[wg11] Minutes of Part 21 E3 DIS testing telecon on June 5, 2914
Thomas Liebich
tl at aec3.de
Tue Jun 10 09:29:04 EDT 2014
Dear all,
only as a side remark. During our IFC (ISO16739) developments we faced
the same issue - a derived unit involving factor and offset cannot be
defined with the current conversion_based_unit.
So we added a subtype to IFC4, which essentially defines the same, as
the proposed extension here for P41.
See
http://www.buildingsmart-tech.org/ifc/IFC4/final/html/link/ifcconversionbasedunitwithoffset.htm
One important consideration: you need to determine, whether to apply the
offset before or after applying the factor. We decided to apply it after.
hope it gives another insight,
regards
Thomas
--
Am 08.06.2014 01:46, schrieb thomas thurman:
> Addendum for Fahrenheit discussion.
> Comments on current approach in part 41 and AP 214:
> 1-part 41 is specific that any unit that is related to a unit
> specified in part 41 shall be a 'conversion_based_unit'.
> 2-conversion_based_unit only converts the magnitude but does not
> provide an offset.
> 3-The use of ISO 13584-20 for such a simple case is long agreed
> overkill; the example in part 41 F 4.4 has EXPRESS
> errors (
> a-the expression_conversion_based_unit shall be a
> conversion_based_unit (see (1) above.) and
> b-the attribute redeclaration in e.g.,
> named_unit_variable.associated_variable_environment is invalid.).
> 4-AP 242 ed1 did not include a structure to support Fahrenheit unit
> exchange.
> 5-AP214 used a context_dependent_unit which is invalid (see (1)
> above). As noted above this was NOT promoted to
> AP 242.
> 6-After several attempts to discover a URL,URN for Fahrenheit I gave
> up and am proposing that SC4 define its own
> (NIST has a guide that includes a definition of Fahrenheit as a note
> (I think) but does not provide a URN or URL.).
> (There are multiple conversion calculators on the web but I am not
> convinced of their persistence.
> In any event use of a conversion calculator opens up its own can of
> worms.)
>
> ==========proposed solution:===========
>
> add a subtype of conversion_based_unit,
> conversion_based_unit_with_offset to provide the offset.
>
> =========current:============
>
> A *conversion_based_unit* is a type of *named_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.named_unit> that
> defines a unit on the basis of a *measure_with_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.measure_with_unit>.
>
>
> NOTE The *value_component*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.measure_with_unit.value_component> attribute
> of the *measure_with_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.measure_with_unit> defines
> the conversion factor.
>
> EXAMPLE An inch is a *conversion_based_unit*. It is from the
> Imperial system, its name is "inch", and it can be related to the
> *si_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.si_unit>,
> millimetre, through a *measure_with_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.measure_with_unit> whose
> value is 25.4 millimetre. A foot is also a *conversion_based_unit*. It
> is from the Imperial system, its name is "foot", and it can be related
> to an *si_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.si_unit>,
> millimetre, either directly or through the unit called "inch".
>
> _EXPRESS specification:_
>
> |*)
> ENTITY conversion_based_unit
> SUBTYPE OF (named_unit
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resource_docs/fundamentals_of_product_description_and_support/sys/18_schema.htm#measure_schema.named_unit>);
> name : label
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resource_docs/fundamentals_of_product_description_and_support/sys/25_schema.htm#support_resource_schema.label>;
> conversion_factor : measure_with_unit
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resource_docs/fundamentals_of_product_description_and_support/sys/18_schema.htm#measure_schema.measure_with_unit>;
> WHERE
> WR1: SELF\named_unit.dimensions =
> derive_dimensional_exponents(conversion_factor\measure_with_unit.unit_component);
> END_ENTITY;
> (*|
>
> _Attribute definitions:_
>
> *name: *the *label*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/support_resource_schema/support_resource_schema.htm#support_resource_schema.label> by
> which the *conversion_based_unit* is known.
>
> *conversion_factor: *the *measure_with_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.measure_with_unit> that
> specifies the physical quantity from which the
> *conversion_based_unit* is derived.
>
> _Formal propositions:_
>
> *WR1: *The dimensional exponents shall be equal to those from the
> *conversion_factor*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.conversion_based_unit.conversion_factor>.
>
>
> =========proposed addition to part 41=========
> A *conversion_based_unit_with_offset* is a type of
> *conversion_based_unit* that includes a scale offset.
>
> EXAMPLE A Fahrenheit is a *conversion_based_unit_with_offset*. It is
> from the Imperial system, its name is "fahrenheit", and it can be
> related to the *si_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.si_unit>,
> degree Celsius, through a *measure_with_unit*
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resources/measure_schema/measure_schema.htm#measure_schema.measure_with_unit> whose
> value is 1.8 degree Celsius. The offset is 32 degree Celsius.
>
> _EXPRESS specification:_
>
> |*)
> ENTITY conversion_based_unit_with_offset
> SUBTYPE OF (*conversion_based_unit*);
> offset : R
> <file:///Users/tom/Documents/2013/SMRLv5/SMRL_v5_rc5/data/resource_docs/fundamentals_of_product_description_and_support/sys/18_schema.htm#measure_schema.measure_with_unit>EAL;
> WHERE
> WR1: offset <> 0.0;
> END_ENTITY;
> (*|
>
> _Attribute definitions:_
>
> *offset: *the value of the offset represented in the units of the
> conversion_factor inherited from the conversion_based_unit.
>
> _Formal propositions:_
>
> *WR1: The offset shall not be zero.*
>
> Note: If the offset is zero, then the supertype *conversion_based_unit
> *is used.
> =========end proposed addition to part 41=========
>
> On Jun 6, 2014, at 3:29 PM, Martin Hardwick <hardwick at steptools.com
> <mailto:hardwick at steptools.com>> wrote:
>
>> *P21 e3 DIS Testing*
>> Minutes of June 5, 2104 Telecon
>>
>>
>> Attendees
>>
>> Martin Hardwick <hardwick at steptools.com> <mailto:hardwick at steptools.com>
>> Robert Lipman <robert.lipman at nist.gov> <mailto:robert.lipman at nist.gov>
>> Michael Benda <mjbenda at rockwellcollins.com>
>> <mailto:mjbenda at rockwellcollins.com>
>> Hedlind Mikael <mikael.hedlind at scania.com>
>> <mailto:mikael.hedlind at scania.com>
>> Dave Loffredo <loffredo at steptools.com> <mailto:loffredo at steptools.com>
>> Ed Paff <ejp at transcendata.com> <mailto:ejp at transcendata.com>
>> Tom Thurman <thomas.r.thurman at imonmail.com>
>> <mailto:thomas.r.thurman at imonmail.com>
>>
>>
>> Apologies for Absence
>>
>> Jochen Fritz <jfritz at steptools.com> <mailto:jfritz at steptools.com>
>>
>> Nicolas Figay <nicolas.figay at eads.net> <mailto:nicolas.figay at eads.net>
>>
>>
>> PMI Splitting
>>
>> ITI has split the sp3_boxy file produced by NIST into a geometry file
>> and a pmi file. STEP Tools has viewed the geometry file and is
>> working on a viewer for the separated pmi file. The two files and the
>> original NIST data can be found at the following URL:
>>
>> ftp://www.steptools.com/private/P21e3_DIS_testing/PMI/
>>
>> STEP Tools is sponsoring the development of open source programs to
>> split and merge P21 files using the Edition 3 specification. The
>> programs are available at the following URL:
>> http://tinyurl.com/thundercode.
>>
>>
>> ZIP Assemblies
>>
>> No visible progress has been made on this test case since the last
>> telecom, but the PMI merge and split code is being updated to handle
>> this case as well.
>>
>> ftp://www.steptools.com/private/P21e3_DIS_testing/ZIP_Assembly/
>>
>> In the last telecom:
>>
>> ·We decided the sub-tree directory names should include the NUAO
>> identifier(s) for their corresponding node in the assembly tree.
>> ·We learned that the LZMA algorithm has been shown to make STEP files
>> three times more compressed than the more commonly used deflate
>> algorithm, but at the cost of an increase in the compression time.
>> ·We learned that each compressed file is required to document the
>> algorithm used for its compression in its header and that different
>> components in the same ZIP can used different compression algorithms.
>> ·We recommended that the choice of the best compression algorithm
>> should be left to the end user and that the standard should be silent
>> on the matter.
>>
>> **
>>
>>
>> Unit Definitions
>>
>> The unit definitions have been merged into a single file called
>> units.stp.
>>
>> ftp://www.steptools.com/private/P21e3_DIS_testing/Units/
>>
>> We continued our discussion on whether unit references should be
>> encoded as a URL or a URN. The following two lines of code show
>> equivalent URL's and URN's.
>>
>> <http://standards.iso.org/iso/10303/tech/reference_data/41/si_base_units.stp#METRE>;
>>
>> <urn:iso:std:iso:10303:-41:tech:unit:metre>;
>>
>> Explanation of URN:
>> urnIndicates this URI is a URN, instead of the more common URL (http)
>> iso URN namespace (other examples are oid, usbn)
>> stdISO standard
>> isooriginating organization (other examples are iec, iso-ies, iso-cie)
>> 10303STEP standard
>> -41part of multipart standard (hyphens required)
>> Techassociated or embedded resource defined by committee that created
>> the standard
>> <the rest>unspecified -- controlled by committee.
>>
>> In the above example, the URN has small advantage over the URL but
>> perhaps the following URN will be acceptable because a file does not
>> have to exist to define a URN.
>>
>> <urn:iso:metre>;
>>
>> It is also conceivable that the following URN might be acceptable
>> even though there are currently small technical issues stopping the
>> deployment of unit definitions for a Fahrenheit measure in a STEP file.
>>
>> <urn:iso:Fahrenheit>;
>>
>> We further discussed the requirements if all the unit definitions are
>> to be defined in STEP files. The definition of literal constants such
>> as PI, and EXPRESS constants such as negri_pi, have both led to
>> extensive additions to the edition 3 format for which other strong
>> use cases are currently lacking.
>>
>> On the other hand the problem might be that we have not been
>> sufficiently ambitious. It was pointed out that if constants can be
>> defined then we are close to allowing expressions to be defined. The
>> use of the PLIB schema for expressions has never been very
>> satisfactory because the expressions are too hard to read when
>> encoded into the STEP files. However, if Part 21 has an expression
>> evaluation capability (probably in the anchors) then potential STEP
>> applications such as parametrics and construction history might
>> become more tractable. One interesting possibility might be to use
>> Modelica as the expression language.
>>
>>
>> Digital Signatures
>>
>> STEP Tools gave a demonstration of how to make a digital signature
>> using a private key and a signing certificate and how to verify that
>> a file has not been edited since it was signed using a public key.
>>
>> ftp://www.steptools.com/private/P21e3_DIS_testing/Digital_signatures/
>>
>> We discussed white space and the issues that might arise if a file is
>> read into Notepad/Wordpad and converted from line-feeds to carriage
>> returns or vice versa. These characters are explicitly excluded from
>> the Part 21 character set so they will not be included when computing
>> and verifying the hash value.
>>
>> We discussed supporting multiple signatures. The easiest procedure is
>> for each signature to be applied to all of the characters that
>> precede that signature. Thus each new signee is also verifying the
>> signature of the previous signees.
>>
>> For this to be consistent it would be best if the signatures came
>> after the END-ISO-10303-21 keyword as shown in the example on the ftp
>> site.
>>
>> ·The next conference call will be held on Friday June 27 at 4PM
>> Paris, 3PM London, 10AM New York and 7AM Seattle.
>>
>>
>> Action Items
>>
>> 1.Complete the first ZIP assembly example.**
>>
>> 2.Consider the best approach for unit definitions: URN's or URLs'?**
>>
>> 3.Consider if Edition 3 should offer any support to the definition of
>> parametrics and if so how?**
>> 4.Extend the digital signatures example to include the creation of
>> signing certificates.
>> 5.Demonstrate signing at one site (ITI) and verification at another
>> site (STEP Tools).
>> As recorded by Martin Hardwick <hardwick at steptools.com>
>> <mailto:hardwick at steptools.com>
>>
>> <Minutes_p21e3_DIS_conference_call_05062014.docx>_______________________________________________
>> wg11 mailing list
>> wg11 at steptools.com <mailto:wg11 at steptools.com>
>> http://lists.steptools.com/mailman/listinfo/wg11
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--
Signature
beste Grüße / kind regards
Dr.-Ing. Thomas Liebich
Geschäftsführer / Director
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