Re: [SMW-devel] Semantic mediawiki and map data

[Markus K. <ma...@se...>]

Hi Kim,

that is a very exciting project. Here are some general hints/comments:

* Jeroen is right, but I would suggest a slightly different approach :-)

* To get started, you can store your data in a Container data item 
(similar to subobjects and records). In essence, this means that each 
value can be an arbitrary collection of property-value pairs. This gives 
you most flexibility and might even be needed for things like polygons 
where you have no limit on the amount of data.

* This does not affect your surface syntax. You can create a datavalue 
(DV) implementation for your datatypes, and the parsing and general I/O 
will be the same in any case. So it will allow you to do many things.

* Once this works and you have a clearer idea what the data looks like, 
it might be useful to have special DIs for it. The advantage of that is 
that the database can handle them more efficiently than general purpose 
container objects. The disadvantage is that the database would need to 
handle them in special ways instead of treating them as general purpose 
container objects. So there is some code needed to get update, 
retrieval, and query answering implemented.

* The special DIs can still support the interface of Container, they can 
even be a special form of containers, and thus create subobjects to 
which further properties could be attached. This does not prevent the 
database from having a special handling for these containers if it is 
known that their structure is of a special form that could be stored in 
a more efficient way. So it is not necessary that your code avoids the 
use of Container-specific code just because of the possible later change 
to a custom DI. In fact, you will certainly need to use the interface of 
Container to get at the data.

* Nischay is currently working on a general overhaul of the SQL database 
backend as part of his Google Summer of Code project. This might 
simplify such extensions in the future by making the code better 
structured and more accessible. So there are good reasons to wait with 
DI extensions now. We will keep this use case in mind -- maybe the code 
that is specific to each DI can be cleanly separated from the rest.

Cheers,

Markus


On 22/05/12 16:08, Jeroen De Dauw wrote:
> Hey,
>
>  > a map can have several polygons
>
> Sure. Which polygons a map shows is determined by the query it
> visualizes the results for. When creating the definitions of the
> polygons you should not have to care to much of which polygon will end
> up on what map. You just create a data structure that makes sense and
> ensure that you can actually do the kind of queries you want to do.
>
>  >  Would this structure be a fair assumption?
>  >
> [[Polygons::[[Polygon::<coords>,<coords>,<coords>...]],[[Polygon::<coords>,<coords>,<coords>...]],[[Polygon::<coords>,<coords>,<coords>...]]]]
>
> No. Imagine you have a pile of articles of cities, then you could have a
> property such as "Has location" (which is of type polygon) which would
> look like:
>
> [[Has location::<coords>,<coords>,<coords>...]]
>
> If you have an article describing an object that has multiple polygons,
> for example a group of islands where each island needs a polygon, then
> you simply have multiple such properties on the page.
>
>  > So say i have a [[Polygon::<coords>,<coords>,<coords>]] property
> defined exactly how can i point additional metadata to this object?
>
> In order to be able to point to an object, it needs to be of type page.
> So you cannot out of the box specify a property such as "Has stroke
> color". One option is to specify the polygons as part of subobjects [0],
> which can then also hold any extra extra info on the polygon.
>
> In a way it would be nicer to not have it as part of the subobject, but
> it actually behave like one, in which case you could have a list of "Has
> point" properties. This way you could do stuff like querying all the
> points in a polygon that have a latitude more then x. This requires some
> more thought I think - anyone more thoughts on this?
>
> [0] http://semantic-mediawiki.org/wiki/Subobject
>
> Cheers
>
> --
> Jeroen De Dauw
> http://www.bn2vs.com
> Don't panic. Don't be evil.
> --
>
>
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