MicroStation's Unit System, and the Interaction between Projected Geographic Coordinate System Units

In MicroStation models, the actual physical size of graphic elements is recorded in "Storage Units". Generally, Storage Units are chosen to be the most natural system for the model content. For geographic data and buildings, that is most often meters or feet. For small scale part design, millimeters, centimeters, or inches might be used. Regardless of the Storage Unit chosen, users can enter data and get measurements in a different unit system by choosing the desired unit system in the Linear Units section of the Working Units page of the Design File Settings dialog:

MicroStation performs the conversion between the Working Units and Storage Units whenever necessary. Working Units can be freely changed to whatever is convenient at any time without changing the physical size of the data within the model. For example, suppose your Storage Units are set to meters, and your Working Units are set to millimeters. With AccuDraw, you draw a line, specifying its length as 320. When MicroStation stores that line in the model, it divides that length by 1000 (the number of millimeters in a meter) and stores the line element with a length of 0.320 meters. When you measure the length of that line, MicroStation multiplies by 1000 (again, the millimeters to meter ratio) and displays the length as 320. If you were to change Working Units to centimeters, and measure the same, line, MicroStation would multiply its length in storage units by 100 this time (the centimeter to meter ratio) and display its length as 32.

The Storage Units can also be changed, by clicking on the Edit button in the Advanced Settings section of the Working Units page. However, as noted in the alert box that is displayed:

"Changing the storage resolution will affect the size of existing elements..." 
Continuing the example above, if you change the Storage Units from meters to centimeters, the line you drew has now been changed to a physical length of 0.320 centimeters! Clearly, changing storage units is not something to be done casually.

MicroStation can make use of Storage Units when referencing one model to another. Since MicroStation knows (through the Storage Units) the actual physical size of the elements in each model, it can calculate the scale factor needed to correctly reference one to another, and that is what it does when you specify that you want to attach a reference with "True Scale" turned on.

Now let's turn to the particular complication that arises in the United States - the fact that there are two different definitions for "foot". In the late 1800's, the length of a meter was established as 39.37 inches. This "foot" is now known as the "U.S. Survey Foot", and it is thus 0.30480060690 m. In 1959, to bring the definition of a foot into conformance with the rest of the international community, the length of the foot was established as exactly 0.3048 meters. This measure of "foot" is known as the "International Foot". Thus the U.S. Survey Foot is 1.00000200 times as long as the International Foot. Although this is a pretty small difference (2 parts per million), it causes problems when long distances are measured (such as displacements from a state plane coordinate system origin) unless care is taken to correctly specify which Foot applies.

Now suppose a project is started, and the seed file is set up with International Foot as both the Storage Units and the Working Units. Assuming that all of the project files are created from this seed file, they will reference each other at a true scale of 1:1 and no apparent problems will arise, even if the data that is entered in the design file is really in Survey Feet! It is only when the project data either references or is referenced from an independent (correct) data source that the error will be noted.

Projected Geographic Coordinate System Units and Model Storage Units

Projected Geographic Coordinate Systems transform geographic coordinates (longitude and latitude) to Cartesian coordinates. The Cartesian coordinates are associated with a particular linear unit system, most commonly meters or feet. This section discusses the interaction between the MicroStation unit system and that of the Projected Geographic Coordinate System, and in particular focuses on the complications caused by the difference between the International Foot and the U.S. Survey Foot.

Frequently, design work is done in a State Plane Coordinate System, but data is entered as linear offsets from some known monument rather than longitude and latitude. As long as the input data is available in the Cartesian system, there isn't a need to assign a Geographic Coordinate System. However, if it becomes desirable to reference data from another geo-located project, assigning a Geographic Coordinate System to your model and using MicroStation's geo-referencing capabilities is a great convenience. Assigning the appropriate state plane GCS to your model is the first step, and that is a good time to consider and verify the models Storage Units.

As mentioned above, each Projected GCS has an associated unit system. The significance of the unit system is that coordinate system parameters that are specified in Cartesian coordinates (such as the False Northing and False Easting) are specified in the GCS's units. The GCS parameters are set up such that when a geographic coordinate (longitude, latitude) is projected using the GCS's mathematical formula, the output is in the GCS's linear units. In the case of US State Plane Coordinate Systems, there are often several different GCS's that are identical except for the linear units. For example, Delaware has two different GCS's that are based on the NAD83 datum. DE83 uses Meters and DE83F uses US survey feet. If you have data that is drawn in the Delaware State Plane, and your Storage Units are set correctly, MicroStation will automatically do the required unit conversions such that either of the two will give correct results. However, if your Storage Unit is set to International Foot, the fact that Delaware has no State Planes based on International Foot should cause you to carefully consider whether you really have data in International Foot or whether it is really US Survey Feet.

When assigning a Projected GCS to a model, MicroStation checks to see whether the model's Storage Units match the linear units of the selected GCS. If they don't match, you have the opportunity to change the Storage Units to make them match. For all of the reasons mentioned above, select that option only after careful consideration.

If you already have graphic data in the model, a dialog box similar to the following appears:

If you select the second option, the Storage Units in the model are changed, and the Working Units may be changed also if the Storage Unit change is between the International Foot and US Survey foot unit systems.

If there are no graphic elements in the file, the dialog box looks like this:

In this case, since there are no graphic elements in the file, the change is less drastic.

If you already have a Geographic Coordinate System assigned, and you change it to a different Geographic Coordinate System with units that don't match, the dialog looks like this:


1. If you elect not to change the Storage Units as you are assigning the GCS, you can still change them at any time using the Advanced Setting section of the Working Units page of the Design File Settings dialog.

2. The unit change can be made only if you are not reprojecting the data. If you need to change the units and also reproject the data, you must change the Storage Units manually, either before or after reprojecting, as appropriate.

3. If you determine that the Storage Units of a particular model are incorrect, you should investigate to see whether the Storage Units of other models that make up the project need to be changed also. As described above, if you've used True Scale to attach references (which is the default) changing the Storage Units of either the reference or the master model affects the position and scale of reference attachments.

4. There are hidden keyins that allow you to change the Storage Units of the currently active model. The keyins are:
SET STORAGEUNIT <storageUnitName>
The keyins are hidden because they bypass the warning alert box (shown above) that is displayed when you edit the Advanced unit settings. As explained above, a change to the Storage Units changes the physical size of every graphic element in the model and must be done very carefully.

These are in addition to the keyin that lets you set your working units
SET UNITS <masterUnitName> <subUnitName>
It is a good idea to set the Working Units when you set the Storage Units. To make the changes set with the keyins permanent, you must Save Settings. That can be done with the FILEDESIGN keyin. Thus, a typical set of commands to change from International Foot to US Survey Foot might be:
SET UNITS "US Survey Foot" "US Survey Inch"