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

From: Clemens Portele <portele@interactive-instruments.de>
Date: Wed, 31 Aug 2016 09:59:59 +0000
To: Jeremy Tandy <jeremy.tandy@gmail.com>, SDW WG Public List <public-sdw-wg@w3.org>
Message-ID: <etPan.57c6aa9f.78883f20.123@interactive-instruments.de>
Hi Jeremy,

regarding linear referencing in GML, GML 3.3 (see http://www.opengeospatial.org/standards/gml) has added in clause 9 rules for using linear referencing - based on ISO 19148.

A simple example from the spec:

<gml:Point gml:id="p1" srsName="#LSRS123">

is a position along a linear spatial reference system, which is specified as

<gmllr:LinearSRS gml:id="LSRS123">
 <gml:LineString srsName="..." srsDimension="3" gml:id="LS_BH18">
  <gml:posList>407829 268621 23.93 407415 268600 8.43</gml:posList>
 <gmllr:LinearReferencingMethod gml:id="LRM001">
  <gmllr:name>chainage</gmllr:name> <!--chainage = measurement in metres —>
  <gmllr:type>absolute</gmllr:type> <!--absolute = measure from start of linear element —>
  <gmllr:units uom="m"/>

The INSPIRE network application schema for networks predates the work on GML 3.3 (and most of the work on ISO 19148) so it does not yet use the GML 3.3 approach.

In general, in my experience GIS software is not well prepared to understand linear referencing unless there is specific support for domains that use this approach like transportation networks, boreholes, etc. For general purpose use like displaying data on a map or spatial queries one will typically include the spatial information also as a geometry in coordinate reference system with a geodetic datum.

So, if we really want include linear referencing in the BP, the limits of what you can do with it in general purpose toolsets should be made clear. Or maybe include as a BP to also provide the spatial information using a CRS with a geodetic datum for the reasons mentioned?

Regarding a representation in RDF, this may be something I will look at later this year, but this may or may not happen. In general, I think there will be similar questions like those discussed above. For example, if you want to query data using GeoSPARQL, you will have to include the spatial geometry in WGS84 (or similar).

Best regards,

On 31 August 2016 at 11:25:06, 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

Received on Wednesday, 31 August 2016 10:00:37 UTC

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