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

From: Jeremy Tandy <jeremy.tandy@gmail.com>
Date: Thu, 01 Sep 2016 06:54:41 +0000

```All. Very useful conversation so far about linear referencing, but nothing
yet on the topic of spatial relations - see the second half of the email I
used to start this thread [1]. I really need some help on this too. Thanks.

Jeremy

[1]: https://lists.w3.org/Archives/Public/public-sdw-wg/2016Aug/0210.html
On Thu, 1 Sep 2016 at 07:51, Jeremy Tandy <jeremy.tandy@gmail.com> wrote:

> @simon ...
>
> > But if you want to compare different properties along a linear feature,
> it makes sense to use linear referencing
>
> for example, in a borehole? This is especially true where you can't
> provide a geographic coordinate because you're not sure where the borehole
> actually goes to any degree of accuracy.
>
> So ...
>
> I think that we're saying that LR should be avoided (by converting LR
> measurements into more general coordinates) except in some special cases
> where:
>
> 1) you want to describe sets of things along a linear element in a network
> - such as in hydrology or samples from a borehole - and it is more
> important to describe the relative positions of those things along the
> linear element
>
> 2) you can't (or don't want to) express the location in geographic
> coordinates to a satisfactory level of precision - for example, in a
> geological survey that uses chainage (distance) between two points.
>
> In both of these cases, it seems sensible to describe the position using a
> 1-dimensional CRS (or LRS).
>
> On Wed, 31 Aug 2016 at 23:55, <Simon.Cox@csiro.au> wrote:
>
>> Ø  … it’s a best practice not to use linear referencing …
>>
>>  … because it makes it difficult to combine with data that uses a
>> different CRS.
>>
>> But if you want to compare different properties along a linear feature,
>> it makes sense to use linear referencing.
>>
>>
>>
>> *From:* Joshua Lieberman [mailto:jlieberman@tumblingwalls.com]
>> *Sent:* Thursday, 1 September 2016 1:48 AM
>> *To:* Frans Knibbe <frans.knibbe@geodan.nl>
>> *Cc:* Jeremy Tandy <jeremy.tandy@gmail.com>; Cox, Simon (L&W, Clayton)
>> <Simon.Cox@csiro.au>; Ed Parsons <eparsons@google.com>; SDW WG Public
>> List <public-sdw-wg@w3.org>
>>
>>
>> *Subject:* Re: Request for help: BP 9 "How to describe relative
>> positions"
>>
>>
>>
>> Well, my opinion is that it’s a best practice not to use linear
>> referencing, but that’s only my opinion, and I’ve been persuaded that a
>> spatial ontology should accommodate it.
>>
>>
>>
>> Josh
>>
>>
>>
>> On Aug 31, 2016, at 11:08 AM, Frans Knibbe <frans.knibbe@geodan.nl>
>> wrote:
>>
>>
>>
>>
>>
>>
>>
>> On 31 August 2016 at 15:19, Joshua Lieberman <
>> jlieberman@tumblingwalls.com> wrote:
>>
>> Yes, but it’s a sort of derived CRS (LRS) that references one or two
>> other feature geometries (curve +/- origin point) in their own CRS, adding
>> a unit of measure.
>>
>>
>>
>> So you agree that there is no need to elaborate on LR in the BP if the
>> spatial ontology (new version of GeoSPARQL) that we are working on offers
>> sufficient hooks to work with LR? That would probably be a relief for the
>> BP editors.
>>
>>
>>
>> I think some recursiveness is hard to avoid in CRS defintions. Coordinate
>> systems are based on datums and datums are based on ellipsoids...
>> Eventually everything in the universe needs to be located in terms of some
>> arbitrary reference system.
>>
>>
>>
>> Regards,
>>
>> Frans
>>
>>
>>
>>
>>
>> There is another way of looking at linear reference measurements, which
>> is that they are observations made directly on a linear feature. In many
>> cases, it is a compact way of recording where some additional measurement
>> has been made, such as a water level, that eventually gets converted to a
>> linear reference point based on an LRS, and from there to an (x,y) point.
>> Both should be possible, depending on the usage.
>>
>>
>>
>> Josh
>>
>>
>>
>>
>>
>> On Aug 31, 2016, at 8:54 AM, Frans Knibbe <frans.knibbe@geodan.nl> wrote:
>>
>>
>>
>> I wonder if Linear Referencing can be seen as a version of the mo
>>
>> re general case of expression location in terms of a well defined CRS.
>> For 3D data, we use two numbers (latitude and longitude) to place a
>> location on a funnily shaped 3D geometry (a model of the surface of the
>> Earth). In LR, we use a single number to place locations on a funnily
>> shaped 2D geometry (e.g. a model of a road or a river). We can use
>> topological relationships to make assertions about those locations and to
>> filter data: Is there an essential difference between asking whether two
>> polygons on Earth touch each other or if two sections of road touch each
>> other?
>>
>>
>>
>> Now if our spatial ontology allows the definition of any kind of CRS in
>> two or three dimensions, and clearly associating that CRS with geometry,
>> and the use of topological relationships, then it could very well be that
>> there is no need to make special arrangements for LR.
>>
>>
>>
>> I do think that organisations that naturally work with LR data, e.g.
>> organisations in the transportation sector, should be able to publish their
>> data on the web and let them be used by whomever it pleases.
>>
>>
>>
>> Regards,
>>
>> Frans
>>
>>
>>
>>
>>
>> On 31 August 2016 at 13:42, Jeremy Tandy <jeremy.tandy@gmail.com> wrote:
>>
>> 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.
>>
>> +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.
>>
>>
>> Jeremy
>>
>>
>>
>> On Wed, 31 Aug 2016 at 12:18, Joshua Lieberman <
>> jlieberman@tumblingwalls.com> wrote:
>>
>> It's also a part of stream hydrology, which is mainly there is a version
>> of it in sdwgeo.
>>
>>
>>
>> Josh
>>
>> Joshua Lieberman, Ph.D.
>>
>> Principal, Tumbling Walls Consultancy
>>
>> Tel/Direct: +1 617-431-6431
>>
>> jlieberman@tumblingwalls.com
>>
>>
>> On Aug 31, 2016, at 06:23, <Simon.Cox@csiro.au> <Simon.Cox@csiro.au>
>> wrote:
>>
>> Well you do see it in navigation systems. Time & distance.
>>
>>
>>
>> *Sent:* Wednesday, 31 August 2016 7:49 PM
>> *To:* Jeremy Tandy <jeremy.tandy@gmail.com>; SDW WG Public List <
>> 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 ?
>>
>>
>>
>> ed
>>
>>
>>
>>
>>
>> On Wed, 31 Aug 2016 at 10:26 Jeremy Tandy <jeremy.tandy@gmail.com> wrote:
>>
>> Hi-
>>
>>
>>
>> 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]
>>
>>
>>
>> ...
>>
>>
>>
>> (1)
>>
>> 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.
>>
>>
>>
>> Questions:
>>
>> 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
>>
>>
>>
>> (2)
>>
>> 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.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> [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
>>
>>
>>
>>
>> --
>>
>> *Ed Parsons *FRGS
>>
>> Google Voice +44 (0)20 7881 4501
>> www.edparsons.com @edparsons
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
```
Received on Thursday, 1 September 2016 06:55:34 UTC

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