Merge Lines
Function Description: The Merge Lines function allows the user to merge two lines connected end to end into one line object.
Steps
1.Click Merge Lines 
button in the Vector Editor, click the two lines that are connected end to end and need to be merged with the left mouse button.
This function can only process two lines with the same point at the beginning and end. For example, the node 1-2 of line 1 and the node 2-3 of line 2 will be merged into a line string with nodes 1-2-3. The details are as follows:
Two lines to be connected
Two lines after connection