[QGIS-Developer] Processing 3.0: Possible change to the Convex Hull algorithm

Neumann, Andreas a.neumann at carto.net
Thu Jun 29 23:36:30 PDT 2017


I also think that the logical solution would be to aggregate the
attributes when grouping and allow the user to specify which aggregate
function to use for each attribute. 

Just randomly using the attributes of the first feature doesn't really
makes sense to me. In 99% of the  

Now that we have aggregate functionality in QGIS - are there technical
reasons for not using these aggregate functions? 

Just a hint, because about 1 year ago we paid Nyall for these aggregate
functions and of course we are interested that these are used more and
more in QGIS ;-) 

BTW: FME from safe software has an aggregate widget/functionality
available whenever something is grouped. QGIS should do the same. I
don't see why not. 


On 2017-06-30 07:47, Matthias Kuhn wrote:

> On 6/29/17 10:21 AM, G. Allegri wrote: 
> 1. Allowing multiple field selection for grouping
> 2. Keeping attributes of first feature when grouping 
> I will accept this criteria, obviously, if it's the preferred solution for the mosts.  
> I just want to report that many users (partecipants to courses or customers) say they find having the "first feature value" misleading.  
> I agree with them. I would set the field values only for the grouping fields, having the same value within the group, and set null for the other fields. 
> The problem raises during a long workflow. At some point you obtain a dataset with unconsisten field values, and it's not always obvious to know when and.which field value was set miningless.

In the long run the most flexible solution will be to allow specifying
the aggregate function applied to each field individually ("first/any",
"mean", "mode", "max"...).

No worries if it's not implemented right now, but please keep the code
modular enough to introduce this easily later down the road. Without
manually editing each algorithm that supports grouping.

A possible approach to this would be a parameter
"ParamterFieldAggregation" that offers a standard gui to configure the
aggregation behavior (fieldA: min; fieldB: mean; fieldC: any; fieldD:
mean(expression("numBirds2017 - numBirds2000")) ). 


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