In article , Matt Whiting wrote:
Dead reckoning is an incredibly important complement to pilotage, and
it's how my in-built (i.e. in-brain) "GPS" gets much better accuracy.
Keep track of time since the last major waypoint or landmark, and it
stops you mis-identifying one ground feature for another, or one airport
for another. It forms a very important cross check when I'm doing
radioless navigation.

Why do you need dead/ded reckoning when you can see the ground?

As I said - dead reckoning (it's dead reckoning by the way, NOT ded
reckoning - deduced reckoning would be a tautology [0]) is used to
form a cross check. Cross checks are always useful, especially over
unfamiliar terrain or an area where several ground features (towns,
lakes etc.) all look very similar.

Dead reckoning does not necessarily mean going to the effort of
calculating precise time/distance calculations with your E6B, it can be
a simple estimation (and that makes the vast majority of my dead
reckoning calculations - either estimating when I'll pass a certain
feature, or cross checking that the ground feature I'm currently over is
what I think it is. In unfamiliar terrain, I like to have three things
confirming my position - the feature looks like it should on the map,
the current time tells me I should be in the vicinity of the feature
(i.e dead reckoning), and another feature I can see on the map appears
where I expect it to be looking out of the window.

As a consequence, I can say with all honesty I have NEVER been lost when
performing pure VFR navigation since being a student pilot. Keeping
track of time (i.e. the dead reckoning part) is how I've turned being
unsure of my position to positive of my position on several occasions.
It's not as if I've only done short cross countries - I've flown coast
to coast in the United States. I've flown a light plane (mostly my old
C140) in 26 states.

ATC (at least here) will occasionally ask you for an estimate, too. If
you've been keeping track of time all along and doing dead reckoning all
along you don't have to tell the controller 'standby' whilst you work it
out because you already know the number he wants.

[0] http://www.absoluteastronomy.com/enc..._reckoning.htm
--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"