All are sources of error except:

$($a$)$ alternate sun and shadow,

$($b$)$ visibility of only the rod's top,

$($c$)$ line of sight passing too high above the ground,

$($d$)$ lines that are too short,

$($e$)$ line of sight passing close to an object like a vehicle, which causes a refracted line of sight, and

$($f$)$ sighting into the sun.