# 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 |
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Last Updated: 21 May, 2020