Merging lines shows up as two parts in vertex list

Question

I've merged multiple lines in ArcMap 10.5.1. When I go into the vertex list it shows up as two different parts. Why is this? I can also explode it as a multipart. Is it possible to turn it into "one" feature, not a multipart?

The merged lines vertices:

Answer 1

This is caused either by

• the fact that the 2 part are "diverging" (if you look at coordinate you see that the vertex n°0 of each part are the same). To be able to turn it in one part you need to have feature that follow each other (so the last vertex of first feature is superposed to the first (n°0) vertex of the second feature). (not sure on this one but can't hurt to fix it)
• the fact that the third coordinate of the common vertex is not the same so your two part appear to be contiguous in 2D but are not in 3D.(more likely cause)
• Or both of the two previous

To get what you want you should revert back to single part then invert one of the feature to get all vertex in the right order and fix the Z coordinate of the common vertex. After that you could merge again and should get a singlepart feature.

Answer 2

I get this issue a lot when merging M-enabled lines in ArcGIS Desktop 10.8. I have ensured that the lines are both oriented in a consecutive direction, that the common endpoints are snapped to each other so they have the same coordinates, the common endpoints have consistent Z values and consistent M values. Still, I sometimes get a multi-part result. I have found a couple other things that have worked for me:

• Before merging, ensure you have saved any edits.
• Before making needed edits and merging, if your data frame is in a different coordinate system from the line data you're using, change the CS of the data frame. I'm not entirely sure why this might solve the problem, but it did for me once when nothing else worked.