[OSGeo Africa] Comparing census and elections datasets

Pregan Pillay PreganP at dbsa.org
Sun Apr 13 23:41:40 PDT 2014

Hi all

I was faced with a similar situation towards the end of last year when I received a request for progress on basic services between 2001 Census and 2011 for Tshwane (13 Main Places).

Some observations:

1. Previous sub places (2001) now main places(2011)
2. Spatial extents not comparable because of new EA' aggregation.
3. Some of the main places and sub-places data needed to be brought in from other provinces. etc.

I believe that Individual line department's may not have the resources to execute such tasks or may simply  keep away as it is not their core business. 

I have been engaging with Stats SA at an operational level on this issue, hopefully taking this further this year. Given the scale of this work if it is to be done for the whole country and need for data at the lowest level I suggest  that Stats SA take responsibility with support from the GIS community. Besides if the data is to be used at an official level then Stats Sa would be more appropriate. Many municipalities and state departments insist on using officially published statistics and prefer not to use data from sector departments. 

The mesozone work of CSIR/HSRC is good start but has its own set of limitations. 

Pregan Pillay
Technical Analyst GIS Services 
Development Bank of Southern Africa
011 313 3602
078 800 55 89

-----Original Message-----
From: africa-bounces at lists.osgeo.org [mailto:africa-bounces at lists.osgeo.org] On Behalf Of Gavin Fleming
Sent: Monday, April 14, 2014 7:53 AM
To: africa at lists.osgeo.org
Subject: Re: [OSGeo Africa] Comparing census and elections datasets

Hi Adi

The CSIR produced (still  produces?) a product called GAP, which aggregates or disaggregates many datasets to a common 'mesozone' layer where each polygon is roughly 50 sq.km and has boundaries based on those of other administrative layers or natural features. A published methodology is used to perform the aggregation / disaggregation. There are all sorts of statistical pitfalls in aggregating and disaggregating data into different areas and this methodology tries to address them. It isn't a simple task to do accurately.


I'd suggest you try and obtain the GAP material and in particular the methodology and mesozone dataset. Doing this would result in more 'interoperable' results since the mesozone units become the ideal unit of comparison.


On 13/04/2014 22:44, Adi Eyal wrote:
> Thanks Hanlie and Charles
> Sounds like that's the only way to go. I was hoping that secretly, 
> StatsSA was remapping census data to various shapefiles. Charles, 
> happy to take a crack at it with you to try to develop a script that 
> will allow this remapping across different boundaries.
> Adi
> On 9 April 2014 10:14, Charles Rethman <rethman at me.com> wrote:
>> Dear Adi and everybody,
>> I've been thinking about this. I think whatever happens you'll need 
>> to do some cookie-cutting.
>> The best solution I think would be to make your algorithm in PostGIS 
>> and map it on QGIS.
>> If you like, we could meet up and discuss the details further--and 
>> post our trials or results back to this forum? Where--Codebridge? 
>> contact me on rethman at me.com.
>> Charles Rethman
>> Wahenga Ltd.
>> Director, Cape Town Office
>> Cell: +27 71 6735502
>> Tel: +27 21 6861510
>> Skype: cwrethman
>> PO Box 23507, Claremont, 7735, South Africa
>> 19 Sans Souci Road, Newlands, 7700, Cape Town
>> On 8 Apr 2014, at 7:12 AM, Adi Eyal <adi at code4sa.org> wrote:
>> Hi All
>> I would like map demographic data from the 2011 census on top of 
>> elections results from 2009. There seem to be two problems with doing 
>> this.
>> 1. Old wards have disappeared and new ones have been created by MDB.
>> 2. In some cases the same ward id has been re-used for completely 
>> different polygons.
>> In short, the obvious approach of joining two datasets won't work. It 
>> is possible (but difficult) to calculate approximations by 
>> overlapping shapes from the two maps and using some sort of algorithm 
>> (possibly assuming uniform distribution of data points) to slice up 
>> one shape and fit it into another.
>> I don't want to go down that route.
>> Is there another to go about this?
>> Thanks
>> Adi
>> --
>> Adi Eyal
>> Director
>> Code for South Africa
>> Promoting informed decision-making
>> phone: +27 78 014 2469
>> skype: adieyalcas
>> linkedin: http://za.linkedin.com/pub/dir/Adi/Eyal
>> web: http://www.code4sa.org
>> twitter: @soapsudtycoon
>> For more information on how to participate in the open data community 
>> in South Africa, go to: http://www.code4sa.org/#community 
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Gavin Fleming
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