As the U.S. celebrates the centennial of its national parks, there has been debate about the value of public recreation lands.

One researcher at the University of Vermont claims to have found a way of quantifying that value, through social media posts.

Laura Sonter, a post-doctoral research associate at the Gund Institute for Ecological Economics from UVM, and her team looked at geotagged photographs of conserved public lands in Vermont on Flickr. They then multiplied the number of photos by the average estimates of per trip economic spending and found that these public lands had contributed some $1.8 billion to the state’s tourist economy in the period from 2007-2014, according to an article published Friday in the journal PLOS ONE.

“Our results reveal where people go to enjoy nature and why certain conserved lands are more popular than others,” Sonter told Travel + Leisure in an email. “This information is powerful—it enables land managers to make more informed decisions on where to undertake future conservation, and how to effectively enhance the existing recreational value of conserved lands.”

Vermonters and out-of-state tourists showed different preferences when it came to public recreation land: Locals tended toward places where the forest had been better conserved, whereas tourists sought out the more accessible lands that were equipped with fresh water or swimming, Sonter said.

The research could help provide a metric for other states to quantify the value of their publicly conserved lands and therefore provide evidence for the necessity of their conservation, according to the study.