To digitize a new line segment parallel to another line, what is the correct first step after marking the first point?

To digitize a new line segment that is parallel to an existing line, the correct first step after marking the first point is to first activate the Constrain Parallel tool. This step establishes the intent to create a line segment that maintains parallelism with another existing line. After the tool is activated, you would then subsequently select the existing line feature to which your new line will align parallelly.

The reasoning behind this method lies in the functionality of GIS software, where activating a constraint tool like Constrain Parallel before selecting the reference line ensures that the software correctly interprets the geometric relationship you are establishing. Once the parallel constraint is active, any subsequent clicks to define the next vertices of the new segment will correctly follow the parallel alignment.

Following this method allows for a seamless workflow while digitizing, ensuring accuracy and consistency in the spatial relationships of the data being worked on.