linemapinfonodessnapping
I want to create a polyline in MapInfo Pro by using the snap to nodes function, which I can do.
But what I am interested to know is that when a node is added to the polyline, could any of the underlying data attributes/fields of the node that was snapped to (eg ID) be captured into the new Mapinfo table that is being created?
You work here with a Graph (Graph Theory) and your confluence point is simply a node with a degree (number of edges adjacent to that node) > 2.
As you mentioned a Python tag, a solution in Python with Fiona (for reading the shapefile) and NetworkX (Graph Theory)
The Polylines
Open the shapefile and convert all the segments of the Polylines in egdes of a Graph
import fiona
import networkx as nx
import itertools # for extracting all the line segments
# creation of an undirected Graph
G = nx.Graph()
for line in fiona.open("lines.shp"):
# convert the line segments to Graph edges
for seg_start, seg_end in itertools.izip(line['geometry']['coordinates'],line['geometry']['coordinates'][1:]):
G.add_edge(seg_start, seg_end)
The Graph with Nodes(x) and edges
Now, in NetworkX, the degrees of a node are obtained by degree (I use Shapely here but you can save the resulting shapefiles with Fiona and even Networkx)
from shapely.geometry import Point, LineString
for node in G.nodes_iter():
if G.degree(node) > 2:
print Point(node) #the nodes
for edge in G.edges(node):
print LineString(edge) # the edges (>2)
Result
You can translate easily the Fiona solution in PyQGIS.
Have a look at the Disaggregate to Lines option in the Disaggregate dropdown in the Edit group on the SPATIAL tab.
That is a feature of MapCAD. Not sure exactly when it was added.
Best Answer
There is no in-built way of doing this AFAIK, however you could use MapBasic to develop a custom drawing tool that would provide this functionality.
It should be noted that this would not be a foolproof method of picking up the actual snapped-to node in the case of having multiple selectable background layers. Unfortunately there is no functionality to return the snapped-to layer or object, you would have to search the locality (within a small tolerance) of the click location for any underlying objects. Where multiple objects are returned you would have to find a way to identifying which one you wanted to use for the attribute information.
If you do want to do this, a good start would be to look up
ToolButton,CommandInfo,SearchPointandSearchInfoin the MapBasic User Guide.