How to convert a graph / edges list into a shapefile (.shp) ?

How to convert a graph / edges list into a shapefile (.shp) ?

Here an example of graph "G", built up with Matlab:

s = [1 1 2 2 2 3 3 3];
t = [2 3 3 4 5 6 7 5];
x = [0 0 1 0 4 3 1];
y = [0 1 0 4 5 0 -1];
G = graph(s,t);
G.Nodes.X = x'; G.Nodes.Y = y';
plot(G,'XData',G.Nodes.X,'YData',G.Nodes.Y)

Answer 1

[Christine Tobler]

There isn't a direct way to do this with a graph object. The mapping toolbox has a shapewrite function which produces a .shp file, but it will first require putting these vectors into one of its data structures (geolineshape or maplineshape might be the right ones for your case).

[Sim]

ok, thanks a lot @Christine Tobler!

Do you have a Minimum Working Example (MWE) to show me ?

Answer 2

[Sim]

@Christine Tobler Since I did not understand exactly how to perform the steps you suggested...

% Matlab graph --> geolineshape or maplineshape --> shapewrite

...I used a workaround, i.e. I imported both the edges list and the x- and y- nodes coordinates to QGIS (a free and open-source cross-platform desktop geographic information system application that supports viewing, editing, printing, and analysis of geospatial data), from where a shapefile can be created.

Thanks to the GIS stackexchange community, I used the following workaround.

First, go to the Python Console's Editor of QGIS:

% open QGIS --> Plugins --> Python Console --> Show Editor

and then, copy and paste the following code (just change the path where to save your shapefile):

# (1) create a layer in QGIS
s_list = [1, 1, 2, 2, 2, 3, 3, 3]
t_list = [2, 3, 3, 4, 5, 6, 7, 5]
x_list = [0, 0, 1, 0, 4, 3, 1]
y_list = [0, 1, 0, 4, 5, 0, -1]
coords = list(zip(x_list, y_list)) # generate a list of lists containing x and y coordinates of every vertex
layer = QgsVectorLayer('LineString?crs=EPSG:4326', 'Layer', 'memory')
layer.startEditing()
for s, e in zip(s_list, t_list): # loop trow a lists of lists containing the start and end vertex index in the coords list 
    feature = QgsFeature()
    start = QgsPointXY(*coords[s - 1]) # start point 
end =  QgsPointXY(*coords[e - 1]) # end point
feature.setGeometry(QgsGeometry().fromPolylineXY([start, end])) # set the geometry to a QgsFeature object
layer.addFeature(feature) # add feature to the layer

layer.commitChanges()
QgsProject.instance().addMapLayer(layer)

# (2) save the layer as shapefile
QgsVectorFileWriter.writeAsVectorFormat(
layer,
'path/to/your/matlab_graph.shp', # set here the path to your file including the file extension
"utf-8", # file encoding
QgsCoordinateReferenceSystem("EPSG:4326"), # set inside quotes the crs id
"ESRI Shapefile" # set here the vector driver
)

Once the shapefile is saved in your folder, you can just open the shapefile in Matlab:

% read the shapefile
A = shaperead('matlab_graph.shp'); 
% extract the coordinates of the edges
x_tmp = vertcat(A.X);
y_tmp = vertcat(A.Y);
x = x_tmp(:,1:2);
y = y_tmp(:,1:2);
% plot the shapefile
plot(x',y','linewidth',2)
xlim([-1, 5])