Passage: The restoration then was carried out between 1907 and 1911, using the principles of anastylosis and led by Theodor van Erp. The first seven months of restoration were occupied with excavating the grounds around the monument to find missing Buddha heads and panel stones. Van Erp dismantled and rebuilt the upper three circular platforms and stupas. Along the way, Van Erp discovered more things he could do to improve the monument; he submitted another proposal, which was approved with the additional cost of 34,600 guilders. At first glance, Borobudur had been restored to its old glory. Van Erp went further by carefully reconstructing the chattra (three-tiered parasol) pinnacle on top of the main stupa. However, he later dismantled the chattra, citing that there were not enough original stones used in reconstructing the pinnacle, which means that the original design of Borobudur's pinnacle is actually unknown. The dismantled chattra now is stored in Karmawibhangga Museum, a few hundred meters north from Borobudur.

Question: What principles were used to proceed with the restoration?
anastylosis