Until recently, archaeology was limited by what a researcher could see while standing on the ground. But light detection and ranging, or lidar, technology has transformed the field, providing a way to scan entire regions for archaeological sites.

With an array of airborne lasers, researchers can peer down through dense forest canopies or pick out the shapes of ancient buildings to discover and map ancient sites across thousands of square miles. A process that once required decades-long mapping expeditions, and slogging through jungles with surveying equipment, can now be done in a matter of days from the relative comfort of an airplane.

But lidar maps are expensive. Takeshi Inomata, an archaeologist at the University of Arizona, recently spent $62,000 on a map that covered 35 square miles, and even was deeply discounted. So he was thrilled last year when he made a major discovery using a lidar map he had found online, in the public domain, entirely for free.

The map, published in 2011 by Mexico’s National Institute of Statistics and Geography, covered 4,440 square miles in the Mexican states of Tabasco and Chiapas. It was made as part of the institute’s mission to create accurate maps to be used by businesses and researchers.