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Aeronautical charts and compass
Rev. 47 — page content was rearranged 13 September 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. Such material is generally in a printed paper format.
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 spot elevations, thus safe operating altitude above terrain can be derived.
Note: at September 2013 two Australian data service providers hold a CASA instrument of approval for some types of digitised aeronautical charts, so private VFR pilots are able to store those charts in an 'electronic flight bag [EFB] system so replacing paper charts, as the primary means of in-flight documentation. Even so, although an EFB is a paper replacement system, it is prudent to carry back-up paper charts.
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°.
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:
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 the parallels appearing on aviation charts 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:
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 0.3 metres. Seconds of arc are generally not used in those aeronautical publications intended for navigation under the Visual Flight Rules; latitude and longitude is expressed in degrees plus minutes to (generally) one decimal place — about 185 metres. 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. 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. 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.
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.
* A datum is the fixed reference or 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. geodetic datums.
For aerial navigation and cartography purposes the shape of the Earth is defined by the WGS84 ellipsoid providing 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.
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 of the continent AUSGeoid09 is 33m below the WGS84 ellipsoid while at the tip of Cape York in the north-east corner it is 72m above the ellipsoid. As shown in the image the geoid and ellipsoid coincide (i.e. zero separation) on a rough line between Port Hedland and Melbourne.
The local value (known as the 'N-value') of the geoid-ellipsoid separation might 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.
*Note: 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.
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 ellipsoid, there has to be a technique to map the image of the surface of the three-dimensional ellipsoid 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 1:1 000 000 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 1:1 000 000, 1:500 000 and 1:250 000. The latter scale means that an actual distance of 2.5 km (250 000 centimetres) is represented by one centimetre on the chart. The 1:1 000 000 scale is a small-scale chart; i.e. it covers a large area but with minimum detail, one centimetre represents 10 km. The 1:500 000 and 1:250 000 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.
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.
'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 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.
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 40–50 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.
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:
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.
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.
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'.
• '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 |
| Operations at non-controlled airfields | Safety during take-off & landing |
|Section 3 of the Flight Planning & Navigation Guide discusses route planning|
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