Filling gaps in vector contour line

Question

I have downloaded bathymetric data but the submarine contours - at 10m intervals - have gaps. Many gaps - the -10m (submarine) contour has over 700 gaps. So I need a programmatic way of either dealing with the gaps i.e. connecting the neighbouring endpoints and thus filling them in; or a way of highlighting the end points of each part so that I can find them and go to them. But I would assume that if I can identify the endpoints of each part, then it should be possible to programmatically connect the two endpoints that are closest together. Unfortunately I do not have the original tif file that the contours came from, nor do I know whence it came. The area I am looking at is offshore SW Ireland to a depth of about 150 m.

EDIT

Here is the image of the MAPZEN of SW Ireland exported into QGIS - note the high numbers for max and min.

Here is an image of an example of the original gapped contour problem

And this image shows what I get when I drag Mapzen into the layers panel. Not much, unless I switch to hillshade.

Answer 1

Probably easier, faster and less error prone than converting lines full of gaps is to recreate the lines from a Digital Elevation Model (DEM). In newer QGIS versions, you have pre-installed Mapzen Global Terrain DEM raster connection under XYZ tiles. This includes submarine terrain/elevation (bathymetry) as well, see:

You can create contours as layer styling, without even downloading anything. Like this, you create contours for visualization only in a few seconds:

If you want to have your contours as separate vector layer, you can download the extent of the data you need from the Mapzen Global Terrain raster layer (right click layer > export) and then locally convert to contours or contour polygons.

Downloading Mapzen Global Terrain to a local GeoTIFF raster file:

Locally downloaded extract of the Mapzen Global Terrain dataset as raster layer and contoour polygons from this raster with one polygon selected (highlighted in yellow):

Answer 2

You can

1. Create start and end points with geometry by expression and the expressions start_point($geometry) and end_point($geometry)
2. Then connect these with Shortest line between features with a Maximum distance of just above your gap distance.

I dont know what your end goal is but you can merge the output with your contours, then dissolve by depth/height field.