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IfcMapConversion.md

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IfcMapConversion

The map conversion deals with transforming the local engineering coordinate system, often called world coordinate system, into the coordinate reference system of the underlying map.

NOTE The IfcMapConversion does not handle the projection of a map from the geodetic coordinate reference system.

For this transformation, IfcMapConversion data are used for:

  1. a scaling of the three axes (x,y,z), by the same IfcMapConversion.Scale
  2. followed by an anti-clockwise rotation about the z-axis of θ, where:

$$ \theta=arctan\left(\frac{XAxisOrdinate}{XAxisAbscissa}\right) $$

  1. and then a translation in (x,y,z) of IfcMapConversion.Eastings, IfcMapConversion.Northings, IfcMapConversion.OrthogonalHeight

With IfcMapConversion, one scale is applied equally to x, y and z, to convert units. With IfcMapConversionScaled, additional different factors multiply x, y and z, to scale coordinates - not units.

NOTE The z-axis of the local engineering coordinate system is always parallel to the z-axis of the map coordinate system.

NOTE The Scale can be used when the length unit for the 3 axes of the map coordinate system are not identical with the length unit established for this project (see IfcProject.UnitsInContext) - for example to convert feet into metres. If omitted, the scale factor 1.0 is assumed.

HISTORY New entity in IFC4

Attributes

Eastings

Specifies the location along the easting of the coordinate system of the target map coordinate reference system.

NOTE for right-handed Cartesian coordinate systems this would establish the location along the x axis.

Northings

Specifies the location along the northing of the coordinate system of the target map coordinate reference system.

NOTE for right-handed Cartesian coordinate systems this would establish the location along the y axis

OrthogonalHeight

Orthogonal height relative to the vertical datum specified.

NOTE for right-handed Cartesian coordinate systems this would establish the location along the z axis

XAxisAbscissa

Specifies the value along the easting axis of the end point of a vector indicating the position of the local x axis of the engineering coordinate reference system.

NOTE for right-handed Cartesian coordinate systems this would establish the location along the x axis

NOTE together with the XAxisOrdinate it provides the direction of the local x axis within the horizontal plane of the map coordinate system

XAxisOrdinate

Specifies the value along the northing axis of the end point of a vector indicating the position of the local x axis of the engineering coordinate reference system.

NOTE for right-handed Cartesian coordinate systems this would establish the location along the y axis

XAxisAbscissa it provides the direction of the local x axis within the horizontal plane of the map coordinate system.

Scale

Scale to be used, when the units of the CRS are not identical to the units of the engineering coordinate system. If omitted, the value of 1.0 is assumed.