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SPACESYNTAX  March 2018

SPACESYNTAX March 2018

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Subject:

Re: angular analysis radius

From:

Jorge Gil <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Thu, 22 Mar 2018 08:20:35 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (51 lines)

Dear José,

That is a complex question that could also be placed on the general 
space syntax mailing list, as here we try to address aspects of the QGIS 
toolkit in particular.

Nevertheless, I'll try a simple answer...

Global analysis is always radius N: to everywhere. Therefore its results 
are the same whatever radius type you use, metric, angular or steps.

Metric type radius can be any value, and there are studies that use 
radius from 100m up.
Keep in mind that long segments with more than 2x the radius don't get 
any integration value for that radius because you can't get anywhere 
from them.
The choice of radius must be linked to the research problem and 
hypotesis being tested. If your problem relates to pedestrian movement, 
400m makes sense as it corresponds to 5 minutes walk, and in much 
walkability literature is regarded as a threshold after which the 
propensity to walk drops. And if your main focus is the car, then larger 
radius make sense. But you might be looking at centrality structures 
irrespective of behaviour, then the choice of radius must come from some 
hypothesis.
In many studies you find very large radius (60km) just because N would 
be too slow to calculate in large models.

Steps radius is rarely used in segment analysis. That would be limiting 
the range of centrality based on the number of segments (maybe axial 
lines, not sure). This is the standard in axial maps as your theory 
considers changes of direction. In principle it can be used as long as 
your hypothesis is that centrality matters up to a certain number of steps.

Angular type radius is even more rare, and the same principle apply: you 
must have a theory or hypothesis to use it. It's also more difficult to 
define and understand, as it sets the limit of degrees that you can 
turn. These sorts of radii are very irregular, allowing very long 
distances (many km) along street alignments, and not going far into 
adjacent grids.

Hope this clarifies things a bit, but as I said, try also the main list 
for further insights.

Best regards,
Jorge


On 22/03/2018 06:44, Jos é Rojas wrote:
> Hi all,
> checking the tutorials on the SS Online Platform I saw that examples use metric radius type and I understand that it could be from 400 to N meters but I want to know if there is a recommended radius for the angular analysis and segment step analysis too. From which radio can a global analysis be differentiated from a local analysis?
> I will appreciate any help.

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