To digitize a line with Raster Snap

Select Place Element.
Turn on Raster Snap.
Select the appropriate color mask.
Choose Point Mode (or Stream Mode).
Choose Line in the Entity option menu.
Enter data points approximately on the raster line to be digitized (in Stream mode, move the cursor approximately along the raster line without pressing the data button). Each point snaps on the center of the raster line, where vertices will be defined.
Reset to end the line string.