To digitize the border of a raster body with Raster Snap
Select Place Element.
Turn on Raster Snap.
Select the appropriate color mask.
Choose Point Mode (or Stream Mode).
Choose Outline in the Entity option menu.
Enter data points approximately on the border to be digitized (in Stream mode, move the cursor approximately along the border line without pressing the data button). Each point snaps on the exact border, where vertices will be defined.
