To convert one raster line

Select Convert Lines.
Select the appropriate color mask.
Choose Line in the Entity option menu.
Turn on or off Auto Continue.
Identify the raster line to be converted by entering a data point along it or at a free end.
The resulting vector element follows the center of the raster line that is converted, and it is now constrained between two break points.

If Auto Continue is off, the procedure is complete.

If Auto Continue is on, the Continue Element tool is automatically selected and a rubber band is drawn from one end of the resulting vector element. Continue with the next steps.
Use the Continue Element tool to continue converting the line as far as you wish and then press Reset.
You are switched to the other end and a rubber band is drawn from there.
Use the Continue Element tool to continue converting the line from the other end as far as you wish and then press Reset.
You are switched back to the Convert Lines tool.