Chart projections & scale

Two VFR charts, two projections:

• VNC (VFR Navigation Chart): a Lambert conformal conic projection at 1:500,000.
• VTA (VFR Terminal Area chart): a transverse Mercator projection at 1:250,000, more detail over a smaller area.

Conformal means angles and the shapes of small features are held true. So a straight line you draw with a ruler is very close to a great-circle (shortest) track, not a rhumb line.

Scale. The smaller the second number, the larger the scale and the more detail: 1:250,000 (VTA) is a larger scale than 1:500,000 (VNC). On a 1:500,000 chart, 1 inch is about 6.86 nm (about 3.43 nm on the 1:250,000 VTA).

Meridians run to true north and converge toward the pole; parallels of latitude cross them. Relief is shown by contours, spot elevations, and colour tints.

Trap: thinking 1:500,000 is the "larger" scale because the number is bigger. It is the opposite: bigger denominator, smaller scale, less detail.

Note: the two scales are in the TC AIM (MAP 2.5.1/2.5.2). The projection TYPE (Lambert conformal conic, transverse Mercator) and what "conformal" buys you are chart-legend and projection theory, not in the local AIM; confirm against the current VNC/VTA legend.

Practice