DWG Units

DWG format does not really have a linear unit concept and it really is a unitless system.  There is no reliable way to determine what units are used in a DWG file.  There are two system variables that suggest units: unit format and design center units.

Unit Format:

Decimal, Architectural, Engineering, Fractional, Scientific

Design Center Units:

Unitless, Inches, Feet, ......, Millimeters, Meters, ......

The two unit formats, Architectural and Engineering, explicitly suggest Inches as the working units.  The other three formats, however, offer no clues at all as what the real world units they imply.  With design center units, on the other hand, one may find it much easier to relate them to the real world units, but they are meant only for Design Center, not for the entire system.  What the design center units do here is that when the units are specified and you are about to insert blocks into the model, the blocks inserted will use the these units.  In many cases, design center units do reflect the file units.  In fact, DWG files created from MicroStation always have design center units set to reflect desired units, ie. they are either DGN units or the units you selected in DWG SaveAs Options.

What this means is that although MicroStation can make a pretty good guess to open your file in correct units, there is no guarantee that it does this right all the times.  The only sure way to get it right is to know exactly what units to be used in a DWG file prior to opening the file in MicroStation.  Once you know for certain what units to be used in the DWG file, make sure that you select appropriate units in the DWG Open Options when you open the DWG file in MicroStation.

One way MicroStation guesses DWG file units is by Design Center Units.  This is the default option.  When Design Center Units in file are unitless, MicroStation will pop up a dialog box to alert you about it, and let you specify correct units.  Another guess MicroStation makes is based on the inch implication of the unit formats Architectural and Engineering.  If you select different units for them, you will get a pop up dialog box alerting you that none inch units are selected and then giving you a chance to confirm your selection.

When you get a DWG file that uses Survey Feet, you can use a DGN seed file pointed to by MS_TRANSEED or directly change the default transeed.dgn to use the Survey Feet for Units of Resolution.  If you do not already have a Survey Feet defined in your units.def, you need to add one.  That will allow you to select it from DWG Open Options.

 

Anonymous
  • For reference attachment, you may want to check active DGN file's unit settings against your DGN seed file for DWG open (transeed.dgn by default) and make sure the unit settings are compatible.  This is in addition to setting the units in DWG Open Options when you attach a DWG file.

  • Very good review.

    I use meters myself but cannot get certain DWG-files into my system correctly. When they are opened directly they seem to have the correct coordinates and elevation, but they are displayed as inches. When using the files as references or imports the digits are changed so they do not fit. Any advices?

  • Cheryl,

    Survey feet can be defined by you via units.def.  Once defined in MicroStation, it will become a valid unit option in the drop down menu you can select.  AutoCAD does not have survey feet, but there should not be actual difference in coordinates or sizes.  The only difference is readout.  It may be a good idea to use Decimal or Fractional unit format in your DWG file such that there is no confusion as to exactly what units you use.  If you use Architectural or Engineering, an ACAD user may interpret the readout as English units.

    HTH,

    Don

  • Okay, Don.  I have a problem with microstation changing my coordinates on a dxf and dwg file when referencing it into a new dgn.  I have read alot of blogs and you appear to be the expert.  I tried the advice for JWinter 10-22-2009 - no luck.  Where should I find a setting for "survey feet"?  I'm in v8.

    Thanks,

    Cheryl