All true, however sonar has shown me things that I would never have found with a marker i.e drop offs that are actually undercut, like a catfish highway on Swan Lake Bluebell. Also my mate taking me straight to an undercut bank in the old river bed at Pitsford Res, Perch hotspot, you couldn't find that by any other means. Sonar also shows bottom hardness, mine does anyway, I'm not saying you can rely on it soley but with a combination of manual work and tech it can be useful on occasion.