# Physical Units in BIS

BIS and iModels always use a well defined system of physical units.

## Physical Units must be SI Base Units or SI Derived Units

All physical property values (including geometry) in a BIS schema must be either:

• SI Base Units (m, kg, s, A, K, mol, cd)
• SI Derived Units without prefixes (Hz, rad, sr N, Pa, J, W, C, V, F, (Omega), S, Wb, T H, C, lm, lx, Bq, Gy, Sv, kat)

It is therefore possible to perform mathematical operations on physical values without considering units:

Mass = Density * Volume; // no unit conversions necessary

Note: in iModels, all coordinate data is stored in meters.

Units with prefixes, such as kilometer, millimeter, kilowatt and millivolt are not allowed.

## Angle Units

Angle units are radians, as that is the SI Derived Unit for angles. Similarly the units for solid angles are steradians.

This may seem awkward, as "90" is a more recognizable value than "1.5707963267948966192313216916398" However, note that storage units are not directly presented to users.

Radians are used to permit consistent mathematical computations.

For example:

(luminous flux in lumens) = (luminous intensity in candela) * (solid angle in steradian)

(energy in Joules) = (torque in N-m) * (rotation in radians)

## Exception for Roll, Pitch and Yaw

Roll, Pitch and Yaw (in GeometricElement3d and ViewDefinition3d, AuxCoordinateSystem3d) are handled specially. They are defined in degrees and declared as unitless in the EC schema.

Similarly, these properties are defined in degrees and declared as unitless in the EC schema:

• Angle in AuxCoordSystem2d
• Rotation in GeometricElement2d
• LensAngle in ViewDefinition3d
• RotationAngle in ViewDefinition2d

## SI Units do not Apply to Names and Text

The SI units requirement only applies to numeric physical and geometry properties; it does not apply to text properties. "90-Degree Elbow" is an acceptable text value.

Next: BIS Schema Validation

Last Updated: 21 May, 2020