What are the differences between the ONSPD and the NSPL?
Introduction
This paper explains the differences in the allocation of postcodes in the ONS
Postcode Directory (ONSPD) and the National Statistics Postcode Lookup (NSPL)
products.
To summarise:
 ONS Postcode Directory (ONSPD) – assigns individual postcodes separately to
each higher level geography
 National Statistics Postcode Lookup (NSPL) – assigns individual postcodes to
Output Areas (OAs), and then using its assigned OA to lookup its higher level
geography.
Methodology:
ONSPD
The ONSPD primarily associates postcodes with a number of other geographies. The
postcode is allocated to each type/level of geography using a point-in-polygon
method. This places each unique postcode in the actual boundary of the geography it
falls in.
Method:
The diagram shows that for each of the different higher level geographies in this
diagram (output areas, wards and parliamentary constituencies) the postcodes are
allocated using a point in polygon process each time.
Example:
To clarify, the example below shows a number of postcodes (red dots) within the
same Output Area but straddles 2 different Westminster Parliamentary Constituency
(ParliCon) boundaries. The postcodes are allocated directly to the ParliCon based on
where the postcode geographically falls within (the point-in-polygon method).
Zooming
in
00MGNW0012
Therefore the postcodes are allocated to the 2 different ParliCon boundaries (C94
and C95) as shown below, regardless of the fact they are within the same output
area:
Postcode
MK13 9HB
MK13 9AP
MK13 9HF
MK13 9HG
MK8 8DD
MK8 8ND
MK8 8DF
ParliCon code
C94
C94
C94
C94
C95
C95
C95
Output Area
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
Although this method is accurate in that postcodes are assigned to the correct
geographical higher level geography, it does not support the Geography Policy for
National Statistics. This is why ONS Geography is now providing a product that does
support the policy – the NSPL.
NSPL:
A key principle of the Geography Policy for National Statistics is that statistics for
higher level geographies should be built from “small area stable statistical building
bricks”. These are Output Areas, or Super Output Areas if not available at OA. The
purpose of the policy is that national statistics are geographically consistent and
comparable.
The policy method recommends first linking postcodes to the Output Areas (using the
ONSPD point-in-polygon method). Postcodes should then be linked to all higher
geographies by using an Output Area to higher geography lookup. The Output Area
to higher geography lookup is created by ONS “best-fitting” 2001 Census population
data for each Output Area into each higher level geography. Where the higher level
geography splits an Output Area, that OA is best-fitted to the higher geography that
the greatest proportion of that OA’s population falls in. If the whole OA is within one
area in the higher geography then this is assigned to that area immediately.
Method:
The diagram shows that postcodes are allocated to Output Areas based on the point
in polygon process. The Output Areas are then allocated to each type/level of
geography based on the population of the Output Area (where the greater proportion
lies, or if split between more than two higher geographies, the greatest proportion of
population).
Example:
To clarify, the example below shows a number of postcodes (red dots) within the
same Output Area but straddles 2 different Westminster Parliamentary Constituency
(ParliCon) boundaries. The postcodes are allocated to the ParliCon boundary based
on where the greatest proportion of the population of the Output Area falls within.
31
18
25
20
24
45
Zooming
in
65
00MGNW0012
The population of the Output Area (00MGNW0012) that falls within ParliCon C94 is 94
(31 +18+25+20) and the populations that falls within ParliCon C95 is 134
(24+45+65). Therefore all the postcodes within this Output Area are allocated to the
ParliCon C95 as it contains the greater proportion of the population.
Postcode
MK13 9HB
MK13 9AP
MK13 9HF
MK13 9HG
MK8 8DD
MK8 8ND
MK8 8DF
ParliCon code
C95
C95
C95
C95
C95
C95
C95
Output Area
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012
00MGNW0012