I'm working on QGIS, How do I intersect a DEM and a polyline to get the longitudinal section of a river?
This is what the 'profile from line' plug-in is for :)
Try the Profile from Line plugin. Make a line shapefile of the "longitudinal section" then run the plugin with your DEM and the line shape loaded. (Be sure to select the DEM raster in the plugin window). It creates a point layer with the DEM value as an attribute column.