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lines with a scale factor of 1, so in
the centre the correction is less
than 1 (0.9996 for the UTM), while
the outer parts have it greater than 1.
The WAC at 1:1,000,000 and the
VNC at 1:500,000 are based on a
two-parallel Lambert Conformal
Conic, whereas the larger scale VTA
at 1:250,000 uses Transverse
Mercator (the Vancouver VTA
covers a bit over 2° of longitude).
The Transverse Mercator's
advantages include accuracy (over
small areas, at least). Wide countries
are split into zones usually no wider
than 6° of longitude, at which point
the distortion becomes unacceptable.
Universal Transverse Mercator
UTM is a metric grid system based
on Transverse Mercator, designed by
the US military, using 60 6°
longitude zones and 20 8° latitude
bands between 80° S to 80° N,
giving 1200 areas overall. Longitude
zones are numbered 1-60 starting at
180°W. Latitude bands are lettered
from C (not I and O) Northwards
from 80°S. Each 6° by 8° area has its
own grid, based on 100,000 m
squares. Each column and row is
lettered, and when the numbers are
used, Eastings are given first.
Polar Stereographic
These charts are used in polar
regions, because the others cannot
cope with convergence that well. To
get the details correct, the paper is
held flat over the top of the Pole and
the imaginary light projected straight
up from the centre of the Earth to it:
On these, rhumb lines are not the
shortest course, and you must use
great circles instead. Since they are
straight lines, meridians are crossed
at different angles.
The Arctic
Up there, it’s darker for longer and
there are fewer navaids. The
compass begins to get unreliable,
and there is increased deviation due
to the aircraft's own magnetic field.
146 JAR Private Pilot Studies
Scale
Assuming a constant scale, the ratio
between distances on a map and the
Earth's surface is expressed as a scale
based on the map's size. For a scale
of 1:500,000 (commonly referred to
as a half-mil), one inch on the map is
equal to 500,000 inches on the
Earth. There are 63,360 inches to the
mile, so an inch on a half-mil map is
7.89 statute miles.
You can tell which chart has a larger
scale by looking at the representative
fractions, obtained by dividing chart
distance by Earth distance. Thus, a
chart distance of one inch divided by
its Earth equivalent of 13.7 nm
would be a 1/1000000 map, and of a
smaller scale than a 1/500000.
A "one-inch map" means one that
uses one inch for one mile. A
"quarter inch map" has 4 miles to
the inch (about the length of the
distance between the joint on your
thumb and the tip, for quick
reference). Not everything on the
map is done to this scale; if it were,
you would hardly see roads and
railways, so they are artificially
expanded to be visible. The centre of
any object is its actual position.
The faster your aeroplane flies, the
less time you have to check the map,
so those made for high and fast
flight, or for instrument flying where
you can't see the ground anyway, will
not have many ground features
marked on them.
If you need to find out what Earth
distance is represented by a chart
distance, multiply the chart distance
by the scale. For example, if asked
what distance is represented by 25
cm on a 1/1,000,000 chart, multiply
25 x 1,000,000 to get 25 million
centimetres. Divide that by 100 to
get metres (250,000), then divide by
100 to get 250 km.
Relief
Information about high ground is
given in various ways.
Contours are lines on a map joining
points of equal height (or elevation)
above sea level, so they are similar to
isobars, in that the closer they are
together, the steeper the slope they
represent:
Some maps may give different
colours or shading to various layers
to make things more obvious,
known as Layer Tinting:
Spot Heights show the elevation of
prominent peaks with small dots.
The actual height will be shown next
to it. The highest one will be
distinguished in some way.
Navigation 147
Otherwise, expect water to be blue,
woods to be green, and railways and
power lines to be black.
Speed & Distance
A knot is a measure of speed, that is
1 nautical mile per hour (it was
originally measured by allowing a
long rope to stretch out behind a
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