A geographer is someone that studies the physical features of the earth and its atmosphere. A geographer uses maps in their line of work. A map is a picture that details an area. A legend can refer to a diagram on a map explaining the symbols on the map. Legends is its plural form. A legend can also mean someone that is famous. This joke is playing on the word "legends" and its different uses.
OLD GEOGRAPHERS never die , they just become legends .