[SoC] GRASS - Network Analysis, 1st week report

[Markus Neteler]

On Fri, May 15, 2009 at 10:15 PM, Daniel Bundala <bundala at gmail.com> wrote:
> Hello list,
>
> I have agreed with my mentor, Wolf, to start working on my GSoC
> project bit earlier so that I can concentrate more on the exams later.
> Well, earlier means last Saturday and so this is my first week report.
> Just a short introduction: I am Daniel, 3rd year at Oxford doing
> Mathematics and Computer Science and some of you may remember me as
> the student working on GRASS v.generalize module two years ago. My
> project this year is about extending GRASS network/vector
> functionality by writing a couple of modules. I have created a wiki
> page: http://grass.osgeo.org/wiki/GSoC_Network_Analysis, which
> contains some useful information such as this email and a link to the
> abstract.
>
> Anyway, after doing boring, but essential, tasks such as downloading
> and compiling GRASS (I use 6.4 developer branch) I have done more
> interesting stuff. I wrote a module for computing weakly and strongly
> connected components. Using this module, I found errors in the
> standard North Caroline datset. At least, I think that these are
> errors. For example, if you zoom to [north: 232051.61663403  south:
> 230521.9965903  east: 640369.10332306  west: 638669.65919416] and let
> GRASS draw roadsmajor map for you, you get something like this:
> http://people.ksp.sk/~dano/grass/wcc1.jpg. (colours correspond to
> different components, which I obtained using my module). Or, the same
> map, but different location [north: 239329.55684214  south:
> 238882.80682937  east: 630466.77182694  west: 629997.32746966], and
> you have another disconnected lines:
> http://people.ksp.sk/~dano/grass/wcc2.jpg
>
> Then, I have also written a module for computing bridges(edges whose
> removal disconnects the graph) in the network and found another,
> probably error. This time, it is in map "railroads". The location is
> [north: 195851.98992388  south: 187604.09950596  east: 669691.16599023
>  west: 663129.724274] and the picture is:
> http://people.ksp.sk/~dano/grass/railroads.jpg. Note that the lines
> are not connected at the point the red arrow points at. Also, it
> seems, but I am not completely sure, that there is something wrong
> with the red segment the green arrow points at. I think, that the
> segment is not topologically connected to the network at both ends,
> although, visually, it seems that it is.

That's great - this tool will help a lot.

Concerning the North Caroline dataset (I am coauthor [1])
just the note that this are official governmental data...!

Perhaps we can fix them in a future release of this dataset package.

[1] http://www.grassbook.org/data_menu3rd.php  (NC in GRASS format)
     http://grass.osgeo.org/download/data.php (NC Subsets and in
common GIS formats)


> Finally, here is a random picture from a module for computing bridges:
> http://people.ksp.sk/~dano/grass/bridges.jpg. (The data is from
> streets_wake map). The red edges are bridges. Strictly speaking, the
> blue edges are bridges as well, but they are not the type of bridge
> you usually look for/consider important. So I wrote another module
> that can identify such chains hanging on the network and remove them.
>
> The code should appear shortly in GRASS addons repository.
>
> That is for the last week. I am not currently stuck at anything and I
> plan to develop another modules over the next week. For exampe, module
> for computing articulation points as the algorithm is almost the same
> as for bridges.

Excellent - please keep up the good work!

Best,
Markus