I have opened a geotiff file with QGIS. The CRS of this file is EPSG:3857 (web mercator). When I use the distance tool in the menu, the computed distance is towards web mercator coordinates and not the "real" distance on the ground. I have found the GeoServer extension to compute the orthodromic distance. It works but it's not really easy to use. Is there a way to change QGIS parameters or anything else that allows to get orthodromic distance with the native tool?
Go Settings -> Project Properties, in CRS tab enable on the fly transformation; in General tab in Measure tool menu choose ellipsoid for distance calculations. See picture:
When you start measurement move cursor to the measurement window: the information about your measurement settings will pop up:
NOTE: ALWAYS check measurement settings in the measure window (like in the picture above) because ellipsoid settings are not always saved (or displayed in Measure tool settings like at the first picture) correctly.