Document id Title Organisation /Author Date Status P6 IFC Schema Extension MSG / Thomas Liebich Final

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1 Document id Title Organisation /Author Date Status P6 IFC Schema Extension MSG / Thomas Liebich Final IFC Alignment Schema This document describes the necessary extensions of IFC to implement the conceptual schema for the IFC Alignment project. The conceptual schema is published as version 1 and with minor updates as version 1.0.1: P6_Alignment_ConceptModel_Documentation_ pdf This version is used as the baseline for developing the IFC schema extension. The development process includes: Adopting the conceptual definitions to the existing framework and schema architecture of IFC within its version IFC4, Using the IFC schema definition language EXPRESS instead of UML as used for the conceptual schema IFC Schema extension The following figures show the IFC Schema extension. The following modules of the IFC4 Architecture are enhanced to include the alignment definitions. Building Controls Domain HVAC Domain Plumbing FireProtection Domain Electrical Domain Structural Elements Domain Architecture Domain Structural Analysis Domain Construction Management Domain Domain layer Shared Bldg Services Elements Shared Component Elements Shared Building Elements Shared Management Elements Shared Facilities Elements Interop layer Control Extension Product Extension Kernel Process Extension Core layer DateTime Material External Reference Geometric Constraint Geometric Model Geometry Presentation Appearance Actor Presentation Definition Profile Presentation Organization Property Representation Quantity Constraint Topology Approval Utility Structural Load Measure Cost layer Figure 1: Extensions to the IFC Schema Architecture 1 Thomas Liebich

2 Figure 2: Addition of IfcAlignment as a positioning element along with grid (IfcProductExtension) 2 Thomas Liebich

3 Figure 3: Addition of alignment segments (IfcGeometricConstraint) 3 Thomas Liebich

4 Figure 4: Explicit 2D horizontal segments for the horizontal alignment (IfcGeometry) 4 Thomas Liebich

5 To Figure 2 Note: The IfcAlignment entity inherits from IfcProduct and thereby has all the capabilities to hold its 2D and 3D geometry, have property sets for additional attribute (like those from the conceptual model, Status, Purpose, Alternative ). It is not clear, what values LinearRefMethod should have, or if this could be handled by the general IFC capability of PredefinedType. To Figure 3 Note: in case of horizontal alignment, the segment curve geometry is defined directly within the geometry resource, as it is defined in the xy plane of the overall engineering coordinate system. In case of vertical alignment, each segment is defined with its shape properties here. 5 Thomas Liebich

6 The IFC Entity Definitions Documentation of each new or enhanced entity within the IFC schema that has been modified as part of the IFC Alignment extension. The alignment definition The new and modified entities as shown in Figure 2. Entity IfcPositioningElement documentation New and abstract entity definition for positioning and annotating elements that are used to position other elements relatively. EXAMPLE A grid is a positioning element to position building components mainly in vertical structures, an alignment is a linear positioning element to position geographic and civil elements mainly in infrastructure works. EXAMPLE An alignment is a linear positioning element for using a linear referencing method to position other items. See ISO Linear referencing for general information about linear referencing methods and expressions. none Entity IfcAlignment documentation An alignment is used to define a reference system to position elements mainly for linear construction works, such as roads, rails, bridges, etc. A single alignment may have: 1) a horizontal alignment defined in the x/y plane of the engineering coordinate system, 2) an accompanying vertical alignment, defined in the distance Along / z coordinates along the horizontal, 3) an 3D alignment, often computed from the horizontal and vertical alignment. Alignments can be related to each other to form an alignment system, such as offset alignments or alignment groups. Multiple vertical alignment may reference the same horizontal alignment. Valid representations of IfcAlignment are: a complete definition as a single horizontal, vertical (represented by their alignment segments) and a 3D alignment a definition as a single horizontal and vertical alignment (represented by their alignment segments) without a generated 3D alignment a definition as only a horizontal alignment (represented by its horizontal alignment segments) without a vertical and 3D alignment 6 Thomas Liebich

7 a definition as only a horizontal alignment by a simple 2D line representation without alignment segments (such as in very early planning phases or as a map representation) a definition as only a 3D alignment (such as coming from a survey) without horizontal and vertical alignment segments Multiple vertical alignments, with corresponding 3D alignments may share a single horizontal alignment and thereby allowing multiple verticals for a single horizontal, while always keeping a tight connection between horizontal, the vertical and the optional 3D alignment. NOTE This release of the specification does not yet support offsets from the shared horizontal alignment An IfcGroup entity can aggregate corresponding alignments and group them together. Alignment can be associated to structures, such as IfcSpatialZone. PredefinedType Specific type of the alignment. Currently unused. LinearRefMethod Linear Referencing Method used to position items along the alignment. Examples are Chainage, Milage, etc. Horizontal Provision of a horizontal alignment being an ordered list of horizontal alignment segments. Vertical Provision of a vertical alignment being an ordered list of vertical alignment segments. Inherited Name Name of the alignment Description Additional descriptive information about the alignment ContainedInStructure Reference to a spatial or linear system in which this aligment is defined. Examples are road, bridge, tunnel or similar. Placement Definition of the alignment coordinate system. This should either be omited, or set to location 0.,0.,0. With no rotation (it shall be identical to the engineering coordinate system of the IfcProject. Representation Reference to potentially multiple geometric representations of the alignment. Two geometric representations are defined: FootPrint Curve2D simple 2D line representation of alignment Axis Curve 3D 3D line representation of 3D alignment NOTE The simple 2D line representation is restricted to be a polyline or poly curve, representing the alignment in 2D space (XY plane of the alignment coordinate system). The 3D line representation is the surveyed or calculated 3D poly line (in the alignment coordinate system) IsDefinedBy Reference to the property sets associated to the alignment. It may include the property Status, defining the alignment as being new, existing, demolish, temporary, other. 7 Thomas Liebich

8 The alignment segment definition (with vertical alignments) The new and modified entities as shown in Figure 3 Entity IfcAlignment2DHorizontal documentation An IfcAlignment2DHorizontal is a linear reference projected onto the horizontal x/y plane. Points along a horizontal alignment have two coordinate values. These are typically x and y in a Cartesian local engineering system. The horizontal alignment is defined by segments that are connected endto-start. The transition at the segment connection is not enforced to be tangential, if the tangential continuity flag is set to false, otherwise a tangential continuity shall be preserved. Based on the context of the project, they are geo-referenced and convertible into Northing and Easting values. NOTE Georeferencing is provided by IfcMapConversion through the IfcGeometricRepresentationContext defined at IfcProject. StartDistAlong The value of the distance along at the start of the horizontal alignment. If omited (standard) it is set to zero. Segment An ordered list of unique horizontal alignment segments, each (but the last) are joint end to start ToAlignment Link to the IfcAlignment for which it defined the horizontal alignment. More then one IfcAlignment can be linked in this case, the horizontal alignment is shared by several alignment with own vertical alignments. Entity IfcAlignment2DVertical documentation An IfcAlignment2DVertical is a height profile along the horizontal alignment. Points along a vertical alignment have two coordinate values. The first value is the distance along the horizontal alignment, the second value is the height according to the project engineering coordinate system. Based on the context of the project, they are geo-referenced and the height value is convertible into orthogonal height above/below the vertical datum. NOTE Georeferencing is provided by IfcMapConversion through the IfcGeometricRepresentationContext defined at IfcProject. Segment An ordered list of unique vertical alignment segments, each (but the last) are joint end to start ToAlignment Link to the IfcAlignment for which it defined the vertical alignment. Only one IfcAlignment can be linked, a vertical alignment can not be shared by several alignments. 8 Thomas Liebich

9 Entity IfcAlignment2DSegment documentation An abstract entity defining common information about horizontal and vertical alignment segments. NOTE The start and end tag are defined as annotations, not as referents along the alignment. Only absolute distance expressions are in scope, not distances ahead or behind a referent, such as a station. However such information can be exchanged as tags. StartTag Tag to annotate the start point of the alignment segment. EndTag Tag to annotate the end point of the alignment segment. TangentialContinuity Connectivity between the continuous segments is not enforced per se to be tangential. Setting "TangentialContinuity" to True means that the current segment shall continue with tangential continuity to the previous one. Entity IfcAlignment2DHorizontalSegment documentation A single horizontal alignment segment with an associated curve geometry. The following segment curve types are defined by the CurveGeometry: line segment circular arc segment clothoidal arc segment For each horizontal segment, the following non-redundant information is provided: the start point (in x/y coordinates) the start direction (in radians or degree, with local x (east) being 0, and increasing counter-clock wise) the segment length the curve parameter needed for circular and clothoidal arc segments The following information can be calculated (and is therefore not exchanged explicitly to avoid redundancy and inconsistencies) the end point (from start point, direction, segment length and curve parameter) the start distance along (from the end distance along of the previous segment, or the start distance along of the horizontal alignment (if it is the first segment) the end distance along (from the start distance and the segment length) the end direction (from the curve parameter, the start direction and the segment length) the point of intersection (from the start direction and the end direction) The following checks can be done to validate the correct exchange: continuity does the calculated end point of the previous segment matches with the provided start point of this segment tangential continuity does the calculated end direction of the previous segment matches with the provided start direction of this segment 9 Thomas Liebich

10 CurveGeometry Geometric representation of the horizontal alignment within the 2D X/Y coordinate space. Inherited ToHorizontal See IfcAlignment2DSegment Link to the IfcAlignment2DHorizontal to which this horizontal segment belongs. Entity IfcAlignment2DVerticalSegment documentation Individual segment along the IfcAlignment2DVertical, being defined in the distance-along/z coordinate space. The vertical alignment is defined by segments that connects end-to-start. The vertical alignment curve geometry is defined in a plane with x = distance along horizontal, the y = height (or elevation). The transition at the segment connection is not enforced to be tangential, if the tangential continuity flag is set to false, otherwise a tangential continuity shall be preserved. The following vertical segment types are defined: line segment circular arc segment parabolic arc segment (so called unsymmetrical parabolic arcs are considered as two connected parabolic arc with different parabola constants) For each vertical segment, the following non-redundant information is provided: the start point (in distance along/ height coordinates) the start gradient (as a ratio measure between 1 < n < -1 (equal to a percentage of 100% < n < -100%, with horizontal being 0, uphill positive, and downhill negative) the length (as horizontal length along the distance along (not the curve segment length)) the curve parameter needed for circular and parabolic arc segments The following information can be calculated (and is therefore not exchanged explicitly to avoid redundancy and inconsistencies) the end distance along (from the distance along and segment length) the end height (from start distance along, gradient, length and curve parameter) the end direction (from start direction, segment length and curve parameter) the point of vertical intersection (from start direction and end direction) The following checks can be done to validate the correct exchange: continuity does the calculated end distance along of the previous segment matches with the provided start distance along of this segment tangential continuity does the calculated end gradient of the previous segment matches with the provided start gradient of this segment 10 Thomas Liebich

11 NOTE Specific subtypes of the IfcAlignment2DVerticalSegment may add specific geometric curve parameters. Connectivity between vertical segments is not necessarily tangential, but this can be enforced as a requirement through the attribute TangentialContinuity. StartDistAlong Distance along the horizontal alignment, measured along the IfcAlignment2DHorizontal given in the length unit of the global IfcUnitAssignment. StartHeight Elevation in Z of the start point relative to the IfcAlignment coordinate system. NOTE It is strongly advised to not offset the IfcAlignment coordinate system from the project engineering coordinate system. StartGradient Gradient of the tangent of the vertical segment at the start point. It is provided as a ratio measure. The ratio is percentage/100 (0.1 is equal to 10%). It has a theoretical range of - < n < using a ratio measure. The equivalent range measured in degree is -90 < n < 90. NOTE For practical application of start gradient, the range of the ratio measure should be within the limits of -1 n 1 (equivalent in degree -45 n 45 ). However larger limits might apply for particular usages. Positive gradient means an increasing height at the start (or uphill), a negative gradient means decreasing height at the start (or downhill). HorizontalLength Length measured as distance along the horizontal alignment of the segment. Inherited See IfcAlignment2DSegment 11 Thomas Liebich

12 illustrations Entity IfcAlignment2DVerSegLine documentation The vertical straight segment is defined as a line using the inherited. inherited none See IfcAlignment2DVerticalSegment Entity IfcAlignment2DVerSegCircularArc documentation The vertical circular arc segment is defined as an arc using the inherited and the following additional curve parameters. Radius radius of the circular arc IsConvex orientation of the circular arc, convex (Boolean= true ) means decreasing gradient along the arc at the beginning, concave (Boolean= false ) means increasing gradient along the arc at the beginning inherited See IfcAlignment2DVerticalSegment 12 Thomas Liebich

13 illustrations EXAMPLE Circular arc with IsConvex=FALSE EXAMPLE Circular arc with IsConvex=TRUE Entity IfcAlignment2DVerSegParabolicArc documentation The vertical parabolic segment is defined as a parabola using the inherited and the following additional curve parameters. 13 Thomas Liebich

14 Constant Parabola constant (determining the steepness of the parabola). The parabola constant is provided by the minimum parabola radius, the true radius of a parabola at its vertical axis (the zero-gradient point of the parabola). IsConvex Figure: radius of curvature at the apex (Wikipedia) The minimum radius is twice the focal length of the parabola (the distance between the focal point and the vertex). Orientation of the parabolic arc, convex (Boolean= true ) means that the minimum radius is the distance between the vertex and the center point along the positive direction of the vertical axis, and concave (Boolean= false ) means along the negative direction of the vertical axis. NOTE Convex normally stands for a valley and concave for a crest. inherited See IfcAlignment2DVerticalSegment illustrations The horizontal alignment segment curves The new and modified entities as shown in Figure 4 14 Thomas Liebich

15 Entity IfcCurveSegment2D documentation The abstract definition of a 2D curve representation. Such 2D curves are used by horizontal alignment segments. Each curve segment is defined by a start point, a start direction, a segment length and additional curve geometry parameter. StartPoint The start point of the 2D curve as x/y coordinates defined by a 2D Cartesian point. StartDirection SegmentLength The direction of the tangent at the start point. Direction value 0.0 indicates a curve with a start tangent along the positive x- axis. Values increases counter-clockwise, and decreases clockwise. Depending on the plane angle unit, either degree or radians, the sensible range is -360 n 360 (or -2π n 2π). Values larger than a full circle (> 360 or > 2 π shall not be used. The length along the curve Entity IfcLineSegment2D documentation The line segment defined using the inherited start point, start distance and segment length parameter. Inherited illustration none See IfcCurveSegment2D Entity IfcCircularArcSegment2D documentation The circular arc segment used the inherited StartPoint as the start of the circular arc, StartDirection as the tangent at the start point and SegmentLength as the arc length. And the following additional : Radius The radius of the circular arc IsCcw (counter-clockwise or clockwise) as the orientation of the 15 Thomas Liebich

16 circular arc with Boolean= true being counter-clockwise, or to the left", and Boolean= false being clockwise, or to the right. Inherited See IfcCurveSegment2D illustrations Entity IfcClothoidalArcSegment2D documentation The clothoidal arc segment used the inherited StartPoint as the start of the circular arc, StartDirection as the tangent at the start point and SegmentLength as the arc length. And the following additional : StartRadius as the radius of the clothoidal arc at the start point. If the radius is not provided by a value, i.e. being NIL it is interpreted as INFINITE the startpoint is at the point, where the clothoid does not have a curvature. IsCcw: (counter-clockwise or clockwise) the orientation of the clothoidal arc with IsCcw= true, if the spiral arc goes counter-clockwise as seen from the start point and start direction, or to the left", and with IsCcw= false if the spiral arc goes clockwise, or to the right. IsEntry (entry or exit) the change in curvature within the spiral arc from the start point, 16 Thomas Liebich

17 Inherited with IsEntry = true, if the arc of the clothoid has increasing curvature, and with IsEntry= false, if the arch of the clothoid has decreasing curvature. ClothoidConstant as the constant A (not A square), that determines the rate of curvature change along the clothoid. See IfcCurveSegment2D illustrations EXAMPLE an clothoidal arc with infinite radius (radius= NILL ), IsCcw= false and IsEntry= true EXAMPLE an clothoidal arc with infinite radius (radius= NILL ), IsCcw= true and IsEntry= true 17 Thomas Liebich

18 EXAMPLE an clothoidal arc with IsCcw= false and IsEntry= true EXAMPLE an clothoidal arc with IsCcw= true and IsEntry= true EXAMPLE an clothoidal arc with IsCcw= true and IsEntry= false 18 Thomas Liebich

19 EXAMPLE an clothoidal arc with IsCcw= false and IsEntry= false EXAMPLE an clothoidal arc with IsCcw= true and IsEntry= false EXAMPLE an clothoidal arc with IsCcw= false and IsEntry= false 19 Thomas Liebich