9.1 Chart Fundamentals: Latitude, Longitude, Variation and Distance

The Shape of the Problem

Aeronautical charts represent a curved earth on flat paper. Everything that follows — the coordinate system, the distance scale, the two norths — exists to manage that translation.

Latitude and Longitude

Latitude measures angular distance north or south of the equator, from 0 degrees at the equator to 90 degrees at each pole. Lines of latitude are called parallels and they run east–west.

Longitude measures angular distance east or west of the prime (Greenwich) meridian, from 0 to 180 degrees. Lines of longitude are called meridians, they run north–south, and they converge at the poles.

The conventions matter because coordinates entered incorrectly put an aircraft somewhere else entirely:

• Latitude is always quoted first, then longitude.
• Each degree is divided into 60 minutes, and each minute into 60 seconds. Decimal minutes are also common in RPA ground stations.
• Australia lies entirely in the southern and eastern hemispheres, so Australian coordinates are always S and E. A negative latitude and a positive longitude is the decimal equivalent.

Worked example. Sydney Airport sits at approximately 33° 56' S, 151° 11' E, or -33.933, 151.183 in decimal degrees. A mission-planning file that produced +33.933 would place the operation in the northern hemisphere near the Chinese coast — which is exactly the kind of error that a habit of checking the hemisphere letters catches.

The latitude-distance relationship

This is the single most useful fact in chart work:

$1\text{ minute of latitude} = 1\text{ nautical mile}$

Because parallels are evenly spaced, the latitude scale printed up the side of a chart is a distance scale. Measure a distance with dividers, transfer it to the latitude scale, and read off nautical miles. One degree of latitude is therefore 60 NM.

Longitude does not work this way. Meridians converge toward the poles, so one minute of longitude equals one nautical mile only at the equator and shrinks as latitude increases. Never measure distance on the longitude scale.

Depicting Height and Elevation

Charts show terrain in several ways at once:

• Contour lines joining points of equal elevation, with the interval stated in the legend.
• Layer tinting — colour bands for elevation ranges, so higher ground is visually obvious.
• Spot heights — a dot with a figure giving the elevation of a specific point in feet AMSL.
• Obstacle symbols with two figures: the top of the obstacle in feet AMSL and, usually in brackets, its height AGL. A tower marked 1240 (485) has its top at 1,240 ft AMSL on ground 755 ft AMSL.
• The maximum elevation figure (MEF) printed in large digits within each grid square, giving the highest terrain or obstacle in that square.

For a remote pilot the obstacle figures are the operationally important ones, because a 485 ft AGL tower reaches well above a 400 ft AGL operating ceiling.

Magnetic Variation

The earth has two norths.

• True north is the geographic north pole — the direction the meridians point, and the reference for charted directions.
• Magnetic north is the direction a compass needle points, and it moves slowly year by year.

Magnetic variation is the angular difference between them at a given place, expressed in degrees east or west. Charts show it as isogonals — dashed lines joining points of equal variation — labelled with the value and the year of measurement.

In Australia, variation is generally east in the eastern states (roughly 10–13° E along the east coast) and swings through zero and to west in parts of Western Australia. The conversion rule is the traditional one:

$\text{Variation east, magnetic least} \qquad \text{Variation west, magnetic best}$

That is, with easterly variation the magnetic direction is a smaller number than the true direction; with westerly variation it is larger.

Worked example. A survey line is drawn on a chart with a true track of 090. Local variation is 12° E. The magnetic track is 090 − 12 = 078 M. If instead the variation were 5° W, the magnetic track would be 090 + 5 = 095 M.

Why an RPA pilot needs this

Three reasons:

1. Forecast winds are in degrees true; reported surface winds are in degrees magnetic. Comparing them without converting introduces an error equal to the local variation.
2. The aircraft's compass reads magnetic, while charted tracks and survey plans are usually true. Mission-planning software handles the conversion, but the pilot needs to know which reference a number is in.
3. The compass calibration the flight controller performs is a calibration against the local magnetic field, which is why travelling several hundred kilometres is a trigger to recalibrate.

Bearing versus heading

• A bearing is the direction of an object from an observer — "the tower bears 045 from the launch point".
• A heading is the direction the aircraft's nose is pointing.

Both must be labelled T (true) or M (magnetic), and the two are related by variation exactly as tracks are. A relative bearing is measured from the aircraft's nose rather than from north, which is what the clock code expresses informally.

Electronic Charts and Drone Safety Apps

Schedule 4 topic 4 also asks about electronic maps and charts and CASA's drone safety apps.

Airservices Australia publishes VTCs, VNCs and other charts electronically, and a range of electronic flight bag applications display them with a moving-map position. CASA-verified drone safety apps are a separate category: they display aerodrome no-fly zones, prohibited, restricted and military operating areas including temporary ones, and approach and departure paths, and they are the tool CASA points operators to before every flight. Some controlled aerodromes also accept airspace authorisation requests through a verified app under the automated airspace authorisations trial.

Their limits are important and examinable:

• A drone safety app is not a legal authority. It shows where you may have a problem; it does not grant permission.
• App data can lag. Temporary restrictions arrive by NOTAM, and the authoritative check is a NAIPS briefing, not an app icon.
• Not every aerodrome is shown. Private and agricultural strips may appear in no published source at all, which is why AC 101-01 advises checking satellite imagery or local knowledge.
• Charts have edition dates. A superseded chart, paper or electronic, must not be used for flight planning.

Use the app as a fast first filter, and the chart plus a NAIPS briefing as the authoritative check.