I would like to get a better understanding of how to assign costs for turns when creating a least-cost path with GRASS network tools, basically because I don't want any line to be used twice for creating the path.

For example with v.net.path, there exists the option to assign turning costs to each line with arc_backward_column and turn_layer.

Question 1: What's the difference between those two?

Then, I question whether any of them would be helpful, because in the wiki it is mentioned that 'Vector directions are defined by the digitizing direction (a-->--b).' However, I do not want to define a direction in according to how lines are digitized (my lines are not), but in terms of the direction that the network graph has already used that line.

Question 2: What is used as direction if the layer is not digitized? Is there a way to consider the direction to the one that the network graph has already used that line for generating a least-cost path?

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