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RE: Request for help: BP 9 "How to describe relative positions"

From: Byron Cochrane <bcochrane@linz.govt.nz>
Date: Thu, 1 Sep 2016 10:45:23 +1200
To: 'Jeremy Tandy' <jeremy.tandy@gmail.com>, 'Joshua Lieberman' <jlieberman@tumblingwalls.com>, "'Simon.Cox@csiro.au'" <Simon.Cox@csiro.au>
CC: "'eparsons@google.com'" <eparsons@google.com>, "'public-sdw-wg@w3.org'" <public-sdw-wg@w3.org>
Message-ID: <666FB8D75E95AE42965A0E76A5E5337E15D79DF8D2@prdlsmmsg01.ad.linz.govt.nz>
In my experience the problem of Linear References is how to express it in a manner that is easy to use in combination with data that is referenced using a CRS.  While linear references often start as a measurement against a path in a network, those measurements become fixed even if the length changes.  So while 1.200 may have been 1.2 km along a path and match the geographic features initial, the path may change due to straightening or adding a diversion, the marker stays in the same physical location.  The distance to the marker may be now be greater or less than 1.2 km.

So from my limited experience, the useful guidance here would be to those publishing LRS data.  The recommendation would be something like including the coordinate value for markers or other ways that make combining LRS data with  CRS representation of data easier.

I would also note that linear referencing is very similar to issues representing the temporal dimension.  Calendars, epochs, etc. are linear references on the temporal dimension, are they not? (But time values have no need to be converted to spatial coordinate values that I can think of.)

I tend to agree with Ed that this is an edge case from our perspective (I would avoid the word “niche”) and need not be covered, but am not ready to put a vote to it quite yet.


From: Jeremy Tandy [mailto:jeremy.tandy@gmail.com]
Sent: Wednesday, 31 August 2016 11:43 p.m.
To: Joshua Lieberman; Simon.Cox@csiro.au
Cc: eparsons@google.com; public-sdw-wg@w3.org
Subject: Re: Request for help: BP 9 "How to describe relative positions"

I take from the discussion so far:

* GML 3.3 does LR by defining a CRS
* LR is pretty specialised
* General GIS tooling does not typically support it; although specific domains may (e.g. transport networks, hydrology, geology, navigation)

I think that @eparsons is inferring that LR is too niche to be considered a "best practice" for spatial data on the web; if data publishers _do_ use LR in their systems, then they should publish the information using a geometry that is computed from their domain-specific specialised tools.

That would certainly give me less to write :-) ... but before concluding this particular topic I'd like to see consensus from the group.

So ...

PROPOSAL: Linear Referencing is too niche to be considered a "best practice" for spatial data on the web; if data publishers _do_ use LR in their systems, then they should publish the information using a geometry that is computed from their domain-specific specialised tools.

Voting please:
+0 (I lack the hands on experience to judge)

Finally, I also note that I still need help on the "spatial relations" topic that was second in my original email. More help required please.


On Wed, 31 Aug 2016 at 12:18, Joshua Lieberman <jlieberman@tumblingwalls.com<mailto:jlieberman@tumblingwalls.com>> wrote:
It's also a part of stream hydrology, which is mainly there is a version of it in sdwgeo.

Joshua Lieberman, Ph.D.
Principal, Tumbling Walls Consultancy
Tel/Direct: +1 617-431-6431

On Aug 31, 2016, at 06:23, <Simon.Cox@csiro.au<mailto:Simon.Cox@csiro.au>> <Simon.Cox@csiro.au<mailto:Simon.Cox@csiro.au>> wrote:
Well you do see it in navigation systems. Time & distance.

From: Ed Parsons [mailto:eparsons@google.com]
Sent: Wednesday, 31 August 2016 7:49 PM
To: Jeremy Tandy <jeremy.tandy@gmail.com<mailto:jeremy.tandy@gmail.com>>; SDW WG Public List <public-sdw-wg@w3.org<mailto:public-sdw-wg@w3.org>>
Subject: Re: Request for help: BP 9 "How to describe relative positions"

I still question the need to include linear referencing, it's another very specialised way to model spatial data and one which is not widely seen on the web ?


On Wed, 31 Aug 2016 at 10:26 Jeremy Tandy <jeremy.tandy@gmail.com<mailto:jeremy.tandy@gmail.com>> wrote:

BP doc section § 10.5.1 "Describing location" [1] is where we intend to provide all the guidance that explains how you should encode location information in a web-friendly way.

This includes BP 8 "Provide geometries on the Web in a usable way" [2] and BP 9 "How to describe relative positions" [3].

(I think it's likely that we will also need a BP to help people choose the right CRS too ...)

We editors envisage BP 9 covering:

(1) Linear referencing
(2) Use of spatial relations [4]


From a quick scan, I see that ISO 19148:2012 covers the topic of Linear Referencing. I don't have access to the ISO document itself, so I've not been able to read the standard ... but reviewing the UML model (accessible here [5]) it seems VERY complicated.

I also note that the INSPIRE Generic Network Model has a simpler implementation of Linear Referencing.

a) are we limited to GML implementations for Linear Referencing?
b) has anyone converted the GML Application Schemas from ISO 19148 and INSPIRE GNM into other formats ... particularly an RDF / OWL ontology?
c) are there any other mechanisms in use for Linear Referencing? e.g. can LR be done with GeoJSON?
d) are people really using ISO 19148:2012 given it's complexity?

INSPIRE's Transport Network specification v3.2 §10.3 "Linear Referencing" states:

“In general it is expected that linear referencing will be used to model the relationships of objects that are associated with an network, but where the position of those associated objects is not known (or required) to a very high level of absolute accuracy ~ better than 1-3m at local level (e.g. traffic accidents, planned works, restrictions).

Where absolute accuracy is required (e.g. the location of drain covers, excavations, line side signalling equipment, masts etc) such objects should be reused, and referenced, if they already exist e.g. as topographic features.”

This seems like the basis of some guidance about when one might use Linear Referencing.

What I need (please!) are some worked examples for Linear Referencing of a point along a linear feature and for Linear Referencing of a length along a linear feature. In the flooding scenario, this might be:
* Location of flotsam / debris (point) blocking a drainage channel that needs to manually cleared
* Location of a flooded section (length) of a road

We also want to demonstrate how spatial relations are used. There are obvious examples of topological relationships such as "this administrative unit _touches_ that administrative unit" (or contains etc.).

I recall that we were going to get the set of topological relationships added to the IANA Link Relations registry [7]. I am not even sure which set of topological relations we should be recommending? GeoSPARQL has me somewhat confused with "Simple Features Relation", "Egenhofer Relation" and "RCC8 Relation". Then there's D9-EIM too ...

Can someone provide me some worked examples using the preferred set of topological relationships?

We also need to illustrate use of _directional_ (e.g. "left", "in front of" and "astern") and _distance_ relations (e.g. "at", "nearby" and "far away"). I don't know of any formalised vocabulary for expressing these things. If there is one, should we be seeking to add these to the IANA Link Relations registry too?

Again, worked examples requested! If you can related them to an urban environment / flooding scenario all the better. (e.g. someone might assert "the flooding is near my house")

Finally, we also need to show people how to express "fuzzy" spatial things. Examples we have elsewhere in the BP doc are "the American West" and "Renaissance Italy". These are spatial things were there is not general agreement about the exact geographic extent, so it is not possible to use a geometry to describe it. What is the best way to describe things like this? Should we use spatial relations e.g. "downtown" _contains_ city districts A, C, D, and G (because "everyone" agrees this) - but we're not saying it's exact geometry because it's a colloquial term used by citizens of our fictional Nieuwhaven.

Again, I'd like to see a worked example.


There's a lot of questions wrapped up in this email. I'm looking for help to resolve them ... preferably with someone in the WG taking the lead to coordinate a response.

I'm also aware that we need to avoid an RDF bias, so it would be good to have examples in other formats too.

Volunteers, please step forward!

Thanks in advance. Jeremy

[1]: http://w3c.github.io/sdw/bp/#bp-expr-geo

[2]: http://w3c.github.io/sdw/bp/#describe-geometry

[3]: http://w3c.github.io/sdw/bp/#relative-position

[4]: http://w3c.github.io/sdw/bp/#spatial-relations

[5]: https://github.com/ISO-TC211/HMMG

[6]: http://inspire.ec.europa.eu/documents/Data_Specifications/INSPIRE_DataSpecification_TN_v3.2.pdf

[7]: http://www.iana.org/assignments/link-relations/link-relations.xhtml


Ed Parsons FRGS
Geospatial Technologist, Google

Google Voice +44 (0)20 7881 4501<tel:%2B44%20%280%2920%207881%204501>
www.edparsons.com<http://www.edparsons.com/> @edparsons

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Received on Wednesday, 31 August 2016 22:46:14 UTC

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