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Charts and compass
Rev. 37 — page content was last changed 17 October 2010
Page edited October 2009 by RA-Aus member Dave Gardiner
|Flight Planning and Navigation|
Ground maps are mandatory for navigation under the Visual Flight Rules. The maps or charts used for air navigation are overlaid with a coordinate grid showing the local meridians of longitude and the parallels of latitude. In aviation, locations are generally defined in terms of latitude and longitude, while chart directions are referenced in relation to true north. Unfortunately the prime navigational instrument — the magnetic compass — aligns itself with the north magnetic pole.
Meridians of longitude are half great circles, perpendicular to the equator, which extend from pole to pole. The meridians are identified by the angle that they subtend with the centre of the Earth — measured in degrees, minutes and seconds — east or west from the prime meridian:
One nautical mile is the length, at the Earth's 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. 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 Mount Beauty airstrip in Victoria is located at S36° 44.1' E147° 10.2'.
It is logical to express 'Lat/long' coordinates with the direction from the equator/prime meridian first (S and E), then a 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.
Incidentally, a 'knot' is a speed of one nautical mile per hour.
The meridians of longitude on such aeronautical charts are straight, equidistant lines, that converge towards the poles. On a southern hemisphere chart the meridian spacing at the bottom of the sheet is a little less than that at the top — about 5 mm on an Australian continent 1:1 000 000 chart. A central meridian on each chart is vertical and the others converge towards it. The parallels of latitude 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 the great circle route subtends with each meridian varies slightly across the chart.
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. The 1:500 000 and 1:250 000 are larger-scale charts that cover progressively smaller areas but with increasing detail.
Directions on air navigation charts are always 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. Directions are usually associated with distances expressed in nautical miles, thus the bearing and distance of a location 55 nm due east would be expressed as bearing 090°/55. For aerial navigation purposes the shape of the Earth is defined by a particular model known as the World Geodetic System 1984 [WGS84] that provides the horizontal datum for the chart coordinate systems. Some Australian charts may also show the Geocentric Datum of Australia [GDA94] as the datum, but for all practical aerial navigation purposes, this is identical to WGS84.
Thus a chart system is built on three basics that must be defined for use:
The Earth's density is not uniform throughout, thus gravitational pull — and consequently msl distance from the geometric centre of the Earth — varies irregularly around the globe. A geoid is a notional surface, within the Earth's gravity field, of equal potential gravity, which describes the Earth's irregular shape. It approximates with msl at the coastline and extends under the continents. The Australian geoid is AUSGeoid98 and is within a metre of the AHD. The geoid is not the same as the ellipsoid (a smooth, slightly flattened sphere), which mathematically represents the Earth's underlying shape. There are many ellipsoids in use but that of most interest to aviators is the WGS84 ellipsoid used by the global navigation satellite system. 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; this is known as the geoid-ellipsoid separation, the extent of which is indicated in the image below.
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 GPS achieves sole-means navigation status. A table of the geoid-ellipsoid separation value for each cell of a two nautical mile (actually two minutes of latitude/longitude) grid covering Australia is produced by the Australian Surveying and Land Information Group [AUSLIG]. For more information visit Geoscience Australia.
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'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
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')
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Also the WAC utilises relief shading of elevated ranges and ridges so that they are more evident. In addition, 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.
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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.
Aircraft magnetic compassAircraft 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 and top it up with 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.
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|Things that are handy to know
• There are other maps such as the Australian NATMAP 250K series. This is intended mainly for surface use, and employs the Transverse Mercator projection with a metric Universal Transverse Mercator grid — rather than latitude/longitude — and the concept of 'grid north'. These maps usually have a line illustration at the side that shows the angular relationship between grid north, true north and magnetic north. The coordinate system is more complex than latitude/longitude but if you are accustomed to that grid system then there is no reason not to use these larger-scale maps for recreational aircraft navigation, particularly with GPS. VTCs, being based on the NATMAP 250K, use the Transverse Mercator projection but with a lat/long coordinate grid.
• Marine navigation charts normally use Mercator (a 16th century Flemish geographer) cylindrical projection where rhumb lines are straight and great circle plots are curved.
• A rhumb line is a line drawn so that it crosses the meridians at a constant angle, but it is not the shortest distance between two points; an aircraft flying a constant heading would be following a rhumb line course. The concept of choice between a great circle route or rhumb line route is interesting but inconsequential to a light aircraft navigator except, perhaps, if planning a direct route from Australia to New Zealand.
|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.
• The ultra-precise WGS84 latitude and longitude of any apparently fixed surface feature in Australia varies with time, because the whole continent is drifting north-west at a rate of about one metre per decade.
• 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.
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. Safety audit | 7. Airmanship & flight discipline |
| 8. En route adjustments | 9. Supplementary techniques | 10. Global Positioning System |
| 11. Using the ADF | 12. Electronic planning & navigation | 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|
Copyright © 2001—2009 John Brandon [contact information]