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## Re: [igraph] Building a graph from a picture

 From: Sadz A Subject: Re: [igraph] Building a graph from a picture Date: Sun, 5 Dec 2010 00:17:50 +0000 (GMT)

Thank you so much for your help, a member on R-sig suggested I look up an extension called SANTAM that seems to be what I am looking for.

Again thank you for all your help.

Kind regards

To: Help for igraph users <address@hidden>
Sent: Fri, 3 December, 2010 23:29:53
Subject: Re: [igraph] Building a graph from a picture

It is clear what you want now.

The crucial bit -- converting raster data to network representations
-- is a spatial task and therefore more appropriate for the
r-sig-spatial list.

I donĀ“t think this operation is trivial. You might try gdistance
(R-Forge) to do this, working with rasters only. However, this will
work with small rasters only.

I am not aware of any package that assigns nodes to road segments
further.

Jacob.

> I'm not sure what you mean by 'how' I want to convert the shapefile to a
> graph:
> The data I have is a road map that I digitised from a satellite image in
> ArcGIS, if I convert it to an ascii that R can read I suppose it would be a
> grid of no-value pixels, with the roads being represented as value pixels.
> I would like nodes to be the points at which two lines cross or a road turns
> at a right angle, and, I would like the edges to simply be how the various
> nodes are connected, they do not need to be directional or contain
> information on flow etc. I would also like to get a matrix out that shows
> how the network is build, for example point a and b are connected by edge 1,
> and point a and c are connected by edge 2.
> I'm very new to network analyses, but I think what I would like to then do
> is compare several of my digitised road maps (after converting them to
> matrices and graphs) by calculating connectivity, network stability etc.
> Thank you
> ps- I will try the R sig spatial list, but I though igraph was used to carry
> out network analysis and draw graphs of networks.
>
> ________________________________
> From: Jacob van Etten <address@hidden>
> To: Help for igraph users <address@hidden>
> Sent: Fri, 3 December, 2010 4:46:59
> Subject: Re: [igraph] Building a graph from a picture
>
> Well, this depends on HOW you want to convert the shapefile to a graph
> (what should the edges and vertices represent?).
>
> I think it would be better to ask this question on the r-sig-spatial
> list, specifying a bit more what type of data you have and what you
> want to do.
>
>> I have some networks in GIS shape file (vector) format (I can change them
>> to
>> raster or Ascii), is there a way to convert them into matrix or graph that
>> can be loaded and analysed with igraph in R?
>> Any help is really appreciated
>> Thank you
>>
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>>
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