JD> Wouldn't it be more interesting to present some evidence on issues
JD> of reliability with axial maps instead of debating it in the
JD> abstract?
GS> ... I think they are [etc.]
I think Jake's point entirely. You *think* they are. I appreciate it's
a valid point to think about the edges rather than the vertices in the
axial graph, but Jake's call here is for a bit of good old fashioned
scientific rigour.
JD> The automated spatial sampling technique provided by VGA resolves
JD> this issue and makes the whole analysis methodology a lot more open
JD> to scrutiny.
GS> Can you explain that, Jake? I don't see well how VGA can deal with
GS> topological distance
The automated spatial sampling *does* make VGA a lot more open to
scrutiny, although it itself is not without it's problems. Ignoring the
isovist field argument which follows in Guido's response (VGA is about
visibility connections in a system, there is no measurement of the
isovist), the key problems with VGA are:
1) resolution --- how does resolution affect the analytic values. This
simply needs a study to resolve the issue (bad pun, I know).
2) topological distance --- Guido is correct, VGA integration is
weighted by large open areas. There are a number of ways around this.
I am looking at the possibility of using entropy or frequency
distribution measures (i.e., consigning the actual area to a secondary
role), Maria Doxa is working on methods which consider how to retrieve
topology directly from VGA, and there are other avenues to be explored,
e.g., area normalised VGA.
3) boundary --- different from resolution, and encompassing the various
levels of visibility graph --- the true "visibility graph" versus the
"permeability graph", also the social aspect of what is important to the
user of the space. My thinking is that there is some work to do on the
fractal dimension of the boundary, and I hope, finding that there are
properties of the visibility graph which scale predictably with the
fractal dimension.
4) 3d --- at the moment, it's entirely 2d. How do we cope with
buildings with stairs (Sheep pointed out that the staircase can
sometimes be considered as a single entity), or even, how do we cope
with inclines and declines. I know Jake has some interesting ideas on
3d VGA.
Alasdair
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