Chart properties and scale

Navigation · Chapter 10 · DGCA CPL and ATPL

What a chart's scale means and how to work out distances with it, and the properties a navigation chart must have.

Remember

Scale = chart length ÷ earth distance, in the same units.
km × 100 000 = cm; NM × 185 200 = cm.
Large scale = small N = more detail, less area.
Projection: a light inside the Reduced Earth throws the grid onto a plane (pole), cylinder (equator) or cone (a parallel).
No chart has constant scale; the one essential property is orthomorphism.
Orthomorphic = right angles + equal scale in all directions at a point.

From the ground to the chart

The real ground • 40 km of country: hills, a river, a town, a motorway, an airfield • Driving across it takes about half an hour
Flatten it and draw symbols • A chart is the ground seen from straight above, flat • Hills become contour lines and colours; every feature becomes a symbol
Now shrink it • Shrink everything by the same amount until the 40 km fit in 8 cm • The outline shows the real ground; the sheet is the chart (drawn much bigger than real)
That squeeze is the scale • 40 km = 4 000 000 cm, drawn as 8 cm • 4 000 000 ÷ 8 = 500 000: every distance is 500 000 times shorter on the chart • Written 1 : 500 000

What scale means

Chart against Earth • 40 km on the ground is 8 cm on this chart • scale compares the two
Same units first • 40 km = 40 × 100 000 = 4 000 000 cm
Divide • 4 000 000 ÷ 8 = 500 000
Three ways to show it • In words: "1 cm to 5 km" • As a representative fraction: 1 : 500 000, first number always 1 • As a scale line on the chart; the latitude scale is one too: 1° = 60 NM