[Qgis-community-team] Choropleth mapping

[Nick Bearman]

Morning David,

It may be that QGIS has considered your data (Value-1) to be String 
rather than Numeric.

The way to check this is right click on the layer, choose Properties and 
Fields. What type is your Value-1 field listed as?

If it is string, you can use the Refactor Tool to change it to Numeric. 
See this example:

The Pop2018 column contains population data, but QGIS thinks it is a 
String (text). QGIS then wouldn’t
be able to display the data, or create a choropleth map with it. We need 
to change the format from String
(text) to Double (number).
• Click Processing > Toolbox.
3
• Type Refactor in the search box.
• Double click on Refactor fields it might take a little while to load.
• Make sure we have our new MidYearEstimates layer in the Input Layer box.
• Change our Pop2018 field from String to Double.
• Click Run.
• When it has finished running, click Close.
This has added a new layer called Refactored. It has a little memory 
chip icon next to it, which means
it isn’t saved anywhere.
• Check that the Refactored layer now has the population data in the 
correct data type.

Once it is Numeric, it should be visible when you setup a choropleth map.

Good luck and let us know how you get on!

Best wishes,
Nick.



On 13/07/2021 02:23, David Rhoads wrote:
> Folks:
>
> Please help me.  I am very frustrated.  I recently downloaded the 
> current long-term release of QGIS (3.16.8).  Two of you kindly 
> suggested documentation to get me started.  One problem, the 
> documentation does not match the current shipping version of QGIS.  In 
> one case, the documentation was for QGIS 1.  In the other case, while 
> supposedly for QGIS 3.16, the screens in the documentation do NOT 
> match what I see in my version of QGIS (I'm running Win 10).
>
> I am starting with a series of districts in a county.  I want to do 
> choropleth mapping in those districts based on numeric values in a CSV 
> file.  I am able to load the two layers, the districts from an SHP 
> file and the numeric values from the CSV file and then to join them.  
> When I try to generate the choropleth map, the name of the field 
> containing the numeric values does not show up when I click on the 
> Symbology tab.  I can see that field in several other tabs, but not in 
> the Symbology tab.
>
> I expect there is an easy solution to this, but so far it has eluded me.
>
> Attached are the Project file and the CSV file containing the data 
> (Value-1) to apply to the polygons.  Join is done on the PrecinctID 
> column.
>
> David
>
> On 7/1/2021 1:20 AM, Evan Derickson wrote:
>> Hi David,
>>
>> You can use the Graduated symbol renderer to make a choropleth map. 
>> The official documentation on this is at 
>> https://docs.qgis.org/3.16/en/docs/gentle_gis_introduction/vector_attribute_data.html#graduated-symbols 
>> <https://docs.qgis.org/3.16/en/docs/gentle_gis_introduction/vector_attribute_data.html#graduated-symbols>, 
>> but you may find something like 
>> https://guides.library.duke.edu/QGIS/Symbolize 
>> <https://guides.library.duke.edu/QGIS/Symbolize> to be more helpful.
>>
>> Best,
>> Evan
>>
>> On Wed, Jun 30, 2021, 14:46 David Rhoads <dgrhoads at dgrhoads.us 
>> <mailto:dgrhoads at dgrhoads.us>> wrote:
>>
>>     Does QGIS support choropleth mapping (i.e. ability to apply
>>     colors to
>>     polygons depending on a numerical value associated with each
>>     polygon)?
>>     Our application has to do with looking at the relative statistics of
>>     voting precincts in Chester County, PA.  We want to easily find
>>     outliers.
>>
>>     If so, where in the documentation is this described?  I've done some
>>     looking and have not been able to find anything related.
>>
>>     Thanks,
>>     David Rhoads
>>     _______________________________________________
>>     Qgis-community-team mailing list for organizing community
>>     resources such as documentation, translation etc..
>>     Qgis-community-team at lists.osgeo.org
>>     <mailto:Qgis-community-team at lists.osgeo.org>
>>     https://lists.osgeo.org/mailman/listinfo/qgis-community-team
>>     <https://lists.osgeo.org/mailman/listinfo/qgis-community-team>
>>
>
>
> _______________________________________________
> Qgis-community-team mailing list for organizing community resources such as documentation, translation etc..
> Qgis-community-team at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/qgis-community-team

-- 
Nick Bearman
Tel / WhatsApp / Signal: +44 (0) 7717745715
nick at geospatialtrainingsolutions.co.uk

Due to my own life/work balance, you may get emails from me outside of normal working hours. Please do not feel any pressure to respond outside of your own working pattern.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-community-team/attachments/20210721/7dd9f1cd/attachment.html>