Christopher Berkey for The New York Times

SAN FRANCISCO, July 26 — On the Web, anyone can be a mapmaker.

April Johnson used a G.P.S. device to help create a map used for the Trace Tribute endurance horse ride near Nashville.

With the help of simple tools introduced by Internet companies recently, millions of people are trying their hand at cartography, drawing on digital maps and annotating them with text, images, sound and videos.

In the process, they are reshaping the world of mapmaking and collectively creating a new kind of atlas that is likely to be both richer and messier than any other.

They are also turning the Web into a medium where maps will play a more central role in how information is organized and found.