Can anyone suggest a method by which I can produce several user-defined contour lines (from a DEM) within a single shapefile using QGIS/GRASS (2.12.x) and its plugins. Each approach I have tried has only allowed my to create contours in specific intervals, rather than at specific elevations
[GIS] Generate multiple contour lines at specific elevations
contourqgis
Best Answer
You can specify levels using the GRASS module, r.contour.level. It's available in the processing toolbox. You can search for it when the dropdown at the bottom of the Processing Toolbox window is switched from "Simplified interface" to "Advanced interface".
The "List of contour levels" parameter takes a comma separated list of elevation levels.
Here is the documentation for the GRASS command r.contours and the corresponding levels parameter.
Its use for this specific task is mentioned in this question.