IfcQuantityLength是一个物理量，它定义了一个派生的长度度量来提供元素的物理属性。它通常由测量方法给出的特定测量规则下元素的物理性质导出。
示例：屋顶结构内的椽可根据其长度进行测量（考虑公共横截面）。长度的实际大小取决于所用的测量方法。
IFC2x中增加的新实体。它替换了以前IFC版本中使用的calcXxx属性。
Formal Propositions
| Rule | Description |
|---|---|
| WR21 | If a unit is given, the unit type shall be a length unit. |
| WR22 | A valid length quantity shall be greater than or equal to zero. |
Attribute inheritance
| # | Attribute | Type | Cardinality | Description | C |
|---|---|---|---|---|---|
| IfcPhysicalQuantity | |||||
| 1 | Name | IfcLabel | [1:1] | Name of the element quantity or measure. The name attribute has to be made recognizable by further agreements. | X |
| 2 | Description | IfcText | [0:1] | Further explanation that might be given to the quantity. | X |
| HasExternalReferences | IfcExternalReferenceRelationship @RelatedResourceObjects |
S[0:?] | Reference to an external reference, e.g. library, classification, or document information, that is associated to the quantity. | X | |
| PartOfComplex | IfcPhysicalComplexQuantity @HasQuantities |
S[0:1] | Reference to a physical complex quantity in which the physical quantity may be contained. | X | |
| IfcPhysicalSimpleQuantity | |||||
| 3 | Unit | IfcNamedUnit | [0:1] | Optional assignment of a unit. If no unit is given, then the global unit assignment, as established at the IfcProject, applies to the quantity measures. | X |
| IfcQuantityLength | |||||
| 4 | LengthValue | IfcLengthMeasure | [1:1] | Length measure value of this quantity. | X |
| 5 | Formula | IfcLabel | [0:1] | A formula by which the quantity has been calculated. It can be assigned in addition to the actual value of the quantity. Formulas could be mathematic calculations (like width x height), database links, or a combination. The formula is for informational purposes only. | X |
EXPRESS Specification
ENTITY IfcQuantityLength
SUBTYPE OF (IfcPhysicalSimpleQuantity);
LengthValue : IfcLengthMeasure;
Formula : OPTIONAL IfcLabel;
WHERE
WR21 : NOT(EXISTS(SELF\IfcPhysicalSimpleQuantity.Unit)) OR (SELF\IfcPhysicalSimpleQuantity.Unit.UnitType = IfcUnitEnum.LENGTHUNIT);
WR22 : LengthValue >= 0.;
END_ENTITY;
原文：https://www.cnblogs.com/herd/p/14397233.html