The article goes into detail about that...

Many forts that Poidebard documented don't even show up in the 1960s and 1970s spy satellite imagery; the Dartmouth team only identified 36 of his original 116. "The attrition of the archaeological record has been substantial and these processes are unlikely to have slowed over the intervening decades," they wrote. They believe further research incorporating higher-resolution or even older satellite imagery should reveal many more Roman forts in the region