The unit of measure in Civil 3D may cause confusion; there are legacy functions, and commands for ACAD and C3D. By default an Imperial and Metric version is installed with two separate icons; if you select one or the other are you then trapped in one unit of measure or the other and why. Inconsistencies can be introduced or preexisted in the templates, in the metric template a subassembly length set to 7.20 Meter when zoomed out has a dimension of 7.20 Inches - documented in this video clip. From a computer graphics standpoint, 1 model space unit is a set number of screen pixels at some resolution, this standard pixel length is then the base designation for a unit of measure, such as meter or feet.
Why is foot the default modelspace unit: From a simple query of Autodesk forum contributors, the consesus is that Autodesk C3D software engineers assumed that a sub-foot unit is not necessary and so selected foot at the base modelspace unit. As a result, elevations in increments of less than a foot are not possible except for the use of a workaround.
The detailed explanation for the difference between metric and imperial versions is simple, it is the "user Profiles used during startup... the difference is the template set as the QNew template, these are Metric and Imperial." blog The literature for Civil 3D ignores the details of units and how to change the settings but provides a solid knowledge foundation to start from; the Civil 3D documentation provides most of the settings and commands, the Wiley Mastering AutoCAD Civil 3D 2010 Authored by James Wedding provides a detailed review of ambient settings, and specific issues are addressed in online forums profile view text, modelunit, and inquiry tool. These three sources provide the form and function of the unit settings but not the behavior. Experienced Civil 3D modellers will tell you it is important to (you must) start with either the metric or imperial template and stay within one without changing the settings - why is apparently not easy to explain.
Metric: Meter modelspace unit, decimal unit of measure type, and meter ambient setting with exception of dimension, that is millimeter
Imperial: In an ad-hoc test for this article, the Imperial template appeared to have two default settings that changed after the file was open for a few minutes.
There are several unit settings, these are modelspace unit, ambient settings, unit of measure type, and scale. These settings have an affect on displayed units, such as stationing unit of measure, Properties unit of measure, Inquiry tool unit of measure, and Quantity takeoff unit of measure.
While feet is the default modelspace unit, the inch modelspace unit of measure does not appear to have detrimental affects on polylines (ACAD primitives) when combined with the engineering unit of measure type. But, as expected, if modeled in inch and the modelspace is changed to feet or the unit of measure is changed to decimal, the displayed units of measure are incorrect. The ancient egyptian Khet - useful for virtually recreating the scope, time, and cost of building the pyramids, a longtime favorite of idle engineers - cannot be added (or any other historical unit) as a custom unit of measure or modelspace unit; this would be a nice and easily written feature - the Khet due to the lack of actual users may make a nice C3D model-space unit, from which all other unit of measures are converted from.
There are several annoying but easily corrected user interface style issues with inch msu, for example the profile view text is too small to see and is difficult but not impossible to resize to a viewable font. Also, the retaining wall given in the imperial tool palate is still metric and the profile view text in metric scale; both are extremely small and therefore unusable; not an issue since the profile is changed with a table and the default retainingwall is not much anyways. Aside from the retaining wall, all the C3D stock subassemblies can be changed from foot to inch in the atc file and then multiplying the default dimensions by 12.
From pragmatic (real world testing) it looks like the modelspeace unit 'inch' does not affect the specific application of the C3D application; certainly for polylines. The C3D algorithm for the corridor composer reads the width of a subassembly as inches, correclty represents this but when the next attached subassembly elevation is calculated with superelevation the inch is assumed foot and the elevation is incorrect. Possibly, other algorithms are similary effected, such as superelevation, grading or pipes - experts in the Autodesk forum have not been able to provide specific issues to expect indicating that it is simply not known without further testing of the application behavior for each specific function.
An example inch model space unit model is posted here http://cife.stanford.edu/wiki/doku.php?id=granite:mer99#integrated_model-based_system
To change the modelspace and display units first identify the Civil 3D unit of measure displays and features you are using - some are given above. Then systematically change each variable and note the effect on each of the displays and features you use. Create a table with each test run as rows and columns for the affected display of features and the variable settings.
Begin with the Metric template, a dimensional 3D model using the dimensions 39.370 inch per meter and then change from metric settings to english settings, then modify these settings to adjust for the inch unit. This process is a deliberate misuse of the C3D application intended to 'shake-out' the details of the units and find the boundaries of the application and the behavior of the units functions and settings. It is a good learning process for students of C3D and should be a course assignment. Issues changing from Metric to Imperial settings with the profile view text height and toolpalette retaining wall are still unresolved and are a 'challenge' task for students.
Wiley Mastering AutoCAD Civil 3D 2010 by James Wedding
Peterson, F. and Fischer, M. “CIFE Wiki Technical Report: AutoCAD Civil 3D Modelspace Units.” CIFE wiki. Stanford University Center for Integrated Facility Engineering. Last accessed 08 July 2011. http://cife.stanford.edu/wiki/doku.php?id=granite:3dmodelling:modelspaceunits