Pierre Charles Francois Dupin (October 6, 1784 - January 18, 1873) was a French Catholic mathematician. He studied geometry with Monge at the ?cole Polytechnique and then became a naval engineer. In 1826 he published a thematic map showing the distribution of illiteracy in France, using shadings (from black to white), the first known instance of what is called a choropleth map today. His mathematical work was in descriptive and differential geometry. He was the discoverer of conjugate tangents to a point on a surface and of the Dupin indicatrix. He died in 1873 at the age of 88.