COMMON SCALES AND USAGE
The information contained on a chart is a function of the chart's scale. The scale is the ratio
between the dimensions of the chart and actual dimensions represented. Thus, if the scale is
1:1,000,000, one inch of the chart represents 1,000,000 inches (about 14 miles) on the ground. If
the scale is 1:500,000, one inch of the chart represents 500,000 inches (about 7 miles) on the
ground. Note a scale 1:500,000 is larger than 1:1,000,000 (a large scale chart gives you a large
amount of detail and a small amount of area).
The choice of scale for an aeronautical chart depends on two factors:
The size of the area to be charted.
The amount of detail to be shown, as limited by the probable use of the landmarks in
The conflict between chart requirements for local and long distance flights has led to the
development of numerous special purpose charts at different scales. The objective is to put as
much information on the chart as possible without cluttering the chart to the degree it is less
useful. The scale of charts used in general long range planning is typically between 1:5,000,000
to 1:1,000,000. For detailed area planning, a scale of 1:500,000 to 1:25,000 is effective. This
coverage is too small for convenient use in aircraft which rapidly "fly off" the chart or fly sorties
of extended range. This necessitates frequent transfers to adjacent charts and subjects the
NFO/WSO/NAV to the difficulty and confusion of continuously paging through chart sheets and
pages during flight.
Some chart types and scales:
Global Navigation Chart (GNC), scale 1:5,000,000 (small scale)
Jet Navigation Chart (JNC), Scale 1:2,000,000
Operational Navigation Chart (ONC), scale 1:1,000,000.
Tactical Pilotage Chart (TPC), scale 1:500,000 (large scale).
GNC and JNC: Used for long range preflight planning.
ONC: Used for medium range preflight planning or used in the air by medium to high
TPC: Used for detailed planning and in-flight use by low to medium altitude aircraft.
CHART LEGEND REVIEW