Lucas is correct. The issue is associated with the difference between a
point and a line based representation. Look for example at the difference
between Axman and Depthmap (in VGA mode). In Axman I stand on an axial line
- say Oxford St at Oxford circus - and wherever I walk on that axial line I
am on a single node in the graph. I walk east and turn the corner onto
Tottenham Ct Rd. I am now on another axial line and I am one step away in
the graph. I walk along the length of TCR and I am always one step away
until I turn onto Torrington Place. Now I am two steps away. I walk along
until I reach Huntley Place. Note that I have now been on three lines (and
so 3 nodes in the graph) but have only been through two changes of direction
(links in the graph or points of intersection between lines). Radius in
Axman refers to the number of nodes involved (ie Torrington Place is within
radius 3 of TCR) rather than the number of links.
Now imagine that I am doing this on a visibility graph. I start at Oxford
Circus on a point which IS a node in the graph. I can see all the way to St
Giles (corner of TCR). This is another point in the Visibility Graph - ie
another node. The relation of visibility between the two is the link. Thus I
have walked along a link in the graph when I reach St Giles. When there I
can see all the way to the corner of Torrington Place - to another point
(node). And so on. Now if we standardise on nodes being the radius, three
nodes (counting the origin) gets me to the corner of Torrington Place but
not along the street to Huntley Place. What is radius three in Axman is (by
convention) radius 2 in Depthmap. This is just a consequence of the move
from a line to a point based representation. The tricky thing here is that
since Alasdair is using the same code to calculate both visibility graphs
and axial graphs in Depthmap, axial radius in depthmap is different to in
axman.
I expect that has made it as clear mud! :-)
Happy Christmas and a merry new year to everyone...
Alan
> -----Original Message-----
> From: [log in to unmask] [mailto:[log in to unmask]] On
> Behalf Of Lucas Figueiredo
> Sent: 19 December 2005 13:38
> To: [log in to unmask]
> Subject: Re: RA or RRA
>
> Hello Sheep.
>
> No one is counting from 0. A single topological step means depth 1.
> There is no topological step from a given line to itself.
>
> I know that arrays are confusing in some old programming languages (I
> also programmed in ANSI C and Pascal) and sometimes we are obliged to
> change outputs because such limitations of data structure.
>
> However, I read lots of papers using R3 and I always understood this
> radius as 3 steps away, not 2. I also think that in my MSc
> dissertation wrote that R3 (3 steps) is the local standard radius
> (which is wrong).
>
> These things (interpretations, implementations) must be clear.
>
> That is why I reinforce that academic software must be properly
> published AND cited - because it is part of the methodology you use in
> experiments. I think we must put it in our software licence "cite it,
> otherwise do not use it".
>
> Regards!
> Lucas
>
>
>
> On 19/12/05, Nick Dalton <[log in to unmask]> wrote:
> > Hi the code in axman was designed to be output compatible with the
> > fortran code. as a 'C' programmer I found it an odd switch ( and
> > still do).
> >
> > People typically count from one (1,2,3,4) it's only C programmers (
> > and their children) that count from 0. Fortran and Pascal (object
> > pascal being the language of Axman) use 1 based indexs for arrays and
> > so number systems. Everything also had to be compatible with the
> > output of the social logic of space ( with the D value).
> >
> > Zero depth makes sense to me but non programmers got there first.
> >
> > so R2 = r3, r3=r4 and r2=r1, which can be said to eliminate a problem (
> no r1).
> >
> > Using R2 (ie old R3 )You may noticed more glitches in radius 2 due to
> > poor micro-structure.
> >
> > sheep
> >
> > >Dear Lucas,
> > >
> > >Yes, this is the same in Depthmap: R2 is the equivalent of R3 in axman.
> > >
> > >I have always said that this makes sense: two steps away is to my
> > >mind R2, not R3.
> > >
> > >As for handling low numbers of lines within (Depthmap's) radius two,
> > >undefined values (nulls) are given in the (small number of) cases
> > >where there are too few lines to calculate RRA.
> > >
> > >Alasdair
> > >
> >
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