incorrect shortest paths solutions when impendance values are particularly small or large

[bonza2]

Description of the bug

I am encountering incorrect shortest path solutions when using the Network.shortest_path (and Network.shortest_paths) method on a network with impedance values of small or large (though still reasonable) orders of magnitude. I have included a very simple toy example below that captures the problem. Essentially, when the impedance values are scaled to a sufficiently small order of magnitude (by experiment seems to be ~1e-4) or large order of magnitude (by experiment seems to be ~1e7), the shortest path solution is no longer correct (and, incidentally, appears to revert to a solution that traverses the least number of nodes, rather than minimizing total impedance, though my example doesn't explore this aspect of the problem).

Environment

• Operating system: Windows 10 Enterprise

• Python version: 3.9.10

• Pandana version: 0.6.1

Paste the code that reproduces the issue here:

import numpy as np
import pandana as pdna
import pandas as pd


nodes = [1, 2, 3, 4, 5]
nodes = pd.DataFrame(
    np.random.uniform(size=(5, 2)), columns=["x", "y"], index=nodes
)

edges = pd.DataFrame(
    [
        [1, 2, 0.001],
        [2, 3, 0.001],
        [3, 4, 0.001],
        [1, 5, 0.002],
        [5, 4, 0.002],
    ],
    columns=["from_node", "to_node", "weight"]
)

net = pdna.Network(nodes["x"], nodes["y"], edges["from_node"], edges["to_node"], edges[["weight"]], twoway=False)

net.shortest_path(1, 4)

>>> array([1, 2, 3, 4], dtype=int64)

The above example produces the expected solution.

Now, use exactly the same nodes and edges, but scale the impedance values down by a factor of 10:

net = pdna.Network(nodes["x"], nodes["y"], edges["from_node"], edges["to_node"], edges[["weight"]]  / 10, twoway=False)

net.shortest_path(1, 4)

>>> array([1, 5, 4], dtype=int64)

In the second example, clearly the more expensive and incorrect route has been chosen. The same behavior is observed at the larger end of impedance values also: for example, using the above example, when the impedance values are 1e7 instead .0001, and 2e7 instead of .0002, the same behavior occurs. I'm at a loss as to why this is happening?

Thank you for your help!

[stefancoe]

I am experiencing the same issue. The returned distance between two nodes of interest is about a mile shorter than the Euclidian distance. NetworkX is able to return a distance greater than the Euclidean distance using the same network. Happy to post code and data if that helps.