Hi
Yes, to be pedantic, when you examine the setup using a Viewer node, the edges do point in the (-1,0,0) and (0,-1,0) directions… If you would like a setup where the grid can be any size, it would be better to normalize the edge vertices and test against those coordinates so you can keep the epsilon small… (testing against (-1,0,0) here for Y lines, but you could test against (0,-1,0) for X lines.), like so:
For these settings:
… for small tweak to Higgsas’ setup:
What the Viewer node sees, somewhere in the spreadsheet:
Doing some arbitrary height mapping with this setup:
Height lines example.blend (94.3 KB)
Hope that makes more sense.
Good luck.