To convert one raster contour line

Select Convert Contour Lines.
Select the appropriate color mask.
Set the active Z for this contour line (ignore the other parameters).
Turn on Auto Continue.
Identify the raster line to be converted.
If the result is a closed shape, the procedure is complete. If not, 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 contour line as far as you can 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 contour line from the other end as far as you can and then press Reset.
You are switched back to the Convert Contour Lines tool.