To illustrate the technique, let us consider an example:

When you isolate relief contours into a black-and-white thematic layer, pixels of combinative colors are unwittingly captured together with them at crossings of relief lines and rivers. Breaks of contours at these points are "untidy" and "shaggy" hindering the tracer from unequivocal selection of line continuation and correct tying of ruptured lines together.

The following technique accelerates vectorizing of relief lines:

1. Vectorize lines of rivers first;
2. Select the rivers with the Group Editor and increase the width of these vector lines up to 3-5 pixels;
3. "Imprint" the Rivers vector layer into the raster layer (image) of relief contours. See Utilities -> Rasterize Vector Data (select black color for filling!);

"Dirty" line ends will be neatly cut off and semiautomatic vectorizing or gap removing after automatic vectorizing will be much quicker and easier.

Raster data may be deleted inside polygons in the same way, for example:

1. Vectorize borders of canyons to a polygonal layer;
2. Select these lines with the Group Editor and increase their width up to 3-5 raster pixels;
3. "Imprint" the Canyons vector layer into the raster layer (image) of relief contours with the Render as Filled Polygons option on.

A lot of short line scraps will not be generated at automatic tracing and thus joining of line ends will be simplified.

You will speed up "tying together" of ruptured lines if cut off their curved ends first with the "scissors" command of the Editor. At that, if the CTRL key is pressed, the shorter line part will be deleted.

The Shares tool of the Group Editor may be also useful sometimes. It deletes line fragments inside an arbitrary polygonal area. You may specify area manually or select any closed polyline as its boundary.

Repeat gap removing after cleaning of "problematic points" with a greater value of the Search Continuation option (about 30 - 50 px).

Repeated joining of line ends should be done PREVIOUS TO line form optimization (Line Forming option)!