How can a barchan dune tell you the direction in a desert?

A desert dune is a dune formed in a desert

Barchan dunes are solitary crescent dunes formed on a flat, hard ground with limited supplies of sand and vegetation. Transverse dunes are a series of long ridges that are found in areas that have a...

The key of a map.

no...not that i know of

Their wavy nature with dips and uneven mounds/hills are caused by the wind pushing sand across the desert.