Aviation sectional charts (maps) and databases used for air navigation are based on true north and south because the planetary rotational poles do not change location on the landscape anywhere near as much as do the magnetic poles. That said, low-tech ways of identifying true north or south include waiting for noon with a stick in the ground to judge the shortest shadow, or waiting for a clear night in order to consult a pole star. By contrast, with a low-tech magnetic compass, one can identify magnetic north or south at any time, hence the presence of a magnetic compass in every cockpit, even today in the GPS era. When onboard electronics fail, pilots can still rely on paper charts and Edwardian-era instruments such as a magnetic compass. There's still the need to convert magnetic to true, however. Sectional charts indicate by dashed magenta lines the number of degrees variation between true and magnetic at any local vicinity as of the printing of that chart. Since the true locations of the magnetic poles meander over time, occasionally quite dramatically, charts and databases are required to be updated at least twice each year. Another reason for frequent chart updates is construction (new tall radio towers, runway renovation, etc.).