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Although the measurement of distance in virtual space can be considered arbitrary, the same is not generally said of the 'real world where buildings, products and material objects are constructed, manufactured or made using one or the other of the two major units of measurement, 'Metric' or 'Imperial'.
The following tutorial explains how and where to find the settings and options to change unit measurement within Blender 3D so the application can be used to make dimensionally accurate objects or material that's analogous to the real world, discussing some caveats of both along the way.
Blender 3D and its system-wide measurement settings are part of the applications "Properties" options, the panel that runs down the right hand side. Clicking the second button along from the left, "Scene", opens the general options applicable to a 'scene' being worked on, including the necessary measurement related settings which are available within the "Units" sub-section. Here the three sub-systems are reside, selected and activated by clicking one of the buttons titled "None" (default), "Metric" and "Imperial" (shown below).
"None" equates to an arbitrary numerical value (but still 'decimal')
"Metric" equates to "millimetres (mm)", "centimetres (cm)", "meters (m)", "kilometres (km)" etc.
"Imperial" equates to "inches ('/in)", "foot ("/ft)", "yard (yd)", "mile (m)" etc.
There are two caveats to keep in mind when changing between "Units" sub-systems in Blender 2.5;
Internally, Blender uses a 'decimal' system so its effectively "metric" both in principle and practice (the only notable difference relates to how distance is referenced, "metres" instead of "units").
Switching between sub-systems is done by directly converting one sub-system to the another.
What both the above points mean in practice is that although you can switch from "Units" to "Metres" and see no change at all (subject to the first point above), changing from either 'decimal' systems to "Imperial" causes relative accuracy problems because, in terms of how measurement is used when working in 3D, "2" units or metres is not the same as "2" yards. In this instance conversion is not simply a 'reference' (as mentioned above) because there are tangible differences between yards and metres - a yard is physically smaller than a metre (1m = 1.09yd = 3ft 3in), so a direct conversion of the default scene and it's contents from units or metres to yards actually equates to 2.187 yards (7ft 2in).
When working in Imperial units then, its best to make sure that the scenes setting are correct before any work is begun. To do this, open a new file and change the "Scene" options to "Imperial" (as described above). Then, press "N" to open the "View Properties" tool panel and change the "Display" sub-section options so "Spacing:" is set to "1yd" - "Spacing: 1yd". Additionally it may be necessary to alter the size of the default cube so a fixed size reference object is available when working.
Blender 3D uses a 'fixed' grid system when using either Imperial or Metric settings, this means that although the "Subdivisions:" values can be changed, they don't, by default, affect the grids display in the 3D view (see below).
Metric sub-divides the grid evenly so there are always 10 minor units (light grey lines) per one major (heavier lines) regardless as to the distance, so kilometres are sub-divided by 100 metre blocks (10 x 100m = 1km) in the same way 1 metre is divided into 10 centimetre blocks (10 x 10cm = 1 metre).
Imperial measurements on the other hand will subdivide slightly oddly in that 'foot' measurements are divided by 12 units (twelve inches per one foot), 'yard' spacing by 3 units (three foot per one yard), 'mile' by 4 'furlong' and so on up the distance scale.
As mentioned above, irrespective of the active measurement mode, Blender itself uses a 'decimal' system globally. This results in numerical values always being displayed relative to decimal divisions of whole numbers. For example, "Spacing: 2m" when changed from Metric to Imperial is displayed as "Spacing: 2.187yd". This can make working with the Imperial system tricky as it typically means using some mental math to calculate what the decimal increments mean in the Imperial system. Blender 3D solves this issue to a limited extent when the "Separate Units" option is activated in "Scene" properties (as shown below).
Design Note: "Separate Units" functions the same way for both Imperial and Metric units
What this setting does is 'convert' the decimal values into a more correct representation within the context of the unit sub-system being used. So "Spacing: 2.187yds" would be broken down into separate 'elements' and displayed in the datablock input field as "Spacing: 2yd 6.7"" - note that the latter part of the information seen in the datablock remains as decimal because smaller increments of Imperial measurement are usually written as fractions, i.e. "2yd 67/10"" (two yards, six and seven-tenths inches), which can't be correctly displayed.
The caveat associated with this means that because of the limitation, even with "Separate Units" active, additional math calculations may be necessary to work out fractions for precision when using the Imperial measurement system for modelling 3D objects in Blender.
The important setting for measurement in Blender is the "Spacing:" option available in the "Display" sub-section of the "View Properties" panel ("N"). General and valid unit 'identifiers' of measurement when typed into the "Dimensions:" and "Spacing:" datablock input fields are;
""" (inches 'quote' )
"'" (foot 'apostrophe')
For example, 1 "millimetre" can be represented as "Spacing: 1mm", "Spacing: 0.1cm" or "Spacing: 0.001m"; 1 "centimetre" as "Spacing: 1cm", "Spacing: 0.01m" and so on. 1 "inch" is be represented as "Spacing: 1"", "Spacing: 0.08'"; 1 "foot" as "Spacing: 1'" and so on. Take care to keep in mind the points raised above about Blender 2.5 using the 'decimal' system when calculating measurement.
Although using Imperial measurements present a problem when switching between subsystems, the discrepancies can be compensated for by telling Blender to use a known fixed value relative to the measurement system being used. This is done by setting a "Scale:" value from within the "Unit" properties subsection (default value is "Scale: 1.0000"). In practice this means using different values depending on the scale required.
Design Note: Object scale values are multiplications of Unit scale.
To set the "Scale:", click the "Scene" Properties button and scroll down to the "Unit" subsection. Here, activate "Imperial" and then in the "Scale:" input field type the appropriate value for the measurement accuracy required; for "inch" use "0.0254", for "foot" use "0.3048", for "yard" use "0.9144" and so on per divisible whole unit. In other words "[unit of measurement] is [n] of a [distance]", e.g. "an 'inch' is '0.0254' of a 'metre'".
The main 3DView grid subdivisions will automatically change to reflect the new settings (note the grids divisions cannot be changed by the user when this mode of measurement is activated).
Design Note: it's important to note that Blender rounds-up/down to the nearest whole number for display purposes so make sure to type the entire four decimal placements, where available, otherwise the system gets skewed out of sync.
Setting "Scale:" to "0.0254" for "inch" based "Unit" measurements. N.B. scene contains a number of basic objects that are "1", "6" and "12" units in size; when using "inch" based measurement this translates in to each being "1 inch", "6 inches" and "12 inches" (1 foot) in size/dimension
Setting "Scale:" to "0.3048" for "foot" based measurements - note Blender rounds the number *up* for display purposes. N.B. using the exact same scene and objects as above, by changing scale to "foot" based measurement each now represents "1 foot", "6 foot" (or "feet") and "12 foot" (or "feet") in scale/dimensions
Setting "Scale:" to "0.9144" for "yard" based measurements - note number is round *down* for display purposes. N.B. using the same scene and objects as previous, setting "yard" as the unit of measurement each objects now represents "1 yard", "6 yards" and "12 yards" in size/dimension. In each instance the objects physically remain the same size, the only change is how the system references their size relative to the "Unit" of measurement set by a "Scale:" value
Although in the example and explanation above the basic premise is in the use of "Metric" or "Imperial" based units of measurement, the flexibility of the system means that any "unit" value could be used so long as the correct parameters are set per the requirements of what's being constructed or represented in Blender. It's important to note here however, that care does need to be taken if or when switching between systems, there should be a general awareness that any physical discrepancies between objects is more likely down to the differing conditions of each unit system rather than problems with mesh objects themselves.