Its funny isn't it? I wonder why people have trouble with this one.
The RA equation puts mean depth onto a 1-0 scale between the deepest and the
shallowest you could possibly have given that number of nodes in the graph.
This is a normalisation.
The RRA equation then relativisies this as compared to the mean depth of a
diamond shaped structure with the given number of nodes. This is an
empirical relativisation ie. not particularly 'theory driven' in that it
works statistically in removing the effects of number of nodes in urban
axial graphs from the average mean depth in a system. There are many other
ways that this could be done, and since there is no pre-existing theoretical
assumption built into this process nothing is lost by ding it a different
way. However, it does allow you to compare some properties of graphs between
maps of different sizes on a more or less comparable basis. Something you
certainly cannot do for the unrelativised RA values.
Alan
>
> On 03/03/06, Bin Jiang <[log in to unmask]> wrote:
> > Usually we compare space syntax measures within a same system, not
> > across different systems. This is my perception. Am I wrong?
>
> Well, if you use local integration for a single axial map you *are*
> comparing different systems because the number of nodes involved to
> calculate this measure vary for each node. It is the same if you
> compare global integration between different maps.
>
> Therefore, if you do not agree that RRA / Diamond Shapes provides some
> help... forget the whole thing about local integration... just does
> not work.
>
> For me it is quite OK.
>
> > I am not convinced by the popular saying that the local integration is a
> > good indicator of pedestrian or vehicle flows. Recently I happened to
> > get some vehicle observation datasets with pressure-sensed techniques
> > (so must be very precise observation). I compared the datasets with
> > local integration, and did not end up with a good correlation (R square
> > value about 0.5).
>
> I have got the same in this paper:
> "Continuity lines: aggregating axial lines to predict vehicular
> movement patterns"
> http://www.mindwalk.com.br/papers/
>
> That is a problem. I produced a continuity map that reveals clearly
> the main street system of my city (Recife, Brazil). But ... correlate
> abstract graph properties with real movement is another issue. There
> are many other factors, such as attractors, street width, etc.
>
> Therefore, this is just a matter what is the number you accept as a
> good proof that the urban grid itself (ignoring the other factors) can
> organise movement patterns.
>
> Regards!
> Lucas Figueiredo
>
> CASA - Centre for Advanced Spatial Analysis
> University College London
> 1-19 Torrington Place
> London WC1E 7HB England
> E-mail: [log in to unmask]
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