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Provide way to cast variables to type geo:wktLiteral in WDQS queries
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Description

It would be good to be able to cast calculated variables to type geo:wktLiteral in WDQS SPARQL queries, to allow calculated co-ordinates as well as retrieved ones to be included in map views of WDQS output, etc.

Currently there seems to be no way to do this.

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Jheald created this task.Mar 13 2017, 12:08 PM
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Jheald added a comment.EditedMar 13 2017, 12:16 PM

Example case:

I wanted to be able to look at the geographic distribution of property P 3616 by grouping into one-degree-by-one-degree squares, and plotting on a map, colour-coded by the number of instances.

The grouping into one-degree-by-one-degree squares is easy enough, but there seemed to be no good way to then plot the centrepoints of these squares on a map -- i.e. no way to cast the calculated variables to a geo:wktLiteral.

I tried the following (full query in query editor):

BIND(CONCAT("Point(",str(?int_lon)," ",str(?int_lat),")") AS ?string) .

But the string itself, although it looks right in the table display, is not recognised as a pair of plottable co-ordinates.

So I hoped there might be a cast function geo:wktLiteral(), by way of analogy with eg xsd:dateTime():

BIND(geo:wktLiteral(?string) AS ?int_coords)

but back comes the error

"Unknown error: unknown function: http://www.opengis.net/ont/geosparql#wktLiteral"

I also tried appending ^^geo:wktLiteral in various places, eg BIND(?string^^geo:wktLiteral) AS ... but this just returns a syntax error.

Would it be possible to implement some kind of casting mechanism? Or alternatively, is there any way I can do this cast with just the sort of string operations available in SPARQL ?

Trying to get round this, by just plotting the values in a scatterplot, without a map, also failed -- though that's likely to be an issue with the scatterplot routine. (Ticket T160325 opened).

Another case where it could be useful to be able to cast calculated co-ordinates back to a geo:wktLiteral would be to allow AVG() co-ordinates to be calculated and plotted, in cases where SAMPLE() is not sufficient.

Jheald updated the task description. (Show Details)Mar 13 2017, 12:29 PM

I don't think though AVG() and other math functions are defined on coordinates.

Perfect. I would never ever have found that.

That was exactly what was was needed to make the query work. (results).

As for AVG(), no, I think it is not defined over geo:wktLiteral. But so long as one can extract the latitudes and longitudes, AVG() those separately, and then cast the result back into a geo:wktLiteral (which with strdt() is now easy), then one can achieve the same result -- to calculate and display the centroid of a set of points.

The one thing I still don't understand is why, clicking on a point in the map view, the fields are presented top to bottom in the opposite order from the order in which they appear as columns -- but that's a question for a different ticket, I think.

Smalyshev closed this task as Resolved.Mar 13 2017, 11:44 PM
Smalyshev claimed this task.