Re: Area of spatial objects

Hi Frans,

Thanks for the follow-up reply. Yes, I could do as you say, perhaps something like this:

<Feature_X> a geo:Feature ;
  geo:hasGeometry [
    # a normal GeoSPARQL Geometry, Point, Polygon etc.
    …
  ],
 [
    geo:dimension 1 ;  # not sure how useful this is
    qudt:numericValue “12345678”^^xsd:decimal ;
    qudt:unit qudt:SquareMeter ;  # more direct than via a CRS
    …
  ]
  …
.


But this still leaves me with no intuitive way to indicate that the geometry here is actually the well-known property “area”. For that I would either need a Geometry subclass or a typing to indicate area. Unless you have other ideas?

Also, you mentioned I might like to try geometries “one with only an area, one with an area and a CRS and one with just coordinates”. How would I indicate, using GeoSPARQL, a Geometry’s CRS without indicating coordinates? Would I use an EMPTY POINT like this geo:asWKT "<http://www.opengis.net/def/crs/EPSG/0/4283> POINT EMPTY”^^geo:wktLiteral or is there a way to indicate a Geometry’s CRS without tying that CRS in to a asWKT or asGML?

Is it possible to indicate an area within the measure dimension of a WKT representation of a geometry leaving all coordinate values empty perhaps?

Thanks,

Nick


From: Frans Knibbe <frans.knibbe@geodan.nl>
Date: Thursday, 8 November 2018 at 7:31 pm
To: Nicholas Car <Nicholas.Car@csiro.au>
Cc: "public-sdwig@w3.org" <public-sdwig@w3.org>
Subject: Re: Area of spatial objects

Hello Nick,

I don't think it would be problem if the coordinates of the geometry are unknown. In that case the property with the coordinates (e.g. geosparql:asWKT) would be absent. Features with a point geometry and an area could be published as a feature having two geometries: one with coordinates and one with an area. The other examples you mentioned could be dealt with the same way. For instance, have a feature with three geometries - one with only an area, one with an area and a CRS and one with just coordinates. There would then be a future option to specify that some of those geometry representations are actually the same thing.

I am not saying this way of publishing your data is better than another, but I do think it is possible. And perhaps this method can be used together with another method, leaving it up to the data consumer how she/he/it wants to query the data.

Regards,
Frans


Op wo 7 nov. 2018 om 13:55 schreef Car, Nicholas (L&W, Dutton Park) <Nicholas.Car@csiro.au>:
Hi Frans,

What if I wanted to publish Features’ areas without also publishing geometries? What about features with a point geometry and an area, no polygon. Also, I have data where an area is given and also a polygon but I don’t know for sure if the area was calculated from the polygon. In fact I have data with an area and an Albers area and a geometry and don’t really know what happened to make which.

So, I want to be able to represent area as a spatial property of a feature, independently of any geometry. For this I will try using Observations and Measurements-style mechanics for this.

I think I might use subclassing of a sosa:ObservableProperty (https://www.w3.org/TR/vocab-ssn/#SOSAObservableProperty) to express spatial properties such as area and then relate then to a geo:Feature which would also, by SOSA logic, be a sosa:FeatureOfInterest. I may invent modelling to relate that spatial property to a geometry, but this wouldn’t be required, just nice to have if known.

Cheers,

Nick

From: Frans Knibbe <frans.knibbe@geodan.nl<mailto:frans.knibbe@geodan.nl>>
Sent: Monday, 5 November 2018 6:22 PM
To: public-sdwig@w3.org<mailto:public-sdwig@w3.org>
Subject: Re: Area of spatial objects


Hi Nick,



I wonder: Doesn't the fact that multiple areas for a single spatial thing are published mean that the areas are calculated from different geometric representations of that spatial thing? That would logically make the area a property of a geometry. Besides, the geometry instance could be used to link to the CRS (e.g. Albers), ideally by URI.



CRS data linked to a geometry could also give access to the basic unit of the CRS (e.g. meter) and through that provide information on the units of properties derived from the geometry (like the area), but that would demand a fair amount of reasoning on the part of the data consumer.



Regards,

Frans