Impact models query


Hi Hrishi,
We noticed that your last update consists of some major changes. Is there any documentation about this update available? Especially, we are wondering whether the decision models are still only for visual purposes or are they included in the processing of change and impact models.


Hi Fred,
At the moment Decision models are only for visual purposes, changing the impact calculations based on Decision models is something that I am working on, but it is not done yet. There is a visual Decision Model plugin:
I think that you know about this, it can be added to the project., but again it is visual not to do with impact.

Your idea is to change the impacts based on the decision model correct?


Yes, this is correct.


Can you remind me what your thoughts are about this? i.e. how do you want to change them? Systems that have higher priority have different impacts?


That depends. Because I don’t know how you compute the impact models exactly this is very difficult to tell. I think we cannot just apply a simple weighted overlay analysis. Such analysis are usually applied for a suitability analysis, which would correspond to our evaluation models rather than to impact models.

For example, the impact model that was created by a developer might already have a minus score because he placed too much new land to be developed in areas where such a development is not feasible. If you applied a simple weighted overlay analysis this would result in even a higher minus score for this model although the developer might be willing to accept some negative impacts in order to achieve his plans.

On the other side, scoring of impact models are mainly based on the evaluation models. As our internal discussions have shown let’s say a developer of “low density residential” areas might have a very different understanding of the definitions of feasible, suitable, capable, etc. compared to an environmentalist. This means, a decision model might cause a shift in the assigned classes of an evaluation system. However, this would be very different to implement because the developer would not want to change the evaluations of all criteria. A parcel is either vacant or not. A change of this criteria does not make sense. In the case of the criteria “in critical area” this might be different.


Hello Fred,
Thank you for this, I have been thinking about this a bit. A couple of points:

  • The evaluation maps are made in a ordinal scale. The administrators of the project decide on the evaluation maps and how they are made. By building a evaluation map, they are building a map of “their opinion” of the state of things for that system.
  • When we make evaluation maps, we ask the administrators to be as objective as possible it works quite nicely: A criterion could be any land that is vacant and has less than 5% slope is suitable for low density housing. This means that the evaluation maps identifies flat vacant lands whereyou can build low density housing, whether to build there or not is a decision made by the design teams.
  • Impacts: We ask the administrator to build simple impacts what happens to agriculture when you build low density housing, is it “good” or “bad” for it. Impact assessement is crude by design, we encourage you to take data out of Geodesignhub and do further detailed analysis. When you assign land for low density housing, it is “bad” impact for agriculture because you are taking away potentially productive agricultural land.
  • Impacts based in Decision models: When you set decision models, you are specifying the systems that are important to you, e.g. a property developer will say “low density and high density housing” are important to me not so much “ecology” , so he does not care if the impact of his projects on ecology is bad, all he wants to do is to ensure that he is putting the projects at the appropriate locations (per the evaluation map) for housing systems as he cares about them more.

Lets discuss this over a Skype call, I am happy to build a plugin to do decision model based impacts, however we need to clarify how it looks like.


Hi Hrishi,
We are working again a lot on GeodesignHub in order to reach the deadline for the IGC project. As we have to upcoming workshops ahead I was wondering whether you made any developments on this subject.




Hi @f_ernst, thank you for your email, I just replied back. We have a dedicated IGC thread for Q&A here: IGC Questions & Answers as well.