, rotated, scaled, stretched etc., complex mathematical transformations are being applied to their geometry. The results are stored back in the drawing database: in AutoCAD with double precision floating point accuracy, in Microstation with 32 bit integer accuracy. Both, math transformation and storage, are REDUCING the accuracy of a drawing. Where an AutoCAD user can safely ignore a 3 or 4 significant digit reduction in accuracy in a drawing that has been modified thousands times over the years (he still has 12 precise digits), the same cannot be said about a Microstation user that has maximum of 10 precise digits and loses 3 of them in complex processing).

To showcase the power of AutoCAD detailing, Autodesk was including in earlier releases of AutoCAD (I think, up to R12) the simple, yet very impressive, 2D drawing file SOLAR.DWG (drawn in 1983 by John Walker, co-founder of Autodesk). You can download this file below (when you finish reading ;).

I will always remember my first "interplanetary" DWG travel that I did in 1985, after learning how to zoom:

• after you open the drawing, you are looking at a whole solar system from outer space, drawn to scale in kilometers as DWG units.
• zooming into blob at the center reveals an inner solar system with a few orbits, a 3-rd of them being Earth's, with the Sun in the center.
• On the Earth orbit you can see a small flickering dot. Zoom in on it, the larger circle is Moon's orbit around Earth, the dot in the middle is Earth.
• On Moon's orbit you can see a flickering dot. Zoom in on it, it's Moon.
• On Moon you can see a flickering dot. Zoom in on it, you will see the crater.
• Near the crater (200 meters from it) you can see small green item. Zoom on it, it's LEM, the landing module that brought Neil Armstrong and his colleagues to the Moon.
• On the leg of LEM you can see a 30cm plaque. Zoom in on it and you can read the message:
  HERE MEN FROM THE PLANET EARTH FIRST SET FOOT UPON THE MOON etc.
  signed by three astronauts and president Nixon

One day, when I have some time I'll add solar.dwg screen shots to this web page.

Now download SOLAR.DWG (don't worry, it's just 10kB, 15 years ago my computer had 20MB hard disk and "huge" CAD files had 200kB). Start your (CAD) engine and take off on your interplanetary trip. Let me know how it went when you come back.

For Microstation accuracy believers, with all their tools starting with "ACCU", for accurate ;) I propose the following form of treatment:
Try to replicate SOLAR.DWG (drawn 17 years ago on AutoCAD!!!), make it into SOLAR.DGN ;) with today's Microstation (XXI century). This 17 year old, 64 bit double precision DWG, can have everything in the Solar System positioned with an accuracy of a few millimeters (you can read 3.7mm text on a plaque). With DGN, the smallest item that can be drawn in this drawing (showing orbit of Pluto,  11916840000 km diameter) is a 3 kilometer line, both endpoints being positioned with 3 kilometer "accuracy" in X, Y and Z direction.

Alex Januszkiewicz
alexj@intelcad.com
Former President / Principal Programmer
IntelCAD Systems / DWG Data Recovery Services

PS. Do I have any Microstation background to make these kind of discussions? Even though I spent the last 15 years programming mostly for AutoCAD, I did a lot of MDL development for Microstation, PDS and Frameworks clients over the last 10 years. To this date I develop, from time to time, commercial grade, complex MDL programs, on site of a large company in Calgary that uses both AutoCAD and Microstation/PDS. I know Microstation and its DGN format in and out.

AutoCAD ^® is a registered trademark of Autodesk, Microstation ^® is a registered trademark of Bentley

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This page was last updated on July 11, 2003