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Aeronautical charts and compass
Rev. 45 page content was last changed 22 January 2013
Flight Planning and Navigation
Pilots operating under the Visual Flight Rules in Australia are
required to carry and have readily accessible in the aircraft the
latest issues of the aeronautical charts and other aeronautical
information relative to the flight.
The charts used for air navigation are overlaid with a
coordinate reference graticule system showing the local meridians
of longitude and the parallels of latitude. In aviation, surface
locations are generally defined in terms of latitude and longitude,
while chart directions in azimuth are referenced in relation to
true north. Topographical charts also indicate terrain elevation by
the use of contour lines and thus safe operating altitude above
terrain can be derived.
The prime navigational direction instrument the magnetic compass
aligns itself with the north magnetic pole and, in Australia, the
variation between the direction to true north and that to magnetic
north can be as much as 13.
Content
2.1 Defining position latitude, longitude, altitude and time
Lateral dimensions Effect of continental drift on precise
location The third positional dimension altitude Universal
Coordinated Time 2.2 Defining the shape of the Earth ellipsoids and
geoids
The World Geodetic System 1984 Chart elevation reference the
height datum The Australian geoid The WGS84 ellipsoid,
geoid-ellipsoid separation and GPS altitude 2.3 Aeronautical
charts
Chart system basics Recommended VFR charts Carriage of flight
documentation Digitised aeronautical charts 2.4 Map topography 2.5
Defining direction the aircraft direct reading magnetic compass
Magnetic variation Compass deviation Things that are handy to
know2.1 Defining position latitude, longitude, altitude and
time
Lateral dimensions
In aerial navigation any point on the Earth's surface may be
precisely defined in terms of a latitude and longitude graticule
reference.
Meridians of longitude are half 'great circles', perpendicular
to the equator, that extend from pole to pole. The meridians are
identified by the angle that they subtend, at the centre of the
Earth, with the prime meridian. That angle is measured in degrees,
minutes and seconds east or west from the prime meridian:
the WGS84 International Reference Meridian or 0 longitude or
prime meridian passes about 102 metres east of the originally
defined Greenwich meridian at the Greenwich Observatory,
England
subsequent meridians are identified as degrees east or west
around to 180
there are 60 minutes of arc in a degree and 60 seconds of arc in
a minute.
Parallels of latitude are 'small circles' drawn around the Earth
starting from the equatorial plane, north and south of the equator
and parallel with it and reducing in circumference toward the
poles. For our purposes we can say that parallels are identified by
the angle that they subtend with the equatorial plane, i.e. they
are geodetic*, measured in degrees, minutes and seconds and whether
they lie north or south of the equator:
the north pole has a latitude of 90N
the south pole has a latitude of 90S
the equator has a latitude of 0 and is a great circle, in that
it is formed by a plane that passes through the Earth's centre,
bisecting the Earth's sphere.
*Note: in satellite navigation latitude is defined in relation
to the Earth's centre of mass and to the ellipsoid, the term used
is 'geocentric latitude'. See 'Latitude and Longitudes in
Geodesy'.
One nautical mile is the length, at the Earth's mean sea level
surface, of one minute of arc of a great circle. The International
Nautical Mile is 1852 metres or 6076.1 feet. Consequently, one
degree of latitude (measured along a meridian) has an equivalent
surface distance of 60 nautical miles, and one second of latitude
is about 31 metres, while 1/100th of a second is about 0.3m.
Seconds of arc are generally not used in aeronautical publications;
latitude and longitude is expressed in degrees plus minutes to
(generally) one decimal place about 185 m. For example the
reference point for Mount Beauty airstrip in Victoria is located at
S36 44.1' E147 10.2'; aerodrome reference points (usually regarded
as the centre of the airfield) are defined in degrees, minutes and
tenths of minutes. However, when necessary, the location of a point
position may be specified much more precisely; some point locations
for instrument landings are required to be specified to 1/100th of
a second.
Some systems may use degrees only, in which case the degrees may
be expressed to five decimal places, e.g. S36.73499
Incidentally, a 'knot' is a speed of one nautical mile per
hour.
It is logical to express 'Lat/long' coordinates with the
direction from the equator/prime meridian first (e.g. S and E),
then a numeral group representing the degrees followed by a group
for the minutes. The symbols for degrees and minutes are omitted,
e.g. S36 44.1 E147 10.2. That is the standard format for geographic
locations in ERSA. However in the global navigation satellite
system (GNSS), and other systems, the northern hemisphere latitude
coordinates may be represented as a positive value and the southern
hemisphere as a negative value, while the longitude coordinates for
the western hemisphere have a negative value and those for the
eastern hemisphere have a positive value, so S36 44.1 E147 10.2 is
represented as 36 44.1 +147 10.2. The positive sign is
usually omitted for the northerly and easterly coordinates.
Effect of continental drift on precise location
The Earth's latitude/longitude reference graticule is regarded
as fixed relative to the Earth as a whole, but the continents are
in motion. The Australian tectonic plate, for example, is moving
north north-east towards the North Pacific at the rate of seven
centimetres per annum*. So, during the last 14 years every fence
post in Australia has moved one metre north north-east and their
precise latitude and longitude reference position has changed, and
will continue to change. Of course these tectonic plate movements
have no discernible effect on aerial navigation but they do
complicate land survey activities. In Australia, to overcome this
the 1994 Geocentric Datum of Australia [GDA94] uses a reference
meridian that is fixed relative to the Australian tectonic plate
rather than the International Reference Meridian. The map
projection for GDA94 is the Map Grid of Australia [MGA94].
*For comparison, it is estimated that the average fingernail
growth is 3.5 cm per annum.
The third positional dimension altitude
Contour lines and spot points on topographical maps provide an
indication of terrain elevation i.e. height above the Australian
Height Datum. The aircraft's altimeter reading provides the
aircraft's vertical position and thus the current height above the
terrain indicated on the chart height above ground level [AGL] or
the terrain clearance may be determined.
Universal Coordinated Time
Time is a most important dimension in aerial navigation; the
reference time is Universal Coordinated Time (symbol UTC a
compromise between the initialisms of the preferred French and
English names) rather than local times. UTC is the time at the
International Reference Meridian and is an average of a large
number of atomic clocks. The suffix 'Z' is used to identify dates
and times as UTC, so it may be referred to as 'Zulu' time the
phonetic for 'Z'.
UTC and the 24-hour clock system rather than local time are used
throughout the aviation information, communication and
meteorological services. UTC is 10 hours behind Australian Eastern
Standard Time, 9.5 hours behind Australian Central Standard Time
and 8 hours behind Australian Western Standard Time. Add an
additional hour in a daylight saving time period.
2.2 Defining the shape of the Earth ellipsoids and geoids
The World Geodetic System 1984
For aerial navigation and cartography purposes the shape of the
Earth is defined by a particular international model or datum*
known as the World Geodetic System 1984 [WGS84] that provides the
standard coordinate frame for navigation/cartography systems. Some
Australian charts may also show the GDA94 as the datum, which is
fixed relative to the Australian tectonic plate as mentioned above,
however for navigation purposes this is compatible with WGS84.
WGS84 also establishes a standard gravitational equipotential
surface or geoid, plus a mathematically (rather than physically)
derived representation of the Earth's underlying shape the WGS84
ellipsoid, centred at the Earth's centre of mass.
* A datum is the fixed starting point of a scale or measurement
system e.g. an aircraft weight and balance pre-flight check. In
this context the plural is datums not data.
Chart elevation reference the height datum
Mean sea level [msl] has been assessed by measuring high tide
and low tide levels in a number of tide gauges over a period of
many years. In Australia the local result is known as the
Australian Height Datum [AHD] and is the zero elevation reference
for Australian aeronautical charts; i.e. the elevations shown on
the charts are height above the AHD.
The Australian geoid
The Earth's density is not uniform throughout, thus gravity and
its perpendicularity and consequently msl distance from the
geometric centre of the Earth varies irregularly around the surface
of the globe. A geoid is a notional surface of equal potential
gravity within the Earth's gravity field, that describes the
Earth's shape and basically follows mean sea level over the oceans
and extends through the continents. Geoids are mainly determined by
specialised satellite measurements, the resulting surface is smooth
with some undularities. The current Australian geoid is AUSGeoid09
but for aerial navigation it can be regarded as equivalent to the
Australian Height Datum. AUSGeoid09 provides the AHD-to-ellipsoid
separations. Check Geoscience Australia for more information about
geodetic datums.
The WGS84 ellipsoid, geoid-ellipsoid separation and GPS
altitude
The geoid is not the same as the ellipsoid (a smooth, slightly
flattened sphere), which is a mathematically (rather than
physically) derived representation of the Earth's underlying shape.
There are many ellipsoids in use but that of most interest to
aviators is the WGS84 ellipsoid that is used by the global
navigation satellite system as its basis for altitude measurement.
The difference in elevation of a particular point on the Earth's
surface when measured against both the ellipsoid and the geoid can
be quite considerable, as much as 100m ; this is known as the
geoid-ellipsoid separation.
In Australia the degree of geoid-ellipsoid separation is quite
unusual. The image below shows the substantial geoid undulation
that slopes across Australia. In the south-west corner the geoid is
33m below the WGS84 ellipsoid while at the tip of Cape York it is
72m above it. As shown in the image the geoid and ellipsoid
coincide (i.e. zero separation) on a rough line between Port
Hedland and Melbourne.
(Image courtesy of Geoscience Australia).
The International Civil Aviation Organization [ICAO] specifies
that the local value (known as the 'N-value') of the
geoid-ellipsoid separation should be shown on aeronautical
navigation charts but the values are not shown on Australian
charts. The local N-value is of little significance to recreational
aviators (although it should be noted that a GPS instrument may
give an apparently incorrect height if the software doesn't adjust
for the local N-value*) but may be of great significance to IFR
pilots and designers of GPS approaches when the GNSS achieves
sole-means navigation status for all flight phases. A table of the
geoid-ellipsoid separation value for each cell of a roughly one
nautical mile square grid covering Australia is produced by
Geoscience Australia's National Geographic Information Group
previously known as AUSLIG. AUSGeoid09 provides the
AHD-to-ellipsoid separations, see the AustGeoid09 on the Geoscience
Australia site.
Some GPS receivers may store just a single N-value for each 10
latitude/longitude graticule cell. As can be seen from the image
above some 10 x 10 degree cells have a 40-50m variation diagonally
across the cell. If the N-value is not used or just approximated,
the calculated GPS altitude may be incorrect.
2.3 Aeronautical charts
Chart system basics
A chart system is built on three basics that must be defined for
use:
the projection employed generally the 'Lambert conformal conic
projection' for air navigation.
the coordinate system latitude and longitude for air
navigation.
the geodetic datum WGS84 [or GDA94] is the standard horizontal
(area) datum for most Australian aeronautical charts and the
Australian Height Datum is the vertical datum.
Note: when using a GPS receiver ensure that these three formats
have been selected correctly, particularly the WGS84 datum.
A map intended for aerial or marine navigation is usually
referred to as a 'chart'. The chart graticule is latitude and
longitude, with the meridians more or less vertical on the sheet
but converging slightly. As the Earth is a sphere, there has to be
a technique to map the image of the surface of the
three-dimensional sphere onto a flat two-dimensional chart without
overly distorting the represented areas. The most suitable
projection technique for world aeronautical charts is the 'Lambert
conformal conic projection'. Although this projection distorts
areas a little, distances anywhere on the chart have the same
scale. The great circle arc* the shortest distance between two
points on the surface of a sphere can be represented reasonably
accurately by the flight planner drawing a straight line between
two points on the chart. However you will note that the angle at
which that straight line crosses each meridian changes because of
the convergence of the meridians.
*Note: the shortest distance between, say, Sydney and Perth, is
a straight line (a tunnel) joining those cities and passing through
the Earth. The great circle route follows that 'tunnel' on the
surface.
The Lambert chart legend will indicate the latitudes of two
'standard parallels'. There is no scale distortion at these
parallels, however scale distortion increases with distance from a
standard parallel. For an explanation of standard parallels see
www.icsm.gov.au/mapping/about_projections.html and look for the
heading 'Multiple standard parallels or central meridians'.
Those meridians of longitude shown on Lambert conformal
aeronautical charts are straight lines, that converge towards the
poles*. On a southern hemisphere chart the meridian spacing between
the meridian lines at the bottom of the sheet is a little less than
that at the top about 5 mm on an Australian 1:1000000 World
Aeronautical Chart. A central meridian drawn on each chart is
vertical and the others converge towards it. The parallels of
latitude as shown on the chart are arcs of circles and cross all
the meridians at right angles because of the slant of the
meridians. If a straight line is drawn diagonally across the chart,
the angle that this great circle route subtends with each meridian
varies slightly across the chart. Aircraft flying very long legs
would alter their heading slightly every 500 nm or so to maintain
the great circle route and thus the shortest distance.
*Note: that convergence of the meridians is why the 'grid' on
such charts is called a 'graticule'; the meridians and parallels do
not form true rectangles, i.e. a 'grid'. If you joined a number of
WACs together by matching parallels and the edge meridians the maps
would form an arc.
On Mercator (a 16th century Flemish geographer) cylindrical
projection charts, straight line plots are 'rhumb lines' and great
circle plots are curved. A rhumb line is a line drawn so that it
crosses the meridians of the Mercator projection at a constant
angle, but it is not the shortest distance between two points; an
aircraft flying a constant track heading would be following a rhumb
line plot. The concept of choice between a great circle route or
rhumb line route is interesting but inconsequential to a light
aircraft navigator because a constant track heading (i.e. a rhumb
line track) is usually flown for each leg; except, perhaps, if
planning a direct route from Australia to New Zealand.
The scales used for aeronautical charts are the representative
fractions 1:1000000, 1:500000 and 1:250000. The latter scale means
that an actual distance of 2.5 km (250 000 centimetres) is
represented by one centimetre on the chart. The 1:1000000 scale is
a small-scale chart; i.e. it covers a large area but with minimum
detail, one centimetre represents 10 km. The 1:500000 and 1:250000
are larger-scale charts that cover progressively smaller areas but
with increasing detail.
The Australian Intergovernmental Committee on Surveying and
Mapping's Fundamentals of Mapping is well worth visiting.
Recommended VFR charts
The paper charts recommended for sport and recreational aviation
VFR flight planning, in-flight navigation and sourcing VHF
radiocommunications data are:
Planning Chart Australia: the PCA is a single sheet showing the
coverage of the WACs (below), the meteorological area forecast
[ARFOR] boundaries, the estimated FIS VHF coverage from both 5000
feet amsl and 10000 feet (but not the frequencies), and the areas
without FIS VHF coverage. The FIS HF communication frequencies are
shown. The spot location of about 700 named airfields is indicated.
PCA is designed to assist in initial VFR flight planning and it is
amended semi-annually. It is of rather limited use in initial
planning of flights below 5000 feet (i.e. most ultralight flights)
in eastern Australia because straight-line tracks between departure
point and destination may be precluded because of the topography,
and there are no indications of such on the PCA. But it is
generally okay for use west of the Dividing Range. Also it is the
only chart that indicates FIS VHF coverage; essential knowledge if
a flight is being planned into the less accessible areas of
Australia.
World Aeronautical Charts: the 43 Australian WACs are small
scale (1:1 000 000 or 1mm=1km), derived from aerial photography,
and designed for pre-flight planning and pilotage. They are part of
an ICAO international series. They do not indicate CTR or PRD, nor
is there any FIA, radiocommunications or radionavigation
information. As the reissue frequency is 24 years (i.e. 50% of the
43 maps are supposed to be re-issued every two years) the base can
be slightly out of date, particularly in regard to the
infrastructure. Amendment lists for each edition are published in
AIP SUP but these amendments generally relate to location of
airstrips and special activities rather than topography or
infrastructure. Each WAC generally covers 6 of longitude and 4 of
latitude. Sheet dimensions are about 70 60 cm and the scale is such
that a real distance of one nautical mile is represented by less
than 2 mm on the chart; thus WACs are really not suited to
low-altitude navigation in slow aircraft, but it is wise to always
have the latest edition of the WAC/WACs relevant to the journey in
the cockpit.
Visual Navigation Charts: the VNCs are a larger scale at 1:500
000 and show airspace information and FIS detail laid over the
topographic base. All VNCs are reissued at six-monthly intervals
but the base topographic detail may not be up to date. They are far
superior to the WACs for both flight planning and pilotage. VNC
sheet dimensions are about 100 60 cm and contain the following
airspace detail:
CTR, CTA dimensions and lower levels
Flight Information Area and Surveillance Information Service
boundaries where available
Flight Information Service and Surveillance Information Service
frequencies and providers
communication and navigation aid frequencies for licensed
airfields
PRD and designated & remote areas.
There are only 15 VNCs, those available covering the more
populous areas of Australia Tasmania to North Queensland, plus
areas around Perth, Adelaide, Darwin and Tindal.
Visual Terminal Charts: the 25 or so VTCs provide both
aeronautical and topographic information around major airports at a
scale of 1:250 000. They are essential for VFR operations in the
vicinity of such airports to avoid violating controlled airspace.
In some cases, these charts show the details of tracks to be flown
and significant landmarks to be used by pilots of VFR aircraft to
avoid inadvertent entry into controlled airspace. All VTCs are
amended and reissued every 6 months. The charts are based on the
NATMAP 250K series maps and use the Universal Transverse Mercator
[UTM] projection but with a latitude/longitude graticule rather
than the normal UTM grid; their dimensions are around 90 50 cm and
show the following details:
PRD areas
CTR and associated CTA dimensions including the lower levels of
the CTA steps surrounding the airport, lanes of entry, ATC check
points
Surveillance Information Service frequencies where available
communication and navigation aid frequencies for licensed
airfields
VFR approach points.
En Route Chart (low level): the ERC-L series is drawn to various
scales to accommodate significant air traffic route areas and shows
controlled airspace, PRD areas, air routes and segment distances,
ATS and radio-navigation services, ATS frequencies and location,
plus communication and navigation aid frequencies for licensed
airfields but no topography. It also indicates those airfields
where VHF radio contact with FIS is possible from the ground.
The FIS area boundaries are shown together with an information
box showing the provider of the flight information service (e.g.
Brisbane Centre), the frequency and the location of the area
transceiver.
The series of eight sheets cover Australia and are intended
primarily for IFR flights conducted below 20000 feet. The multitude
of air routes that radiate from major cities make the charts
difficult to read but they are the only chart series that show all
the FIS frequencies, PRD areas and give indications of sports
aviation activities thus they are an essential document for VFR
navigation. Each route segment is a great circle route with the
magnetic track angles measured at the end points rather than the
middle of the segment, which is why there is an apparent
discrepancy in the reciprocal track angles. Reissue frequency is
twice per year.
PCA, WAC, VNC, VTC and ERC-L can be purchased from the
Airservices Australia online store. You can see the coverage for
each sheet in each series by clicking 'Coverage Map' on their
'Aviation charts' page. These charts can also be ordered from pilot
supply shops. Possibly the TPCs may be purchased from the National
Mapping Division of Geoscience Australia.
Satellite and aerial images of the Earth's surface are also
available via the Google Earth and Google Map geobrowsers and
provide help in flight planning; for example, the ability to locate
an unlisted airstrip and establish the exact lat/long coordinates
for entry into a GPS.
Carriage of flight documentation
AIP ENR 1.10 para 5.1 states:
'Pilots are required to carry, and have readily accessible in
the aircraft, the latest editions of the aeronautical maps, charts
and other aeronautical information and instructions, published: a.
in AIP, or b. by an organisation approved by CASA,that are
applicable to the route to be flown, and any alternative route that
may be flown, on that flight.'
(The AIP entry is an extract from CAR 233 'Responsibility of
pilot in command before flight')
Digitised aeronautical charts
The WAC, VNC, VTC and ERC-L charts, and others for flight under
the instrument flight rules [IFR] , are also available in digitised
format raster or vector images for use in tablet computers with
flight planning software and for inflight use with portable
electronic devices with moving map software. They have the same
reissue frequency as the paper charts. This is discussed in the
'Electronic planning and electronic flight bag' module.
2.4 Map topography
Aircraft operating under the VFR must navigate by visual
reference to the ground. The lower the level at which a flight is
planned, the more important it is that the pilot is able to
visualise a three-dimensional image of the terrain from the
graphical details presented by the two-dimensional topographic
chart by the usage of colour, symbols and lettering. To assist this
visualisation, WACs and VNCs display tinted topographic contours
signifying surface areas between the 660 feet (200 m) and 1639 feet
elevations, 1640+ feet (500 m), 3280+ feet (1000 m), 4920+ feet
(1500 m) and 6560+ feet (2000 m) levels. The shape of the contours
and the width between them indicates the form of the land and the
gradient. The closer the contour lines (i.e. the narrower the
colour bands) are to each other, the steeper the gradient.
Also the WAC utilises relief shading of elevated ranges and
ridges so that they are more evident. In addition, spot elevations
are shown and the highest spot elevation within each chart
graticule is recorded in a bolder lettering than other spot
elevations. The graticule on the WACs and VNCs is spaced at 30
minutes of latitude and 30 minutes of longitude: 30 nm in latitude
and, for much of Australia, around 24 nm in longitude.
The contours on VTCs are at 500+, 1000+, 2000+, 3000+, 4000+ and
5000+ feet amsl, but in addition all areas are shaded purple where
there is less than 500 feet of clearance between the terrain and
the lower limit of the overlying controlled airspace. Like WAC and
VNC, the highest spot elevation within each chart graticule is
shown in a bolder type than other spot elevations. The graticule is
spaced at 10 minutes of latitude and 10 minutes of longitude: 10 nm
in latitude and around 8 nm in longitude. The VTCs generally cover
an area within a 4050 nm radius from the major airport and are the
essential chart for visual navigation within that area.
Vegetation is usually not shown on WACs, nor are many structures
except for towers and similar obstructions to low-flying aircraft;
although grain silos which are an excellent navigation aid usually
associated with a railroad are shown. Railroads, power transmission
lines and some roads are depicted.
2.5 Defining direction the aircraft direct reading magnetic
compass
Sport and recreational navigation under the VFR is basically
azimuth and distance and is also short-range i.e. each leg is
usually less than 500 nm or so. Directions in azimuth are usually
expressed as the angular distance from the north pole true north in
whole degrees from 0 at north clockwise to 360; i.e. north is both
0 and 360 (though is usually expressed as 360). For example, the
direction due east from any particular location is 090. These
directions may be described as bearings, headings, courses or
tracks depending on the application. Direction is usually paired
with distance expressed in nautical miles, thus the bearing and
distance of a location 55 nm due east would be expressed as bearing
090/55.
However, the prime navigational direction instrument the
magnetic compass aligns itself with the north magnetic pole and, in
Australia, the variation between the direction to true north and
that to magnetic north can be as much as 13, so there is a need to
define directions in terms of 'degrees true" or 'degrees
magnetic'.
Civil Aviation Order 20-18 specifies just four mandatory flight
and navigational instruments for flights under the day Visual
Flight Rules. These basic instruments are:
an airspeed indicating system
an altimeter, with a readily adjustable pressure datum setting
scale graduated in millibars
an accurate timepiece indicating the time in hours, minutes and
seconds, which may be carried on the person of the pilot and
a direct reading magnetic compass.
If the aircraft is a Light Sport Aircraft for which a current
special certificate of airworthiness or an experimental certificate
has been issued it need not carry the individual instruments as
defined above, if equipment is carried that provides a pilot with
the same information, i.e. an electronic flight display.
Magnetic variation
The simple direct reading compass is essentially a bar magnet
freely suspended in a lubricating fluid designed to damp out
oscillations, vibrations and swings caused by aircraft
accelerations. The bar magnet, which may be a needle or part of a
circular compass card, aligns itself with the Earth's local
magnetic lines of force with the north-seeking end pointing roughly
north. The Earth's magnetic field is systematically surveyed so
that the difference between the direction at which a compass points
magnetic north and the direction of true north is measured. That
difference is called variation, or declination if you are of a
scientific bent, and is expressed in degrees of arc east or west of
true north. The magnetic lines of force at any location may also be
substantially varied by local magnetic anomalies substantial iron
ore deposits for example. Lines on a chart joining locations with
equal magnetic variation are isogonals, or isogonic lines, and are
shown on WACs and VNCs as dashed purple lines at half-degree
intervals. The local variation may also be shown numerically on
some charts. The isogonals on Australian charts vary from 3 west in
the south-west corner of the continent to 13 east on the eastern
coast.
This means that if you want to fly from A to B, the direction
ascertained from the chart will be relative to true north the true
course and let's say it is due west, 270. If you then set 270 on
the aircraft compass and fly that heading then your track over the
ground will not be due west but will vary according to the
variation. Let's say the variation is 10 east then the true course
you are flying will be 280. This small complication requires that
when you have finally calculated the true course you have to fly to
get from A to B, after allowing for the effects of wind, then you
need to convert it to a magnetic heading. The conversion rule used
for at least the past 70 years is: "Variation east, magnetic
heading least; variation west, magnetic heading best". So if the
local variation is 12 east the magnetic heading will be the true
course minus 12; e.g. true course 010, magnetic heading 358. If the
variation is 2 west the magnetic heading will be the true course
plus 2; e.g. true course 010, magnetic heading 12.
For all wind velocities, given in meteorological forecasts and
actuals, the directions are relative to true north, except if you
happen to hear a broadcast from a CTR tower controller (or an
Automatic Terminal Information System [ATIS] broadcast) who
provides the wind direction as magnetic, because the airfield
runway numbers are relative to magnetic north. The air route
directions shown on ERC-L are also relative to magnetic north.
Compass deviation
Aircraft compasses are also deflected by magnetic fields within
the aircraft, some related to ferrous engine/structural metals,
others related to electrical currents. These aircraft magnetic
fields produce heading errors compass deviation which vary
according to the aircraft course, either reducing or increasing the
Earth's magnetic field. These errors can be quite significant, 30
or more, and any magnetic field within about one metre of the
compass may have a discernible effect. Mobile telephones in the
cockpit may also affect the compass. Compass error is the
combination of variation and deviation adjustment necessary to
determine the compass heading that will provide the true
course.
A bar magnet aircraft compass will have screw-adjustable
compensating magnets to negate or at least reduce the effect of
these magnetic fields. The compass and aircraft must be 'swung' to
make these adjustments, and the residual deviation errors noted on
a compass correction card displayed in the cockpit. Residual
deviation errors should not exceed 10 at any compass point. The
procedure for 'swinging the compass' is time-consuming and
difficult but necessary. We will go further into compass deviation
in the 'En route adjustments' module.
Airfield runway numbers are stated as their magnetic heading
rounded off to the (supposedly) nearest 10; thus an east-west
runway will be numbered 09/27. The ERSA entry in the "Physical
characteristics" section for the airfield usually shows the actual
magnetic heading following the runway numbers, but only for one
direction. For example at Dubbo aerodrome '05/23 043' indicates the
actual magnetic heading for runway 05 is 043 magnetic, and
consequently 223 for runway 23. Thus, when stationary and
accurately lined up for take-off on such a runway, you can measure
deviation on that heading; but make sure the compass has stopped
moving. Flying to a few airfields and checking deviation at various
runway headings is one way of producing a compass correction card.
Always make sure the compass fluid level is okay. A vacuum chamber
for de-aerating the compass fluid must be used in the re-filling
process using the proper fluid, not alcohol.
Bar magnet compasses are also affected by vibrations, aircraft
accelerations and inertia when turning; thus they tend to be
shifting constantly. Compass acceleration errors are most apparent
when the aircraft is on an east/west heading and least apparent
when on a north/south heading. The turning errors require the pilot
to make an undershoot/overshoot adjustment when changing heading.
To overcome these errors, normally the magnetic compass is
accompanied by a gyroscopic instrument that indicates the direction
in which the aircraft is heading, without being subject to external
forces. This electrically or suction-operated directional gyro [DG]
or direction indicator [DI] is initially aligned with the compass
before take-off and needs to be realigned occasionally during
flight; however, few ultralights are equipped with DGs.
Electronic flight information systems [EFIS or 'glass cockpits']
are now becoming much cheaper and thus a reasonable proposition for
amateur-built light aircraft. These systems use solid-state
electronic componentry plus software to present a cockpit display
incorporating the functions of most single flight instruments. In
such systems magnetic field strength sensors (magnetometers) are
used to provide a three-dimensional magnetic compass that displays
magnetic heading without acceleration, attitude or turning errors;
thus it also incorporates the DG facility. The simple direct
reading magnetic compass must still be part of the aircraft
equipment.
Things that are handy to know
There are other maps such as the Australian NATMAP 250K series.
This is intended mainly for surface use, with the GDA 94 datum and
using the Map Grid of Australia projection (MGA 94) which conforms
with the Universal Transverse Mercator (UTM) projection with
usually the metric UTM Eastings and Northings grid rather than a
latitude/longitude graticule. This coordinate system is more
complex than latitude/longitude the Earth's surface is divided
longitudinally into 60 six-degree numbered zones and Eastings are
measured in metres (to 3 decimal places) from the central meridian
of each zone. Northings start from zero at the equator in the
northern hemisphere but, for the southern hemisphere, start from a
10000000m base Northing at the equator. See
en.wikipedia.org/wiki/UTM_coordinate_system.
Fortunately the digitised NATMAP 250K series are also available
with a latitude/longitude graticule, so these larger-scale (1:250
000) maps could be used for the limited leg distance of
recreational aircraft navigation, particularly with GPS. Each map
covers an area of 1.5 longitude and 1 latitude. VTCs, being based
on the NATMAP 250K, use the Transverse Mercator projection with a
lat/long graticule. Some UTM maps may show the lat/long graticule*
in one colour with the UTM grid* in another.
*Note: 'grids' are rectangular in shape; the 'graticule' is not
the meridian lines converge poleward.
The digitised NATMAP 250K series may be purchased from
Geoscience Australia. The 513 maps of the NATMAP 250K series are
available on DVD for about $100 which is less than 3% of the cost
of the paper series and well worth having as home reference
material even if you don't use them for aeronautical navigation.
They are in ECW format and software is supplied for viewing and for
export to geoTIFF, TIFF, JPEG, PNG, bitmap or OziExplorer format.
Image resolution is 200 dpi and the pixel size is around 30 metres
with a positional accuracy of 127 metres. The 'Map Viewer' software
supplied is currently (2012) confined to Windows operating systems.
View 'About NATMAP Digital Maps 2008'.
Stuff you don't need to know
Maps that lack contours, like street maps, are planimetric; i.e.
flat.
'Large scale' maps are those with a scale of 1:70 000 or
less.
Groundschool Flight Planning & Navigation Guide| Guide
content | 1. Australian airspace regulations | [2. Charts &
compass] | 3. Route planning | | 4. Effect of wind | 5. Flight plan
completion | 6. Pre-flight safety and legality check | 7.
Airmanship & flight discipline | | 8. En route adjustments | 9.
Supplementary techniques | 10. En route navigation using the GNSS |
| 11. Using the ADF | 12. Electronic flight planning & the EFB
| 13. ADS-B surveillance technology | Supplementary documents|
Operations at non-controlled airfields | Safety during take-off
& landing | Section 3 of the Flight Planning & Navigation
Guide discusses route planning
Copyright 20012013 John Brandon [contact information]
