This Client Server article is republished in its entirety from 2003 for reference purposes.
By Nelson Hobdell, Technical Support Analyst, Bentley Corporate Office
06 October 2003
Pre-MicroStation V8 files and working unit labels
Prior to the release of MicroStation V8, working unit labels and their meanings were in the hands of the user. "M" may have represented "meter" to one user, "mile" to another. This metadata was not expressly defined, nor carried along with the DGN file, except in cases where map definition and reprojection programs such as MicroStation GeoCoordinator or MGE's Projection Manager were used.
Working units in MicroStation V8
Migrating DGN data from the earlier MicroStation format to the MicroStation V8 format makes the understanding and interpretation of your working unit labels-as well as the mechanics of the upgrade-imperative to achieving accurate results.
When migrating earlier files to the MicroStation V8 format, Master and Sub Units are no longer merely labels with implied meaning; they are definitive cues in the upgrade process.
Current de facto known unit label upgrades are as follows:
ui ..............microinches
ti................tenth of an inch
i, in "..........inches
tn................tenth of a foot
f, ft, ' , '-......feet*
yd...............yards
mi..............miles
um.............micrometers
mm............millimeters
cm.............centimeters
dm, dc.......decimeters
m................meters
hm..............hectometers
km..............kilometers
all others...meter
* Note: The 'feet: definition above refers to International Feet, not U.S. Survey Feet.
What if I have legacy files which are not defined with one of the units listed above?
Obviously converting to meters is not always the desired choice, so MicroStation incorporates the ability to define custom unit definitions for labels not covered in the list above. The variable MS_CUSTOMUNITDEF will designate a file containing these definitions. A sample of the file is located in .../Workspace/System/data/ as Units.def.
Within this file, if the record of choice is uncommented (by removing the # symbol), the definition will be enabled.
When enabled, definitions created and enabled within a custom units_def.ini file also will supplant any of the de facto listings above during a file conversion to MicroStation V8 from an earlier version.
International Foot versus US Survey Foot
One common source of confusion is the ‘FT' label or tic (‘) marking used to represent ‘foot' in many earlier version files.
Typically, users intend these labels to represent:
The U.S. Survey Foot: 1.0 FT = 0.3048006096012191900 meters
The International Foot: 1.0 FT = 0.3048000000000000000 meters
The ClarkeFoot is a third, but seldom-used, consideration.
While not typically crucial on a mechanical drawing scale, or with coordinates close to 0,0, respectable differences may accrue when using large coordinates for civil or mapping purposes. Consider the following example.
Set up a new file within a pre-V8 version of MicroStation
Label the master unit ‘FT'
- Settings -> Design File -> Working Units
Create a line from ‘0,0' to ‘100,100' via the Key-in window.
- Place Line; xy=0,0; xy=100,100
If you choose, label the end points using the XYZtxt Label Points tool.
Edit the delivered units.def example for your MicroStation V8 installation, and enable the ‘Survey Foot' record by removing the preceding # symbol. The resulting line will appear as follows:
FT,ft'; Survey Foot; Survey Feet; 3937.0; 1200.0; 1; 1
Open the pre-V8 file using MicroStation V8 and choose to upgrade the file to the V8 format.
The units.def file's ‘FT' label definition will be engaged, and the resulting Master Unit name and label will be "Survey Feet FT."
( Snapping to the end points of your line will reveal that the coordinates are in fact 0,0 survey feet and 100,100 survey feet, respectively.
If, instead, your pre-V8 label represented International Feet, we can observe the error by changing the Master and Sub Unit within the file.
Use the Settings->Working Units dialog or key in: set units "Foot"
Snap to each end point of your line and notice that the readout for 0,0 remains constant, but that the opposite end of the line has become 100.000200,100.000200 International Feet.
Implications for MicroStation GeoGraphics users
With the advent of MicroStation GeoGraphics 8.1, MicroStation GeoCoordinator was integrated as ‘Coordinate System Tools' within MicroStation GeoGraphics. As a reprojection package, GeoCoordinator allowed definition of the coordinate system's unit (representing the Master Unit of the design file) independent of the labels defined by MicroStation's Working Units dialog. This information was stored separately from the design files label as extra metadata.
To account for proper interpretation of this data, the GeoGraphics EC includes a separate definition file: custom_units.def, which is automatically selected when the user is operating within the GeoGraphics workspace(s) via the definition of MS_CUSTOMUNITDEF. Default interpretation of the ‘FT' record is set to the U.S. Survey foot, assuming that is the intention of the majority of mapping users.
To further ensure that users are confident with their working unit selection, and as a stop-gap against accidental mis-definition, GeoGraphics' Coordinate System Tools will cross-check a defined coordinate system with the design file's working unit name and working unit labels. Any inconsistency will be flagged, and the user will be alerted by the request to examine or change the current working unit name and labels.
Coming Attractions
Look for additional "hard-coded" custom unit definitions to appear in future releases of MicroStation V8.
For additional information on Working Units within MicroStation V8, see http://selectservices.bentley.com/technotes/faqs/6206.htm
See Also
MicroStation Desktop TechNotes and FAQs
Geospatial Product TechNotes And FAQs
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