I have one line layer which is made up of multiple segments. I want each segment to be a new line feature so that I can select them individually using select features by area, as in the picture.
It looks from the graphic like you're using QGIS, in which case you have a couple of steps, depending on whether you're looking at a polygon or a polyline.
If your layer is a polygon - start at 1
If your layer is a line - start at 2
- Open Vector > Geometry Tool > Polygons to lines : this will convert the polygon to lines.
- Open the processing toolbox > Explode Lines (you can find it by searching 'explode') : this will split all of your lines at their vertices to give you individual lines so you'll be able to select features by area and just get the one line.