[QGIS-ZA-user] aggregate crime attribute data from SAPS Station boundaries to Wards

Wed Oct 3 06:04:22 PDT 2018

Hello Brian,

What Gerbrand says is absolutely correct, but taking your assumption 
that crimes occur the same over the total area of the ward you would 
need to work out the crimes/km2 or so first. From there on it is a 
pretty straight forward exercise with QGIS. That involves a few steps 
however:

 1. Add the two layers to your map
 2. Set your project projection to Zangi:Zangi:CPNO31 (and area units to
    km2, if that's what you want)
 3. Then use the vector function "Union" on the two layers to create a
    new polygon layer which essentially is a combination of attributes
    on the two
 4. The you will notice that there are a few polygons that are not
    "unioned" as they don't overlap (e.g. policing areas outside your
    wards): make the new layer editable and delete those. You should
    then have 367 polygons with "unioned" attributes (I noticed that
    there are a few small ones inside the wards/policing areas due to
    not-adjoining polygons..these should be sorted out first!).
 5. Then use the field calculator to create new columns for each year
    category by taking the "number of murders" / $area to get the
    murders per km2 (scary!!)
 6. Then use the Vector Analysis Tool "Statistics by categories" (you
    are on QGIS 3?) to do a statistic on each of the new year columns
    using the ward ID or number (or both) as Field with categories where
    you would get min, max, mean, etc. but the mean is a pretty good
    assumption in murders/km2 as it is a ratio.
 7. Then you join the Union table with your results for each year and
    save the layer (using selections) to a new table with only the
    columns you want. You could convert the murders/km2 back to the
    total murders in the area (inverse) beforehand.
 8. "Bob's your Uncle"

Hope his helps!

Immo

On 03/10/2018 14:20, Gerbrand Mans via qgis-za-user wrote:
> Hi Brian
>
> You need a proxy variable to predict what the proportional 
> contribution from each ward would have been to get to the total murder 
> rate for the station area.
>
> One way will be to use socio-econ profiling, but you do run into 
> certain biases which may become problematic when you need to explain 
> your method to your users of the output. Else it is just going to be a 
> straight proportional redistribution to wards based on total 
> population per ward if you assume the probability is the same across 
> the police station area.
>
> Best rgds
> Gerbrand Mans
> >>> Brian O'Leary via qgis-za-user <qgis-za-user at lists.osgeo.org> 03/10/2018 12:26 >>>
> Hi Gavin,
>
> attached are the Durban police station boundaries with murder 
> attributes for the last 10 years. Also attached are the Ward 
> boundaries. Any advise on how to produce an output table with murder 
> by ward would be much appreciated.
>
> Thanks
> Brian
>
> On Wed, Oct 3, 2018 at 8:53 AM Gavin Fleming via qgis-za-user 
> <qgis-za-user at lists.osgeo.org <mailto:qgis-za-user at lists.osgeo.org>> 
> wrote:
>
>     Hi Brian
>
>     Please share a few overlapping station and ward features with the
>     list.
>
>     thanks
>
>     Gavin
>
>>     On 02 Oct 2018, at 10:57, Brian O'Leary via qgis-za-user
>>     <qgis-za-user at lists.osgeo.org
>>     <mailto:qgis-za-user at lists.osgeo.org>> wrote:
>>
>>     Dear QGis SA,
>>
>>     I would like to know if there is a QGis tool that can assist with
>>     the following task. I want to transfer crime attribute data from
>>     SAPS Station boundaries to Ward boundaries, assuming that crime
>>     is evenly distributed within the SAPS Station boundary.
>>
>>     Previously I have done this task in Arc View using a script
>>     called DataPart.avx which was written by Jianwei Dou in 2002. The
>>     below simplified sketch illustrates what I am trying to do in
>>     Qgis. The SAPS Station shape has the data attributes and the Ward
>>     shape is the overlay or boundary shape. The output that I need is
>>     an attribute table that has calculated the % intersection of the
>>     SAPS Station by the ward boundaries and aggregated the crime
>>     attributes to a Ward table based on the % intersection. In the
>>     below sketch the ward boundaries intersect the SAPS Station
>>     boundary by 50% in both cases. As there were 100 crimes in the
>>     SAPS Station boundary the output table lists Wards 1 and 2 with
>>     50 crimes in each.
>>
>>     <image.png>
>>     Any advise most welcome.
>>
>>     Regards
>>     Brian O'Leary
>>
>>
>>
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