Observations & Measurements (O&M) is an international standard for modeling observation events and describing their relations to the target spatial objects under observation, the measured properties & measurement procedure, and the captured data resulting from those observation events. It’s based on Geography Markup Language (GML), another standard by the Open Geospatial Consortium (OGC) enabling a common base for it’s notation of location based information.
In addition to the most obvious cases of representing records of scientific measurement data, the O&M model is also used for modeling predicted information like weather forecasts. Because of it’s general ability to model perceived values of spatial objects’ properties at specific times, it’s a good fit for many kinds of application domains where it’s necessary to capture time-based changes on objects of interest.
The basic O&M observation event concepts.
The O&M conceptual model is published both as an Open Geospatial Consortium (OGC) Abstract Specification Topic 20 and as an ISO standard with number ISO/DIS 19156. The XML implementation of O&M model is also an OGC standard “Observations and Measurements – XML Implementation“. The origins of the O&M is in the Sensor Web Enablement (SWE) initiative of the OGC. It was needed as the common standardized data model for handling the measurement events occurring in different kinds of sensors from thermometers inside an industrial process to satellites taking images of the Earth from the space. Together with other SWE framework open standards like SensorML and Sensor Observation Service (SOS), O&M provides a system-independent, Internet-enabled ways of data exchange between different parts of sensor networks and other systems using the captured sensor information.
Even though the O&M model was originally created for modeling measurement acts that have already happened, there is no technical difficulty in using the same model also for describing acts of estimating the values of some properties of spatial objects at some point in the future. After all, event the most precise measurements are still only estimations of the actual properties of the target objects, limited by the method and the tools used, as well as our capabilities of interpreting the measurement results. The captured data itself is often very similar in both measurement and prediction cases, so it makes sense to try to store and deliver those data sets using the the same basic concepts.
One of the facts that makes the O&M model interesting right now is the increasing affordability of IP-based environmental sensors: these days almost anyone can afford to buy a basic weather observation station, place it in their backyard, and plug it in the Internet for sharing the data with others. Or buy an Internet-connected web camera. This also means that it’s becoming possible for anyone to gather and refine detailed environmental information about the world around us, both locally and globally. What used to the playground of the big, closed national and international institutes and governmental offices, is now opening up also to ordinary citizens. Of course this also means, like in everything based on the Internet, that as the amount of information and the heterogeneity of the sources producing it grows, the quality range of the available information also inevitably becomes wider.
The Sensor Web movement is so promising that also the organizations that used to deploy and maintain their own sensor networks with their proprietary data and control interfaces built for their specific software and hardware systems, are moving towards these open standards. Even though they might not put their data publicly in the Internet, they definitely want to take advantage of the IP-based networks for communicating, and they’s love to be able to easily switch between two sensor equipment boxes made by different vendors in plug-and-play fashion. The extra network traffic caused by a higher level communication protocols and more verbose content encoding is less and less of an issue in this ever more broadband world of ours.
Still, it would be nice if the increasing amounts of sensor data collected by publicly funded organizations would also be made available to the public, wouldn’t it? In many cases it already is available for someone who knows where to ask. Sometimes it’s even freely available in the Internet, like the various public web cams, but mostly it’s still accessible only to professionals. This is bound to change gradually however as international legislation aiming at data opening and harmonization, like the EU INSPIRE directive in Europe, is being implemented around the world. The O&M concepts form the basis of the EU-widely harmonized INSPIRE data models for meteorological, climatological and air quality information, as well as for physical oceanographic measurement data and the structure and the capabilities of the various environmental observation networks. This basically means that in the near future the ordinary citizens will be able to access the environmental data provided by the government officials in pretty much using the same protocols and data formats that they’re used to while accessing their neighbor’s off-the-self sensor equipment. Ain’t that cool?
I’m currently involved in the international expert teams on behalf of our customer the Finnish Meteorological Institute for creating the data specifications and writing guidelines for some of the O&M based INSPIRE data sets. We’re currently finalizing our work on the guidelines documents, but the actual work to make the INSPIRE spatial data infrastructure reality goes on of course. Fortunately there are deadlines: initial bunch of the view and download services for these INSPIRE data sets should be publicly available in May 2013, and even the last bits should be fully in INSPIRE compliant shape by 2020.
XLink is a widely used W3C standard for creating links between XML documents or fragments inside those documents, similar to HTML a tag. The problem is that for historical reasons there several slightly different XML Schema grammar definitions for the same XLink namespace “http://www.w3.org/1999/xlink” published by different standardization authorities, like World Wide Web Consortium (W3C), Open Geospatial Consortium (OGC), Organization for the Advancement of Structured Information Standards (OASIS) and XBRL International (XBRL stands for eXtensible Business Reporting Language).
Two different XML Schema versions for the same namespace = trouble
If your software is using XML documents referring to different versions of XML Schema files for the same namespace, you are heading for trouble: A well-behaving, validating XML parser will probably only load the XML Schema files once for each namespace, meaning that it will use different schema files for validating XLinks depending on the order in which it happens to read the input XML files. If this happens, there is a high probability that part of your XML files will not validate if parsed together, even though they will if parsed separately.
It could also happen that an XML parser caches the downloaded XML Schema files locally to avoid excess network traffic and thus improve it’s parsing efficiency. Say that your software first validates a bunch of XBRL 2.1 documents, and caches the schema file XBRL schema refers to as the definition of the XLink namespace. Then the same software tries to validate a GML 3.2 document. When the parser notices that the XLink schema is also used in the GML schema files (AssociationAttributeGroup), it will not download the referred OGC XLink schema, but use the cached XBRL version instead. This will result in validation error, because the attributeGroup named “xlink:simpleLink” used by the GML schema does not exist in the XBRL version of the XLink schema. So one day the same files do validate, the other day they do not.
XLink in OGC standards
The W3C Xlink 1.0 version published in June 2001 was accompanied with only the DTD version of the defined linking attributes, and the users of XML Schema language were left on their own (probably because the W3C XML Schema 1.0 had only just been published at the time). The OGC version of the XLink XML Schema file was always meant to be a temporary measure, to be replaced by an official W3C version when one would become available. Unfortunately W3C XLink Schema version eventually published in May 2010 along with the standard version 1.1, was not exactly the same as the OGC version, a decision which is now causing a major headache for the OGC.
The differences between the two are quite small, but irritating. Both versions use almost identical definitions for the XLink attributes (href, type, role etc.), but they have grouped the typical sets of these attributes using different group names: the OGC XLink schema has an attributeGroup named “simpleLink” where as in the W3C XLink 1.1 schema the corresponding attribute groups is called “simpleAttrs”. Both contain the same set of attributes: the mandatory “type” and “href”, and the optional “role”, “arcrole”, “title”, “show” and “actuate”. This means that the actual XML documents using either version of these XML Schema files do validate against the other schema, but the XML Schema files containing references to these named attribute groups will not.
As an example, to make the GML 3.1.1 to use the W3C XLink schema, in addition to changing the schemaLocation attribute to point from the OGC schema repository to the W3C one, all references to the attribute group “simpleLink” would have to be changed to “simpleAttrs”. For GML 3.2.1 the problem is even somewhat more complicated, because it refers to the XML schemas for the ISO standard 19139 “Geographic information — Metadata — XML schema implementation” a.k.a GMD, which too are currently pointing the the OGC version of the XLink XML Schema.
OGC is moving towards W3C XLink 1.1
This issue has been acknowledged by the OGC, and as far as I know, they are currently taking action in moving from using their own version of XLink schema into using the W3C XLink version 1.1. That said, the OGC has not yet officially declared how and when the changes will be made, but it seems obvious that they will require changes to the existing XML Schema files for published OGC standard versions stored in the OGC XML Schema repository. Because it’s commonplace, and even recommended, to make local copies of the XML Schema files for making XML validation more efficient and robust, this would mean that there would be different versions of the OGC standard XML schema files out there, until everybody would replace their local versions with the modified ones. Needless to say, this kind of maneuver needs careful planning and efficient communication to all the OGC standard users around the world, to minimized the problems during the transition period.
So why would OGC risk making such change to the XML Schema files of the published standards? Why not just do the change to the new versions of the standard and leave the existing standards to use the OGC version of the XLink schema? So for example the GML 4.0 would to the transition from the OGC XLink into the W3C XLink definition, while the GML 3.3 as well as all the other published OGC standard versions would still use the OGC XLink schema.
The problem is that hanging on to both versions of the XLink schema would probably cause even more trouble for the XML validators than trying to change all the schemas simultaneously: it would increase the probability of a validator encountering different XLink schema versions, and there would be no end to this misery in sight. It might also happen that even newly created XML languages would start using the OGC XLink schema, because they would want to be schema compatible with the older OGC standards. So the only way to eventually make the nuisance disappear, would be to abandon the OGC version of the Xlink schema once and for all.
Prepare for the change
As mentioned, the OGC is still planning the best way to make the XLink transition as smooth as possible for all OGC standards users. If they do decide to go for the full-scale, once-and-for-all transition my personal guess is that something like this will happen:
- All the OGC standards using the OGC version of the XLink Schema files will be listed, and they will be converted to using the W3C XLink 1.1 Schema. The namespaces of these changed schema versions would be left unchanged, but they would initially be published in a dedicated, temporary schema repository.
- The users of those standards are urged to test their systems by using the alternative, modified versions of those OGC Schemas instead of the OGC official versions, for validating their documents. One way to do this is to use XML Catalog technique instructing the XML validators to retrieve the schema files for those specific namespaces from an alternative URL address. The main purpose of this testing period is to reveal possible compatibility problems with other XML schema languages using a non-W3C version of XLink: It could well be that some other schemas used for XML validation by the same parsers no longer validate after the OGC XLink schema is no longer available. The pre-configured XML Catalog file could be provided by the OGC to ease the transition.
- A hard transition day would be announced by the OGC: on that day (or night, more likely) all the affected OGC schema files in the official will be replaced by the modified versions pointing to the W3C XLink Schema only. At the same time the OGC XLink Schema file would probably be removed from the OGC schema repository. The users should take action to ensure that their systems no longer point to the old OGC schema versions of the OGC XLink schema from that point on, and that the possible local versions of the changed schemas are replaced with the new ones.
- The files in the temporary schema repository may co-exist for some (pre-announced) time after the transition date. This gives the XML Catalog users more time to update the URLs of the modified Schema files from the temporary location (back) to the official location.
In general organizations should not publish an XML Schema for the namespace they do not own, because the governance of such schemas becomes complicated: if the owner of the namespace (w3c in this case) decides to make a change to it’s schemas, it might not be able to do so because the other organization “hosting” the schemas does not want to or is unable to make those changes. The case of W3C and OGC XLink schemas is a perfect example of the problems even a slight lack of coordination in such issues may cause.
Edit 27th Jan: Carl Reed, the CEO of OGC confirmed to me yesterday that there will be official OGC announcement considering the problem as well as actions to be taken “in the next month or so”. The actual transition day is expected in late summer to early autumn this year.
Edit 13th April: The OGC Architecture Board has to make the XLink schema switch-over in July 2012, most likely during the weekend of 21st July. See my follow-up post for more information.
Working (and blogging) at Cafe Piritta today for a change. Very good lunch, a bit pricey though. I’m about to leave for Vienna today to meet with the INSPIRE Thematic Working Group for atmospheric and meteorological data ( Atmospheric Conditions & Meteorological Geographical Features themes, TWG AC-MF in the INSPIRE jargon). I’m privileged to be a member in the group on half of our customer the Finnish Meteorological Institute.
Interesting two days to go through the comments on the Data Specification version 2.0 of our theme, and to figure out how to proceed with the final version of the spec. We’ve received a bunch of comments, which is good, because it means that the EU member states are interested in what the INSPIRE requirements for the meteorological and atmospheric data are going to be like. The task we’re facing is challenging, because the expectations span both data policy (what data to include) and the technology (how to share that data in interoperable, INSPIRE compatible way). I just wish we could focus just on technology, because that alone is enough to occupy our minds for some time.
My personal goal for the Data Specification would be to make it an instantly usable guideline on how to implement an INSPIRE compliant OGC Web Service interfaces intended for the tech-savy people working for the different institutions dealing with meteorological data.The current version of the specification is not straightforward enough. If I want to publish an INSPIRE compatible Download Service for delivering meteorological observation data from ground observation stations, what service do I use? The answer is probably an OGC Web Feature Service 2.0 with INSPIRE additions, but this is not explicit in the spec. The data model for the met data is specified, but how it should be mapped to the View Services (Web Map Service) and Download Services (WFS, possibly Web Coverage Service or Sensor Observation Service) is not very clear.
There are separate guideline documents for implementing INSPIRE Discovery Services, View Services and Download Services, but they may not be a perfect fit for the themes involved with measurement data, like our AC-MF or the Oceanographic Geographic Features. Our data model is based on a general ISO/OGC Observations & Measurements (O&M) model, which is a good, solid framework to build on, but it also means the you can extract only very little information about the actual measurement data from the model itself. Thus the model does not really tell you what data is expected, unlike in the themes like Transport Networks, where the kinds of roads, crossings etc have different classes in the model. We only have “Observation” instances, which could as well contain data originated from ground observations, weather radar or numerical forecast models, and thus need to be more specific about the expected data types and suitable service interfaces than most other themes.
Well, a challenge it may be, but pursue it we must nevertheless. I must say it’s a relief to be aquatinted with some hard-core experts in the area of O&M to ask for advice. Like in most contexts, it’s nice to know you’re not alone 🙂