[Requests] OGC Web Coverage Service - Transaction operation extension - Comment

Timothy Astle timothy.astle at caris.com
Fri Aug 7 08:56:05 EDT 2015


Bringing this back to my question on requirement 8, Peter, can it be 
adjusted from:

     _Requirement 8 _
     After completion of a successful InsertCoverage request, the identifier of
     the coverage established in the server’s offering shall be listed as an
     existing coverage in this WCS service’s Capabilities document.

to something like:

     _Requirement 8 _
     After completion of a successful InsertCoverage request, the identifier of
     the coverage established in the server’s offering shall be made available as
     an available coverage in this WCS.  This can either be done by adding an entry
     in the service's capabilities content, or by updating the content of another
     system referenced by the service capabilities OtherSource element.

This will allow implementers to adopt WCS-T with other types of discovery sources and choose not to list the content in the service capabilities.  Else the support of transaction would forbid the ability to delegate to an OtherSource as the requirement is written.  I understand the idea behind the requirement, but it seems constraining.

Tim


On 07/08/2015 5:20 AM, Peter Baumann wrote:
> Hm, that's a different ballpark. At the bottom I add a few use cases which WCS
> operators typically need for their users.
> But first an analysis:
>
> - preamble: OpenSearch is for human users directly talking to the data. The gap
> XPath fills, however, is mainly that clients need some information for operation
> and maybe generate client info (such as pop-up menus) - in other words, the
> XPath is machine-generated and hidden from the humans.
>
> - data model. OpenSearch does not know about the structure and semantics of the
> Capabilities document.
> Ex: retrieving a URL OpenSearch cannot determine whether it is the Native CRS of
> some coverage or just some CRS supported.
> Note that the XPath approach takes the complete WCS offering and hence allows
> drilling down into specific coverages (see examples).
>
> - query model.
>      - It looks like it allows conjunctive queries (only AND connections), but
> not disjunctive queries (OR connections), likely also no negation of predicates,
> and no nesting (ie: parentheses). The XPath approach relies on the established
> definitions of W3C which allows OR connections, predicates, etc.
>      - OpenXPath allows to address only part of a Capabilities document, and for
> WCS it would not allow the coverages to be addresses; XPath allows both
>      - XPath can deliver XML or just plain text lines, it's up to the user
>      - XPath is fully W3C conformant whereas bbox and start/end introduce new
> query concepts (and separation of time and space does not work with coverages)
>
> - complexity.
>      - OpenSearch is rather complex (6 parameters) whereas XPath is simple and
> straightforward (1 parameter defined by just 4 requirements).
>      - you introduce a specific requirement on the data kept in the server,
> whereas the XPath approach simply assumes document order, something inherent to
> the definition of XML and Capabilities.
>      - OpenSearch introduces a series of new parameters, XPath has only one.
>
> - generality:
>      the XPath proposal likewise can be mapped to any underlying physical
> representation, but can make use of the structural knowledge.
>
> - implementation:
>      XPath engines are readily available, no code modification needed
>
> So wrt expressive power your proposal is inbetween XPath (document-oriented
> extraction) and SQL/WCPS/etc (query language).
> What I wanted - based on user needs - is not something that makes Capabilities
> appear a catalog, but to obtain the technical information necessary for finding,
> recognizing, and binding WCSs. Hence, both proposals address different needs and
> hence follow different approaches.
>
> So here the use cases:
>
> •    “All WCS Extensions supported by this server”
> XPath request:  /Capabilities/ServiceIdentification/Profile/text()
> Sample response:
> http://www.opengis.net/spec/GMLCOV/1.0/conf/gml-coverage
> http://www.opengis.net/spec/GMLJP2/2.0
> http://www.opengis.net/spec/WCS_coverage-encoding_geotiff/1.0
>
> •    “All formats supported by this server”
> XPath request:
> /Capabilities/ServiceIdentification/ServiceMetadata/formatSupported/text()
> Sample response:
>      application/netcdf
>      image/jp2
>
> •     “Identifiers of all coverages offered”
> XPath request: /Capabilities/Contents/CoverageSummary/CoverageId/text()
> Sample response:
>      NASA_NIGHT_EARTH
>      NASA_NIGHT_EARTH_SCALED_SHALLOW_TOPO
>
> •     “spatial extent of coverage X”
> XPath request: /Capabilities/OfferedCoverage/coverage[@id=“X”]/boundedBy
> Sample response (you see the CRS coming along):
> <boundedBy>
>      <Envelope srsName="http://www.opengis.net/def/crs/EPSG/0/4326"
>          axisLabels="Lat Long" uomLabels="deg deg" srsDimension="2">
>          <lowerCorner>
>              -179.9933330866212 -0.0133335802515
>          </lowerCorner>
>          <upperCorner>
>              0.0133338271788 359.9933332102485
>          </upperCorner>
>      </Envelope>
> </boundedBy>
>
> •    “Native CRS of coverage X”
> XPath request:
> /CoverageOfferings/OfferedCoverage/coverage[@id=“X”]/boundedBy/Envelope/@srsName
> Sample response:
>      http://www.opengis.net/def/crs/EPSG/0/4326
>
> Some more:
> * "Does this service support REST?"
> * "Is there anything ground-level temperature measurement available in
> (x0,y0,x1,y1) on this server?"
> * "Which version of WCS-T is supported by this server?"
>
> Ahem, so much about thoughts while reading the document.
> Sorry for the lengthy reply,
> Peter
>
>
> On 2015-08-06 18:36, Panagiotis (Peter) A. Vretanos wrote:
>> Simon at al.
>>
>> Already done ... but we used an OpenSearch API rather than XPath letting the
>> underlying server pick nay query technology it likes.
>>
>> https://portal.opengeospatial.org/files/?artifact_id=54580
>>
>> Ciao.
>>
>> On 08/05/2015 05:56 PM, Simon.Cox at csiro.au wrote:
>>> ØTherefore, we have prepared a WCS Extension which I want to propose for voting
>>> at the Nottingham TC. This extension introduces a very simple concept: an XPath
>>> expression can be submitted which is evaluated on the Capabilities document on
>>> server side. This allows applications to get exactly what they need, without
>>> excess data transfer. If you feel that this is a way forward, maybe you want to
>>> support this in the upcoming TC meeting - any such support is greatly
>>> appreciated!
>>>
>>> And perhaps better still, submit this idea as an OWS Common CR or as input to
>>> the upcoming revision.
>>>
>>> *From:*Requests
>>> [mailto:requests-bounces+simon.cox=csiro.au at lists.opengeospatial.org] *On Behalf
>>> Of *Peter Baumann
>>> *Sent:* Wednesday, 5 August 2015 5:31 PM
>>> *To:* Timothy Astle <timothy.astle at caris.com>; requests at lists.opengeospatial.org
>>> *Subject:* Re: [Requests] OGC Web Coverage Service - Transaction operation
>>> extension - Comment
>>>
>>> Dear Timothy,
>>>
>>> after completion of the RFC period I'm coming back to your (very thoughtful)
>>> questions below.
>>>
>>> Re Req 8: This actually is a known issue with OWS Common. A service provider can
>>> decide which elements to expose in a Capabilities document, which has been
>>> considered sufficient in the past. Alas, it does not solve the issue - practice
>>> shows that either you get not enough information, or too much (such as all
>>> coverages listed). In fact, the Capabilities mechanism is way too static and
>>> inflexible.
>>>
>>> OGC has started thinking about how to fix and modernize, but there is no relief
>>> on the horizon soon. Therefore, we have prepared a WCS Extension which I want to
>>> propose for voting at the Nottingham TC. This extension introduces a very simple
>>> concept: an XPath expression can be submitted which is evaluated on the
>>> Capabilities document on server side. This allows applications to get exactly
>>> what they need, without excess data transfer. If you feel that this is a way
>>> forward, maybe you want to support this in the upcoming TC meeting - any such
>>> support is greatly appreciated!
>>>
>>> Re References to grids: Core and extensions have been formulated very carefully
>>> so as to allow all coverage types defined, which includes point clouds and
>>> meshes. Currently, there is one extension which specifically refers to grids,
>>> that is WCS Scaling: change of resolution is an operation meaningful on grids
>>> only (at least that is my current understanding). Concretely speaking about
>>> WCS-T, a point cloud (MultiPointCoverage) or mesh (MultiCurveCoverage or
>>> MultiSurfaceCoverage or MultiSolidCoverage) can be inserted and deleted, and can
>>> even be updated. Of course, update of a mesh requires more complex consistency
>>> constraints to be respected, but that is inherent to the definition of such mesh
>>> types, so does not have to be treated separately here.
>>>
>>> Actually, there is nothing in the mask parameter that confines us to a grid.
>>> Consider a point cloud, for example, The mask in this case will contain the
>>> direct positions of the points to be updated, associated with a range value of
>>> "1" (likely in this case the "0" points will simply be left out), and this
>>> guides the server on the points to be updated - say, with a new temperature
>>> value. Hence, I'd claim that this definition works on all coverage types -
>>> kindly let me know should I miss something here, it's a good opportunity to
>>> catch any and all hiccups.
>>>
>>> And obviously there is one: searching for the root cause of your concern I find
>>> occurrences of "maskGrid", a leftover forgotten in an earlier renaming action. I
>>> have corrected this, and it actually resolves an inconsistency - good catch!
>>>
>>> Thank you again for the encouraging assessment and your thoughtful comments.
>>> Looking forward to further discussion in future.
>>>
>>> best regards,
>>> Peter
>>>
>>> On 2015-06-26 20:48, Timothy Astle wrote:
>>>
>>>      A couple of questions:
>>>
>>>      _Requirement 8 _
>>>      /After completion of a successful InsertCoverage request, the identifier of
>>>      the coverage established in the server’s offering shall be listed as an
>>>      existing coverage in this WCS service’s Capabilities document./
>>>
>>>      OGC 06-121r9 (OGC Web Services Common Standard) still allows for the idea of
>>>      an "otherSource" for finding OWS content.  Is the above requirement
>>>      mandating thatwhen implementing a WCS-T, you must describe the coverage in
>>>      the Capabilities' CoverageSummary list?  I'm concerned that when dealing
>>>      with a large amount of coverages, that could really degrade the performance
>>>      of a system.
>>>
>>>      _References to Grids_
>>>
>>>      I definitely see value to this transactional extension to WCS.  I'm
>>>      definitely interested to watch WCS grow beyond gridded data.  In this
>>>      particular extension, I seereferences to gridsand grid-related terminology
>>>      and concepts.  I'm concerned that if / when point clouds or TINsare
>>>      introduced as extensions, that this specification might not makesense in
>>>      some contexts.  For example, if a WCS was created that specialized in TINs,
>>>      the optional grid parameters wouldn't really apply.  Is the intent that this
>>>      is a "grid transactional operation extension" or a "general coverage
>>>      operation extension"?  Would a future point cloud transactional extension be
>>>      created for point cloud operations?  I guess I'm just asking for
>>> clarification.
>>>
>>>      Thank you for your time and consideration.
>>>
>>>      --
>>>
>>>      Tim Astle
>>>      Development Manager for Web Technologies
>>>
>>>      *CARIS* <http://www.caris.com>
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>>> -- 
>>>
>>> Dr. Peter Baumann
>>>
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-- 
Tim Astle
Development Manager for Web Technologies

*CARIS* <http://www.caris.com>
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