Following talks held in 1989, on 21 March 1990 a border treaty was signed between King Fahd of Saudi Arabian Oman's Sultan Qaboos at Hafr al-Batin, and then ratified in May 1991.[1][2] This agreement finalised the border at the 1955 modified Riyadh line.[11] On-the-ground demarcation then followed with the assistance of the German aerial photography company Hansa Luftbild, being completed in 1995.[7]