[Qgis-user] Re: python-shapely

[Carson Farmer]

I was just about do so...

The cartogram plugin requires a numeric input field (although I just 
realized that I haven't filtered out non-numeric values from the 
drop-down list). [The error that you described is caused by using 
non-numeric input fields.] This numeric field is your 'false area 
field'. What the plugin tries to do is create a continuous cartogram by 
recursively altering the shape of each polygon in an effort to have the 
area of the polygon reach the specified 'false area'. The algorithm, 
based on Dougenik, J. A, N. R. Chrisman, and D. R. Niemeyer. 1985. 'An 
algorithm to construct continuous cartograms.' Professional Geographer 
37:75-81, is essentially trading off between the actual area of the 
polygon, and the user specified area.

For into on cartograms, check out:
http://en.wikipedia.org/wiki/Cartogram
or
http://www.ncgia.ucsb.edu/projects/Cartogram_Central/

Hope that clears things, up...

On a related note, does anyone know how to filter fields by type (i.e. 
only select numeric fields (int, double, etc.)?

Cheers,

Carson
Paolo Cavallini wrote:
> Carson Farmer ha scritto:
>
>   
>> In an effort to ensure that credit is given where credit is due, I just
>> wanted to clarify that the 'spatial_operations' plugin is the work of
>> 'tgwong' and not 'cfarmer', though this is certainly a flattering mistake!
>>     
>
> oops, sorry 'tgwong'!
> BTW, could you explain the use of the cartogram plugin?
> pc
>