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date! The International Date Line is
where a change of date is officially
made, being mainly the 180°
meridian which bends to
accommodate certain islands in the
South Sea and parts of Siberia.
Canadian time zones are:
Zone Convert (UTC-)
Newfoundland 3.5
Atlantic 4
Eastern 5
Central 6
Mountain 7
Pacific 8
They don't necessarily coincide with
the correct longitude lines, but with
province boundaries, for
convenience (some places in BC, like
Fort Nelson, use Alberta time).
Deviation
We saw above that the magnetism
from the Earth will vary the
direction displayed by a compass.
The aircraft's magnetism, created
from large amounts of metal mixed
with electric currents, will do the
same thing, called deviation, which is
applied to the magnetic heading to
get Compass North.
The phrase here is Deviation West,
Compass Best, Deviation East, Compass
Least. This means that if the
deviation is to the left of the
magnetic North, the difference
should be added to the course to get
the correct magnetic heading.
Deviations will be displayed on a
small correction card next to the
compass, and are obtained after a
compass swing, a complex procedure
normally done by an engineer. There
will be an area on every aerodrome
well away from buildings, etc. set
aside for this purpose. Allowing for
deviation is called compensation.
Maps & Charts
The words map and chart are
nowadays used interchangeably but,
officially, a chart will show parallels
and meridians with minimum
topographical features, and be used
for plotting. A map will show greater
detail of the Earth's surface.
The point about them both is that
their representation of the Earth's
surface is only accurate within a
relatively small area, since you are
trying to show a 3 dimensional
object on a 2 dimensional surface.
The further from the centre of projection
you go, the more the distortion is
but, to all intents and purposes, it
can mostly be ignored. You can see
the problem if you flatten a globe:
There are many ways of
compromising for this, and each
suits a different purpose, so lines
drawn on maps based on different
projections will not cross through
the same places.
The quality of orthomorphism, that all
charts should strive for, means the
scale must be correct on all
directions within a very small area.
144 Canadian Private Pilot Studies
In addition, parallels must always
cross meridians at right angles.
Otherwise, no chart is perfect, as
you will find when you fold them:
Lambert's Conformal
Imagine the Earth with a light
shining at the centre, then place a
cone on the top. Where the cone
meets the earth, the shadows of the
land formations will be accurate, but
will be out of shape the further
North and South you go.
This is the conic projection, the basis of
the Lambert Conformal, and is what
most of the charts used today are
based on, as the meridians will be
straight, even if they converge
towards the North:
Great circles are assumed to be
straight lines (actually they are very
shallow curves), and rhumb lines will
be curves concave to the nearer pole.
Johannes Lambert overcame the
problem of scale expansion in the
18th century by pushing the
imaginary cone further into the
Earth's surface, so it cuts in two
places:
This gives it two Standard Parallels, or
points where scale is correctly
shown. To be sure, there is a slight
contraction between them, but this is
considered insignificant (1% or less)
if two-thirds of the chart are
between the Parallels.
Mercator
The Mercator projection does things
differently. Instead of a cone, the
Earth is surrounded with a vertical
cylinder, touching at the Equator.
Meridians now do not converge, so
rhumb lines will be accurate, but
distance between latitude lines
increases away from the centre (not
significant below about 300 nm, but
always use the scale near the distance
to be measured):
Navigation 145
Again, shapes will be accurate where
the cylinder touches the surface, but
the distortion will be much greater
the further away (as a point of
interest, Mercator was the first chart
to be used for maritime navigation in
the 16th century). Since rhumb lines
on this projection are straight lines, it
follows that great circles must be
curved, in this case, concave to the
　
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