Tag: sparql

A comment from @frabcus earlier today alerted me to the fact that the Scraperwiki team had taken me up on my suggestion that they make the Python YQL library available in the Scraperwiki environment, so I thought I ought to come up with an example of using it…

YQL provides a general purpose standard query interface “to the web”, interfacing with all manner of native APIs and providing a common way of querying with them, and receiving responses from them. YQL is extensible too – If there isn’t a wrapper for your favourite API, you can write one yourself and submit it to the community. (For a good overview of the rationale for, and philosophy behind YQL, see Christian Heilmann’s the Why of YQL.)

Browsing through the various community tables, I found one for handling SPARQL queries. The YQL wrapper expects a SPARQL query and an endpoint URL, and will return the results in the YQL standard form. (Here’s an example SPARQL query in the YQL developer console using the data.gov.uk education datastore.)

The YQL query format is:
select * from sparql.search where query=”YOUR_SPARQL_QUERY” and service=”SPARQL_ENDPOINT_URL”
and can be called in Python YQL in the following way (Python YQL usage):

def run_sparql_query(query, endpoint):
    y = yql.Public()
    query='select * from sparql.search where query="'+query+'" and service="'+endpoint+'"'
    env = "http://datatables.org/alltables.env"
    return y.execute(query, env=env)

For a couple of weeks now, I’ve been look for an opportunity to try to do something – anything – with the newly released Ordnance Survey Linked Data (read @gothwin’s introduction to it for more details: /location /location /location – exploring Ordnance Survey Linked Data – Part 2).

One of the things the OS Linked Data looks exceedingly good for is acting as glue, mapping between different representations for geographical and organisational areas; the data can also return regions that neighbour on a region, which could make for some interesting “next door to each other” ward, district or county level comparisons.

One of the most obvious ways in to the data is via a postcode. The following Linked Data query to the ordnance survey SPARQL endpoint (http://api.talis.com/stores/ordnance-survey/services/sparql) returns the OS district ID, ward and district name that a postcode exists in:
PREFIX skos: <http://www.w3.org/2004/02/skos/core#&gt;
PREFIX postcode: < http://data.ordnancesurvey.co.uk/ontology/postcode/&gt;

select ?district ?wardname ?districtname where { <http://data.ordnancesurvey.co.uk/id/postcodeunit/MK76AA&gt;
postcode:district ?district; postcode:ward ?ward.
?district skos:prefLabel ?districtname.
?ward skos:prefLabel ?wardname
}

Here is is running in the YQL developer console:

(Just by the by, we can create a query alias for that query if we want, by changing the postcode (MK76AA in the example to @postcode. This gives us a URL argument/variable called postcode whose value gets substituted in to the query whenever we call it:

http://query.yahooapis.com/v1/public/yql/psychemedia/ospostcodelookupdemo1?postcode=INSERTPOSTCODEHERE&env=http://datatables.org/alltables.env

[Note we manually need to add the environment variable &env=http://datatables.org/alltables.env to the URL created by the query alias generator/wizard.]

So… that’s SPARQL in YQL – but how can we use it in Scraperwiki… The newly added YQL wrapper makes it easy.. here’s an example, based on the above:

os_endpoint='http://api.talis.com/stores/ordnance-survey/services/sparql'

os_query='''
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX postcode: <http://data.ordnancesurvey.co.uk/ontology/postcode/>

select ?district ?wardname ?districtname where {
<http://data.ordnancesurvey.co.uk/id/postcodeunit/MAGIC_POSTCODE> postcode:district ?district; postcode:ward ?ward.
?district skos:prefLabel ?districtname.
?ward skos:prefLabel ?wardname
}
'''
postcode="MK7 6AA"

os_query=os_query.replace('MAGIC_POSTCODE',postcode.replace(' ',''))

def run_sparql_query(query, endpoint):
    y = yql.Public()
    query='select * from sparql.search where query="'+query+'" and service="'+endpoint+'"'
    env = "http://datatables.org/alltables.env"
    return y.execute(query, env=env)

result=run_sparql_query(os_query, os_endpoint)

for row in result.rows:
    print postcode,'is in the',row['result']['wardname']['value'],'ward of',row['result']['districtname']['value']
    record={ "id":postcode, "ward":row['result']['wardname']['value'],"district":row['result']['districtname']['value']}
    scraperwiki.datastore.save(["id"], record) 

I use the MAGICPOSTCODE substitution to give me the freedom to create a procedure that will take in a postcode argument and add it in to the query. Note that I am probably breaking all sorts of Linked Data rule by constructing the URL that uniquely identifies (reifies?) the postcode in the ordnance survey URL namespace (that is, I construct something like <http://data.ordnancesurvey.co.uk/id/postcodeunit/MK76AA&gt;, which contravenes the “URIs are opaque” rule that some folk advocate, but I’m a pragmatist;-)

Anyway, here’s a Scraperwiki example that scrapes a postcode from a web page, and looks up some of its details via the OS: simple Ordnance Survey Linked Data postcode lookup

The next thing I wanted to do was use two different Linked Data services. Here’s the setting. Suppose I know a postcode, and I want to lookup all the secondary schools in the council area that postcode exists in. How do I do that?

The data.gov.uk education datastore lets you look up schools in a council area given the council ID. Simon Hume gives some example queries to the education datastore here: Using SPARQL & the data.gov.uk school data. The following is a typical example:

prefix sch-ont: <http://education.data.gov.uk/def/school/&gt;

SELECT ?name ?reference ?date WHERE {
?school a sch-ont:School;
sch-ont:establishmentName ?name;
sch-ont:uniqueReferenceNumber ?reference ;
sch-ont:districtAdministrative <http://statistics.data.gov.uk/id/local-authority-district/00MG&gt; ;
sch-ont:openDate ?date ;
sch-ont:phaseOfEducation .
}

Here, the secondary schools are being identified according to the district area they are in (00MG in this case).

But all I have is the postcode… Can Linked Data help me get from MK7 6AA to 00MG (or more specifically, from <http://data.ordnancesurvey.co.uk/id/postcodeunit/MAGIC_POSTCODE&gt; to <http://statistics.data.gov.uk/id/local-authority-district/00MG&gt;?)

Here’s what the OS knows about a postcode:

If we click on the District link, we can see what the OS knows about a district:

The Census Code corresponds to the local council id code used in the Education datastore (thanks to John Goodwin for pointing that out…). The identifier doesn’t provide a Linked Data URI, but we can construct one out of the code value:
<http://statistics.data.gov.uk/id/local-authority-district/MAGIC_DISTRICTCODE&gt;

(Note that the statistics.data.gov.uk lookup on the district code does include a sameas URL link back to the OS identifier.)

Here’s how we can get hold of the district code – it’s the dmingeo:hasCensusCode you’re looking for:

PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX admingeo: <http://data.ordnancesurvey.co.uk/ontology/admingeo/>
PREFIX postcode: <http://data.ordnancesurvey.co.uk/ontology/postcode/>

select ?district ?nsdistrict ?wardname ?districtname where {
<http://data.ordnancesurvey.co.uk/id/postcodeunit/MAGIC_POSTCODE> postcode:district ?district; postcode:ward ?ward.
?district skos:prefLabel ?districtname.
?ward skos:prefLabel ?wardname .
?district admingeo:hasCensusCode ?nsdistrict.
}
'''

postcode='MK7 6AA'
os_endpoint='http://api.talis.com/stores/ordnance-survey/services/sparql'
os_query=os_query.replace('MAGIC_POSTCODE',postcode.replace(' ',''))

result=run_sparql_query(os_query, os_endpoint)

for row in result.rows:
    print row['result']['nsdistrict']['value']
    districtcode=row['result']['nsdistrict']['value']
    print postcode,'is in the',row['result']['wardname']['value'],'ward of',row['result']['districtname']['value']
    record={ "id":postcode, "ward":row['result']['wardname']['value'],"district":row['result']['districtname']['value']} 

So what does that mean… well. we managed to look up the district code from a postcode using the Ordnance Survey API, which means we can insert that code into a lookup on the education datastore to find schools in that council area:

def run_sparql_query(query, endpoint):
    '''
    # The following string replacement construction may be handy
    query = 'select * from flickr.photos.search where text=@text limit 3';
    y.execute(query, {"text": "panda"})
    '''
    y = yql.Public()
    query='select * from sparql.search where query="'+query+'" and service="'+endpoint+'"'
    env = "http://datatables.org/alltables.env"
    return y.execute(query, env=env)

edu_endpoint='http://services.data.gov.uk/education/sparql'    

edu_query='''
prefix sch-ont:  <http://education.data.gov.uk/def/school/>

SELECT ?name ?reference ?date WHERE {
?school a sch-ont:School;
sch-ont:establishmentName ?name;
sch-ont:uniqueReferenceNumber ?reference ;
sch-ont:districtAdministrative <http://statistics.data.gov.uk/id/local-authority-district/MAGIC_DISTRICTCODE> ;
sch-ont:openDate ?date ;
sch-ont:phaseOfEducation <http://education.data.gov.uk/def/school/PhaseOfEducation_Secondary>.
}
'''
districtcode='00MG'
edu_query=edu_query.replace('MAGIC_DISTRICTCODE',districtcode)
result=run_sparql_query(edu_query, edu_endpoint)
for row in result.rows:
    for school in row['result']:
        print school['name']['value'],school['reference']['value'],school['date']['value']
        record={ "id":school['reference']['value'],"name":school['name']['value'],"openingDate":school['date']['value']}
        scraperwiki.datastore.save(["id"], record) 
​

Here’s a Scraperwiki example showing the two separate Linked Data calls chained together (click on the “Edit” tab to see the code).

Okay – so that easy enough (?!;-). We’ve seen how:
– Scraperwiki supports calls to YQL;
– how to make SPARQL/Linked Data queries from Scraperwiki using YQL;
– how to get data from one Linked Data query and use it in another.

A big problem though is how do you know whether there is a linked data path from a data element in one Linked Data store (e.g. from a postcode lookup in the Ordnance Survey data) through to another datastore (e.g. district area codes in the education datastore), where you is a mere mortal and not a Linked Data guru?! Answers on the back of a postcard, please, or via the comments below;-)

PS whilst doing a little digging around, I came across some geo-referencing guidance on the National Statistcics website that suggests that postcode areas might change over time (they also publish current and previous postcode info). So what do we assume about the status (currency, validity) of the Ordnance Survey postcode data?

PPS Just by the by, this may be useful to folk looking for Linked Data context around local councils: @pezholio’s First steps to councils publishing their own linked data