Clairvoyance / arcane eye do help.
One can see distances of several thousands ft. under certain conditions. However, the complication handling battles of large scales prevents most DMs from letting out these conditions.
Sorry, but your anwser if wrong.
line of sight: Two creatures can see each other if they have line
of sight to each other. To determine line of sight, draw an imaginary
line between your space and the target’s space. If any such line is
clear (not blocked), then you have line of sight to the creature (and it
has line of sight to you). The line is clear if it doesn’t intersect or
even touch squares that block line of sight. If you can’t see the target
(for instance, if you’re blind or the target is invisible), you can’t have
line of sight to it even if you could draw an unblocked line between
your space and the target’s
You probably won't have line of sight to someone under your Scrying or Clairvoyance spell since the line is drawn between
your space and the creature,
not between your spell effect's space and the creature.