Highlight all tiles with pipes on them and in the current "pipe layer"
When placing pipes it would place the next obvious pipe.
If the mapper connects a straight horizontal and vertical pipe path then it should make the intersection a + pipe.
The mapper should also be able to place a pipe on an "elevated layer", which simply would mean that all other pipes shouldn't be considered except pipes on this layer.
A sanity check, unless this snap to mode is a toggle, is to ensure it only replaces pipe groups that are same name, so you don't mix scrubber and air supply, for example.
Pipe Placement Mode
If the mapper connects a straight horizontal and vertical pipe path then it should make the intersection a + pipe.
The mapper should also be able to place a pipe on an "elevated layer", which simply would mean that all other pipes shouldn't be considered except pipes on this layer.
The layer is just an abstraction.