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Learning hub · General Navigation · Lesson 2

Charts, latitude, longitude and distance

Every navigation sum starts with two questions: where is this place, and how far is it? The answers come from the grid of latitude and longitude and from the chart you hold. This lesson shows how the grid works, why a chart always bends the truth a little, and how to measure a distance without a calculator.

About 12 minutes, plus the quiz. Part of General Navigation. It comes before time, speed, distance and magnetic variation.

1. The grid on the Earth

The Earth is almost a sphere, a little flattened at the poles. For navigation at student level you can treat it as a sphere. Two sets of lines are drawn on it:

  • Parallels of latitude are circles parallel to the equator. Latitude is the angle north or south of the equator, from 0° to 90°.
  • Meridians of longitude run from pole to pole. Longitude is the angle east or west of the Greenwich meridian, from 0° to 180°.

A position is written latitude first, for example 45°30′N 025°15′E. Each degree has 60 minutes. Romania lies at roughly 44° to 48° north and 20° to 30° east.

The nautical mile (NM) is defined as 1852 metres, and it was chosen to match one minute of latitude. So one degree of latitude is about 60 NM, and a speed of one knot is one nautical mile per hour. Longitude is different: meridians meet at the poles, so a degree of longitude gets shorter as you go north. At the equator it is 60 NM, and elsewhere it is 60 NM multiplied by the cosine of the latitude:

LatitudeCosineOne degree of longitude
0°1.0060 NM
30°0.8752 NM
45°0.7142 NM
60°0.5030 NM

2. Great circles and rhumb lines

A great circle is any circle on the sphere whose centre is the centre of the Earth. The equator and every meridian are great circles. The shortest path between two points on the surface is part of a great circle. Because the meridians converge, a great circle crosses each meridian at a different angle, so the track changes along the way.

A rhumb line crosses every meridian at the same angle. You can fly it on one constant true track, which is simple, but it is longer than the great circle unless you fly due north or south or along the equator.

On the short legs of a VFR flight the difference is tiny and you can ignore it. You draw a straight line on the chart and measure the track. Because the meridians converge, the angle is slightly different at each end, so measure it where the line crosses a meridian near the middle of the leg.

3. Why charts are drawn the way they are

You cannot flatten a sphere without stretching something. A projection is the method used to draw the curved surface on flat paper, and every projection chooses what to keep correct.

Many aeronautical charts for mid-latitudes, such as VFR charts of Europe, use the Lambert conformal conic projection. Imagine a cone laid over the globe like a hat, cutting it along two parallels called the standard parallels. Its useful properties are:

  • It is conformal: angles and shapes are correct locally, so a track or bearing measured on the chart is right.
  • Meridians are straight lines that converge towards the pole, and parallels are curved arcs.
  • The scale is nearly constant across the chart and is exact along the standard parallels.
  • A straight line drawn on it is very close to a great circle.

Other charts use other projections, such as the Mercator, on which a rhumb line is straight, so the printed legend always tells you which one you have.

4. Scale and measuring distance

The scale compares the chart with the ground. At 1:500,000, one unit on the chart is 500,000 of the same unit on the ground. That gives some useful figures:

  • 1 cm on the chart is 5 km on the ground.
  • 1 NM (1852 m) is about 3.7 mm on the chart.
  • A 1:250,000 chart is twice as detailed, so 1 cm is 2.5 km.

You can measure with a ruler or with a plotter that has a nautical mile scale. If you use a ruler in centimetres, multiply by the scale, then divide kilometres by 1.852 to get nautical miles. You can also read the distance off the latitude scale at the edge of the chart, because one minute of latitude is one NM. Always use the latitude scale, never the longitude scale, and take it from about the same latitude as your line. In an app, the scale changes as you zoom, so use the app measuring tool or the scale bar and not a ruler held against the screen.

Worked examples (invented points):

  1. From 45°10′N to 45°52′N on the same meridian: 52 − 10 = 42 minutes of latitude, so 42 NM.
  2. Due east along 45°N from 025°00′E to 026°30′E: that is 90′ of longitude. One minute of longitude at 45°N is about 0.71 NM, so 90 × 0.71 ≈ 64 NM.
  3. A leg measures 14.8 cm on a 1:500,000 chart: 14.8 × 5 = 74 km, and 74 ÷ 1.852 ≈ 40 NM.

5. Symbols, elevations and the legend

A chart is only useful if you can read it. Every chart has a legend that explains its symbols: aerodromes, airspace boundaries and classes, obstacles, power lines, railways, water and built-up areas. Read it before you fly with a chart you do not know.

Check the units of the heights. Elevations and obstacle heights can be shown in feet or metres, and they are measured above mean sea level, while airspace limits may be given as altitudes, flight levels or heights above the ground. The legend says which is which. Spot heights, contour tints and the highest obstacles help you stay clear of terrain, so pick them out along your route.

Finally, look at the date of the chart. Airspace, obstacles and aerodromes change, and the chart in your bag may be out of date.

For training only. A chart in this lesson, in a video or on a screen is not for real navigation. Use current official charts and the AIP for real flights, and check your school’s manual for the chart you will actually use.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. Two aerodromes lie on the same meridian. One is at 44°48′N and the other at 46°03′N. How far apart are they, in nautical miles?
    Show the answer
    1. Subtract the latitudes: 46°03′ − 44°48′ = 1°15′.
    2. Change to minutes: 1°15′ = 75′.
    3. One minute of latitude is one nautical mile, so the distance is 75 NM.
  2. A leg on a 1:500,000 chart measures 18.5 cm. Your groundspeed will be 90 kt. What is the distance in nautical miles, and about how long will the leg take?
    Show the answer
    1. 18.5 cm × 5 km per cm = 92.5 km.
    2. 92.5 ÷ 1.852 ≈ 50 NM.
    3. Time = 50 ÷ 90 hours ≈ 0.556 h, and 0.556 × 60 ≈ 33 minutes.

    The distance is about 50 NM.

  3. You fly due east along the 45°N parallel from 024°00′E to 025°30′E. Roughly how far is that, and why is it not simply 90 NM?
    Show the answer
    1. The change in longitude is 1°30′ = 90′.
    2. At 45°N a minute of longitude is only about 0.71 NM, because the meridians have drawn closer together (cosine 45° ≈ 0.71).
    3. 90 × 0.71 ≈ 64 NM.

    It is not 90 NM because that figure would only be true at the equator.

7. Watch

This video from Aviation Theory covers the shape of the Earth and the geographic coordinates, which is the grid in the first part of this lesson.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

The same channel has a video on cartography, projections and scales, which goes with the parts on projections and chart scale.

Open on YouTube if the player does not load. Not made by VectorEFB.

One more from the same channel, on great circles and rhumb lines.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. One nautical mile is, on average, equal to:

A nautical mile (1852 m) matches one minute of latitude, so 60 NM make one degree of latitude. A minute of longitude is only one NM at the equator.

2. A great circle is useful in navigation because it is:

The shortest distance between two points on a sphere lies along a great circle. A line crossing all meridians at the same angle is a rhumb line.

3. A conformal chart projection, such as the Lambert conformal conic, keeps which property correct locally?

Conformal means angles and shapes are correct locally, so a track measured on the chart is right. Area and distance cannot both be perfect on a flat chart.

4. On a 1:500,000 chart, how many kilometres does 6 cm represent?

1 cm is 5 km at this scale, so 6 cm is 30 km, which is a little over 16 NM.

5. You want to measure a distance on a paper chart using the scale at its edge. Which scale do you use?

One minute of latitude equals one NM everywhere. The length of a minute of longitude changes with latitude, so the longitude scale gives the wrong distance.

Next: time, speed, distance and magnetic variation. Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.