Annex D

(Normative)

Arc Option

D.1 Introduction

This annex contains an option which allows complete chains to be composed of arc and string spatial objects. Unless stated otherwise in this annex, all requirements of the body of this part also apply when using this option.

D.2 Spatial Objects

The following table indicates spatial object requirements which differ from that of section 2.1 of this profile.

Object Representation Code	Required	Optional	Not Permitted
LS - String		x
AC - Circular Arc		x
AE - Elliptical Arc		x
AU - Uniform B-spline		x
AB - Piecewise Bezier		x

At least one of the four arc objects (AC, AE, AU, AB) is required.

All arc and string objects must be components of complete chain objects which are components of 2-D manifolds.

D.3 Relationship Between Modules and 2-D Manifolds

In addition to the requirements of section 4.7 (a), for objects particular to one 2-D manifold there shall be:

• zero or one Line module for optional simple object type LS;
• zero or one Arc module for optional simple object type AC;
• zero or one Arc module for optional simple object type AE;
• zero or one Arc module for optional simple object type AU;
• and zero or one Arc module for optional simple object type AB.

There shall be at least one Arc module for a particular 2-D manifold in a transfer using this option.

D.4 Transfer Module Specification

The following table contains inclusion/exclusion, and cardinality rules for additional modules permitted by this annex. The standardized modules names are included, along with the minimum number and the maximum number of occurrences of the module type. A lowercase "n" indicates that the upper limit is user defined. Any lowercase letters or dots in the module name has the meaning explained in Section 4, Standard Module Names.

Module Type	Name	Min. No.	Max. No.
Line1	LS..	0	n
Arc	AC.. AE.. AU.. AB..	0 0 0 0	n n n n

1 This Line module is in addition to the Line module required for Complete Chain (LE) objects as described in section 5.

D.5 Module Restrictions/Requirements: Identification Module

To indicate that this annex is being used, the Profile Identification subfield shall include "/D" in the manner described in section 5.8.2 of Part 4.

D.6 Module Restrictions/Requirements: Line Modules

D.6.1 Chain Component ID

Complete chains (LE object type) in transfers using this annex shall use this field to reference arcs and strings which are components of the chain. All arcs and strings in the transfer referenced by complete chains with this field.

D.6.2 Spatial Address

Complete chains (LE object type) shall always include the Spatial Address field, even if the chain is composed of strings or arcs referenced by the Chain Component ID. If a chain is composed of strings and (or) arcs, an encoder shall convert these strings and arcs into a series of vertices which shall be transferred in the Spatial Address field.

D.6.3 Object Representation Codes

Complete chains (LE object type) and strings (LS object type) shall not be included in the same module.

D.7 Module Restrictions/Requirements: Arc Modules

D.7.1 Object Representation Codes

Arc objects with different object representation codes shall not be included in the same module.

D.7.2 ISO 8211 Tag

The ISO 8211 tag for the Primary Field of the Arc module shall be ARCC. This is because all tags in an ISO 8211 file must be the same length (all other tags in the Arc module are four characters.)