I have a closed contour line and want to edit it by removing some branches (valleys). Deleting single coordinates is easy. For larger branches with many coordinates I haven't found a method to delete many coordinates at once (to keep the closed line).

With the other method of cutting of branches (Delete Coordinate - Split Branch) on both sides of the branch I have the problem that I cannot manage to close the line again: I snap both end coordinates of the line with Snap to Lines, apply Join Lines, but when I go back into Edit mode and move one of the coordinates that are on top of each other they are still not connected. It seems I need more than one line for Join Lines. How to close the line? (And I don't want to create areas from the polygon!)

Could you post a screenshot or two to help explain what you mean?

In the second part (paragraph 2), are you by any chance editing the line drawing in a map window, with a compressed image underneath? I find that snapping is often ineffective in this context, and I believe it's a common problem (an old bug that either hasn't been fixed, or has recently come back). The same operations work as expected if performed in the drawing's own window.

tjhb: I just want to split a line at two points, remove the branch in between and join the line again.

Indeed I was editing the drawing in a map window with a lot of other layers - including images - underneath. In the meantime I have drawn a simple line in a drawing, snapped the ends and Join Lines worked fine. Back to my original project I was more sucessful now in joining the lines. The crucial point seems indeed to be the snapping, and editing in a drawing rather than in a map layer is better too. I still sometimes fail - lines not joined - and have to improve my workflow with careful snapping.

We're definitely seeing the same glitch in the second part then.

For your objective, here's what I think is an efficient way of doing it.

You start with a contour line (L1), which includes a side valley that you want to chop off, while maintaining the continuity of the contour.

In the drawing's own window:

1. Turn on snap to lines, and draw a new line (L2) across the mouth of the valley. (A chord between two coordinates of L1.)

2. Select L1 for editing.

3. Press S, or the Split button on the editing toolbar.

4. Click on L1, then on L2. (This splits L1 into branches at its intersections with L2.)

5. With L1 selected, use the transform [Selection] Decompose.

6. Select and delete the line that describes the valley.

7. Select the remainder of L1, together with L2, and use the transform [Selection] Join lines.

Your workflow is fine with using a line to split off the branches and then Decompose (I haven't used this before.)

Still sometimes after Join Lines lines were not joined at one point and I had to repeat. As I finished the job now I didn't check the problem closely but I had the impression that JoinLines worked better when the line was in Edit mode.

Join lines is probably subject to the precision setting of your drawing, so if the gap is greater than the precision setting, join lines probably won't work. You can modify precision to something greater than the gap (measure it with the tracker tool to get a reasonable value) before running join lines. Afterwards, return precision to the default you are happy with overall.