In MicroStation, problems can arise when users do not understand the difference between US Survey Foot and International Foot.
Usually, the problems are not immediately obvious. The difference between an International Foot (usually known as just "Foot" in MicroStation) and a US Survey Foot is very small (2 parts per million, or .0002%). 2 parts per million is insignificant for most relative distances. However for coordinate values in the 100,000's or 1,000,000's in map projection-based coordinate systems such as U.S. State Plane, confusing the foot definitions can produce feet of shift because scaling is relative to the coordinate origin. The foot standard for such coordinate systems in the U.S. is the US Survey Foot. Often all of the models that make up a particular project are set to International Foot, even if they all should be set to US Survey Foot.
In previous versions of MicroStation V8, US Survey Foot was not enabled by default. This was done to avoid confusion. So, when users selected Foot, they were getting International Foot.
MicroStation gets the units it shows to the user from a file called "units.def". In that file is a definition for US Survey Foot, but it is commented off. We provided users with instructions on how to enable US Survey Foot by editing the supplied units.def file and "uncommenting" the definition.
The problems that come about because of an International/US Survey Foot mismatch can show up when you attach references, when either the reference or the master has incorrect storage unit settings, or when the data is "interfaced" to outside data. Examples of the latter are attaching a Geographic Coordinate System (a capability added in MicroStation V8i that was previously possible only with the Geospatial Extension) or perhaps doing construction stakeout.
There is nothing at all wrong with MicroStation's Unit system. It handles International Foot and US Survey Foot just fine, and if it discovered that a file is set to International Foot when it really should have been set to US Survey Foot (or vice versa), it is possible to correct the units at any time.
The issue is one of user education, and of making it easier for users to do it right. Prior to MicroStation V8i, you had to first realize that there is a difference between International Feet and Survey Feet, then change our delivered units.def to enable US Survey Feet, and then select the right foot.
In MicroStation V8i, you can now do the following:
- US Survey Foot (as well as US Survey Mile and US Survey Inch) units are now "turned on" by default -- the name that displays is "US Survey Foot". In the user interface where units are chosen, International Foot-based units are separated from the US Survey Foot-based units with a line separator:
- When you select a Geographic Coordinate System, MicroStation checks to see whether the storage units of your model matches the units of the Geographic Coordinate System, and if it does not, it gives you an opportunity to change the Storage Units of your model.
- Keyins exist to set the Storage Units so that a batch process can be used to correct the units for a group of files if it is discovered that all models in a project should be changed from International Foot to US Survey Foot (or vice versa).
More information regarding how to address this can be found in the [[8948|MicroStation's Unit
System, and the Interaction between Projected Geographic Coordinate System
Units and Model Storage Units]] article.