[Back to Blog](/blog) # Unit Conversion and Exporting to IFC
Ilya Kovaldov April 30, 2020 [\#ifc](/blog/tag/ifc)
Sometimes members have questions about unit conversion when exporting to an .ifc file using the TB\_IfcExport module. This article contains an example of working with units and exporting them to an .ifc file.
#### Create a Project Unit Assignment
The OdIfcExportImpl::createDefaultUnits() function creates an IfcUnitAssignment. This is a long list of units that translates from internal BimRv units to known units.
##### Conversion-Based Unit
For example, consider the algorithm for creating a conversion-based unit using length units.
1\. Get the internal units using UnitsTracking Manager and FormatOptions:
```
OdBmUnitsTrackingPtr pUnitsTracking = m_database->getAppInfo(OdBm::ManagerType::UnitsTracking);
OdBmUnitsElemPtr pUnitsElem = pUnitsTracking->getUnitsElemId().safeOpenObject();
OdBmAUnitsPtr pAUnits = pUnitsElem->getUnits();
OdBmFormatOptionsPtrArray aFormatOptions;
pAUnits->getFormatOptionsArr(aFormatOptions);
OdDAIObjectId lenSIBaseUnit;
{
bool lenConversionBased = false;
bool lenUseDefault = false;
OdAnsiString lenConvName;
OdAnsiString lenUnitType = "LENGTHUNIT";
OdAnsiString lenPrefix;
OdAnsiString lenUnitName = "METRE";
OdBmFormatOptionsPtr pFormatOptions = aFormatOptions[OdBm::UnitType::UT_Length];
switch (pFormatOptions->getDisplayUnits())
{
case OdBm::DisplayUnitType::DUT_METERS:
case OdBm::DisplayUnitType::DUT_METERS_CENTIMETERS:
break;
case OdBm::DisplayUnitType::DUT_CENTIMETERS:
lenPrefix = "CENTI";
break;
case OdBm::DisplayUnitType::DUT_MILLIMETERS:
lenPrefix = "MILLI";
break;
case OdBm::DisplayUnitType::DUT_DECIMAL_FEET:
case OdBm::DisplayUnitType::DUT_FEET_FRACTIONAL_INCHES:
{
if (exportToCOBIE)
lenConvName = "foot";
else
lenConvName = "FOOT";
lenConversionBased = true;
}
break;
case OdBm::DisplayUnitType::DUT_FRACTIONAL_INCHES:
case OdBm::DisplayUnitType::DUT_DECIMAL_INCHES:
{
if (exportToCOBIE)
lenConvName = "inch";
else
lenConvName = "INCH";
}
lenConversionBased = true;
break;
default:
{
//Couldn't find display unit type conversion -- assuming foot
if (exportToCOBIE)
lenConvName = "foot";
else
lenConvName = "FOOT";
lenConversionBased = true;
lenUseDefault = true;
}
break;
}
```
2\. Create a base SI unit instance and SI unit with prefix instance if necessary:
```
OdDAIObjectId lenSIUnit = OdInstanceExporter::createSIUnit(m_model, lenUnitType, lenPrefix,
lenUnitName);
if (lenPrefix.isEmpty())
lenSIBaseUnit = lenSIUnit;
else
lenSIBaseUnit = OdInstanceExporter::createSIUnit(m_model, lenUnitType, NULL, lenUnitName);
```
3\. Get the length scale factor using BmUnitUtils:
```
double lengthScaleFactor = OdBmUnitUtils::convertFromInternalUnits(1.0, lenUseDefault ?
OdBm::DisplayUnitType::DUT_DECIMAL_FEET : pFormatOptions->getDisplayUnits());
// If BIM Units differ from SI Units, for example Inches or Foots using for length measure, creating // Base Conversion Unit. First getting Conversion Scale Factor using OdBmUnitUtils:
if (lenConversionBased)
{
double lengthSIScaleFactor = OdBmUnitUtils::convertFromInternalUnits(1.0,
OdBm::DisplayUnitType::DUT_METERS) / lengthScaleFactor;
```
4\. Create a dimensional dexponents instance (one dimension for length):
```
OdDAIObjectId lenDims = OdInstanceExporter::createDimensionalExponents(m_model, 1, 0, 0,
0, 0, 0, 0);
```
5\. Create a measure-with unit, which is an object that determines which unit to convert (SI length unit) and how to convert (lengthSIScaleFactor ratio):
```
ODIFC_CREATE_AS_RATIO_MEASURE(supVal, lengthSIScaleFactor)
OdDAIObjectId lenConvFactor = OdInstanceExporter::createMeasureWithUnit(m_model, supVal, lenSIUnit);
```
6\. Create a unit of measure based on conversions whose attributes are a dimensional exponent object and measure-with unit object, created before.
```
lenSIUnit = OdInstanceExporter::createConversionBasedUnit(m_model, lenDims, lenUnitType,
lenConvName, lenConvFactor);
```
7\. Add the SI unit object to a UnitSet array:
```
unitSet.push_back(lenSIUnit);
```
8\. Add the SI unit and scale factors to a units cache:
```
OdExporterCacheManager::getUnitsCache()->addUnit(OdBm::UnitType::UT_Length,
lenSIUnit,lengthScaleFactor, 0.0);
```
##### Expression Unit
For example, consider the algorithm for creating an expression unit using mass density units.
1\. Create a mass SI unit:
```
OdDAIObjectId massSIUnit;
{
massSIUnit = createSIUnit(OdBm::UnitType::UT_Mass, "MASSUNIT", "GRAM", "KILO",
OdBm::DisplayUnitType::DUT_UNDEFINED);
unitSet.push_back(massSIUnit);
```
}
2\. Create derived unit elements using kilograms to the power of 1 and meters to the power of -3. Attributes of the derived unit elements are SI unit objects created above: mass SI Unit and length SI unit.
```
OdDAIObjectIds elements;
elements.push_back(OdInstanceExporter::createDerivedUnitElement(m_model, massSIUnit, 1));
elements.push_back(OdInstanceExporter::createDerivedUnitElement(m_model, lenSIBaseUnit, -3));
```
3\. Create a kg/(m^3) derived unit using derived unit elements:
```
OdDAIObjectId massDensityUnit = OdInstanceExporter::createDerivedUnit(m_model, elements, "MASSDENSITYUNIT", NULL);
```
4\. Add the created derived unit and scale factor to project a UnitSet array into the units cache:
```
unitSet.push_back(massDensityUnit);
double massDensityFactor = OdBmUnitUtils::convertFromInternalUnits(1.0, OdBm::DisplayUnitType::DUT_KILOGRAMS_PER_CUBIC_METER);
OdExporterCacheManager::getUnitsCache()->addUnit(OdBm::UnitType::UT_MassDensity, massDensityUnit,
massDensityFactor, 0.0);
```
5\. Repeat in the same way for filling other units: area, volume, plane angle, time, frequency, temperature, etc.
6\. Finally, after filling the UnitSet array, create a project UnitAssignment instance:
```
OdInstanceExporter::createUnitAssignment(m_model, unitSet);
```
##### Use UnitsCache and UnitUtils for Unit Conversion in the Export Process
In the process of exporting objects, convert the units of measurement obtained from BimRv API to IFC project units. A cache and utilities for using this cache are created for these purposes. Consider their application in the next example of a level export.
```
// …
// some code to fill levels array from database
// …
OdBmLevelPtr level = levels[ii];
double elev = level->getElevation(); // elevation in internal units
```
1\. Convert elevation from BimRv units to IFC units using ScaleLength() util:
```
double elevation = OdUnitUtil::ScaleLength(elev);
```
2\. Use the converted elevation to create local placement of a building storey and a building storey:
```
OdGeVector3d orig(0.0, 0.0, elevation);
OdDAIObjectId placement = OdExporterUtil::createLocalPlacement(m_model, buildingPlacement, &orig,
NULL, NULL);
OdDAIObjectId buildingStorey = OdInstanceExporter::createBuildingStorey(m_model, level,
OdExporterCacheManager::getOwnerHistoryHandle(),
bsObjectType, placement, "ELEMENT", elevation);
```
##### Export Result
As a result of the export, a file contains the following lines.
1\. Length units (METRE in SI and FOOT in BimRv database):
```
#21=IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.);
#22=IFCDIMENSIONALEXPONENTS(1,0,0,0,0,0,0);
#23=IFCMEASUREWITHUNIT(IFCRATIOMEASURE(0.30480000000000035),#21);
#24=IFCCONVERSIONBASEDUNIT(#22,.LENGTHUNIT.,'FOOT',#23);
```
2\. Mass and mass density units:
```
#37=IFCSIUNIT(*,.MASSUNIT.,.KILO.,.GRAM.);
#38=IFCDERIVEDUNITELEMENT(#37,1);
#39=IFCDERIVEDUNITELEMENT(#21,-3);
#40=IFCDERIVEDUNIT((#38,#39),.MASSDENSITYUNIT.,'');
```
3\. Project units assignment:
```
#73=IFCUNITASSIGNMENT((#24,#28,#32,#36,#37,#40,#41,#42,#44,#48,#52,#53,#54,#55,#56,#57,#58,#59,#64,#67,#68,#72));
```
4\. Building storey:
```
#76=IFCCARTESIANPOINT((0.00000000000000000,0.00000000000000000,-800.00000000000000));
#77=IFCAXIS2PLACEMENT3D(#76,$,$);
#78=IFCLOCALPLACEMENT(#16,#77);
#79=IFCBUILDINGSTOREY('1i_YYDONP3AgUe36R0EAL1',#20,'Foundation','','8mm Head',#78,$,'Foundation',.ELEMENT.,-800.00000000000068);
```
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