by sim » Mon Apr 12, 2010 2:42 pm
All of the above Merik except like runway numbers magnetic headings tend to be prefered for air nav over geographical headings as explained below,
Aviation sectionals (maps / charts) and databases used for air navigation are based on True north rather than magnetic north, and the constant and significant slight changes in the actual location of magnetic north and local irregularities in the planet's magnetic field require that charts and databases be updated at least 2 times per year to reflect the current magnetic variation correction from True north. For example, as of March 2010, near San Francisco the magnetic north is about 14.3 degrees east of True north, with the difference decreasing by about 6 minutes of arc per year.
When plotting a course, a pilot in most small planes will plot a trip using true north on a sectional (map), then, convert the true north bearings to magnetic north for in-plane navigation use (which rely on cockpit instruments that read magnetic north).
Radionavigation aids located on the ground, such as VORs, are also checked and updated to keep them aligned with magnetic north to allow pilots to use their magnetic compasses for accurate and reliable in-plane navigation
GPS systems used for air navigation can use magnetic north or true north. In order to make them more compatible with systems that depend on magnetic north, magnetic north is often chosen, at the pilot's preference. The GPS receiver natively reads in true north, but can elegantly calculate magnetic north based on its true position and data tables calculate the current location and direction of the north magnetic pole and (potentially) any local variations, if the GPS is set to use magnetic compass readings.
WikiP on air nav. Cheers, sim