TJS Frequently Asked Questions

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Frequently Asked Questions


Why would I use TJS? I can program this type of thing myself.

Why not just modify existing standards, such as WFS (Web Feature Service) and GML (Geography Markup Language) to support TJS functionality?

Is TJS the same as Geolinking?

Do we have to share our whole database? Our organization has a lot of sensitive information that not everybody should have access to.

Do we have to give away our data for free?

How can my organization make its attribute data accessible?

Can I join multiple attribute to a common geometry at the same time?

Where is the Join operation performed?

Can a WPS request values of a TJS join (e.g. where polygons have been populated with corresponding attribute values) for further aggregation i.e. scale up / normalize, to calculate mean / average / totalsum / trends, algorithmic and geostatistical rendering, interpolation with other thematic layers?

Can TJS be used to tie WPS output values to WFS geometry?


Why would I use TJS? I can program this type of thing myself.

The main reason for using a standard or specification such as TJS is interoperability, which allows for easy sharing of information. By adopting a common standard to format and transfer the terabytes of data existing in multiple organizations on multiple systems, the data become easily accessible through the internet for all to benefit.

Table joining technology is endorsed by the Canadian Geospatial Data Infrastructure (CGDI), which in its tutorial gives some practical, day-to-day reasons for using CGDI-endorsed standards and specifications for geospatial data and services:

Widening the focus, the CGDI also gives the following reasons for supporting standards and specifications:

TJS was developed by the OpenGIS Consortium (OGC).

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Why not just modify existing standards, such as WFS (Web Feature Service) and GML (Geography Markup Language) to support TJS functionality?

There are a number of reasons why TJS was created as a new specification.

Note that the use of TJS does not preclude the use of GML. A TJS can be set up to merge a GDAS stream with a GML stream to produce an amended GML stream that incorporates the attribute information provided by GDAS.

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Is TJS the same as Geolinking?

Yes. TJS is the successor to a suite of technology that was known as geolinking. TJS was developed over the period from 1999 to 2010. During that time it underwent a number of name changes. It was originally known as the Geographic Data Access Service and the Geolinking Service, then merged to form the Geographic Linkage Service, and is now known as the Table Joining Service. The fundamental concepts have remained unchanged, but TJS has incorporated a substantial number of improvements from the past ten years of interoperability experiments and testing.

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Do we have to share our whole database? Our organization has a lot of sensitive information that not everybody should have access to.

Not everything has to be shared – you can select which data you wish to publish. Perhaps the best way to ensure security of your data is to implement TJS on a dedicated, isolated network, to which access is limited.

For example, Agriculture Canada works with some sensitive agricultural census data accessible only by its own scientists and analysts. TJS is used to make this data accessible to them via an internal network. Only the aggregated data or results of the modelling are made available to the public, via a separate TJS instance.

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Do we have to give away our data for free?

No, using TJS does not mean that the data must be given away for free. In the case of government departments, it is encouraged to freely distribute the data to the taxpayers that support them. However, for commercial enterprises, an e-commerce module can be layered on top of TJS.

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How can my organization make its attribute data accessible via TJS?

There are three things you need to do to make your geolinked data Web-accessible:

  1. Document the data. Determine which data and associated metadata you wish to publish. Depending on how your databases are structured and which TJS software you use, you may have some data restructuring and documentation work to do.
  2. Create the Application Program Interfaces (APIs) to your data. TJS relies on a lightweight XML schema called GDAS to exchange attribute information. If your needs are fairly simple, you might just want to create a GDAS file manually and publish it to a GDAS registry. If your datasets are more substantial, you can either have a developer create APIs to your corporate database system(s) to produce XML streams according to the TJS standard, or install software that supports TJS and populate its associated databases.
  3. Register your data. Use this registry to advertise your TJS instance. Search for data using this client.

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Can I join multiple attribute to a common geometry at the same time?

Yes, you can. Multiple attributes from a single table can be joined to a common geometry in one operation, although the server may restrict the number of attributes that can be joined simultaneously. This restriction is advertised in the server's Capabilities document.

Joining data from multiple tables will require separate operations for each table. In order to create a single output that consists of data joined from multiple tables, the input data would have to be merged into a single GDAS representation prior to the TJS Join operation. Such merging could be accomplished via a WPS.

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Where is the Join operation performed?

The Join operation is mediated by the TJS server, but can actually take place on another server, and the results of the join operation can be made available in a variety of formats.

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Can a WPS request values of a TJS join (e.g. where polygons have been populated with corresponding attribute values) for further aggregation i.e. scale up / normalize, to calculate mean / average / totalsum / trends, algorithmic and geostatistical rendering, interpolation with other thematic layers?

In order to use the results of a TJS join in further geostatistical manipulations, the TJS server performing the join should be configured to output data via WFS.

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Can TJS be used to tie WPS output values to WFS geometry?

TJS can be used to tie outputs from a WPS to the geometry hosted by a WFS. In order to do so, the outputs from the WPS must be in GDAS format, which is defined in the TJS spec. Also, the TJS performing the JoinData operation must be able to manipulate the database that supports the WFS server.

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