A GPS campaign of measures on the site of Heliopolis was implemented in order to document precisely the orientation of the remaining structures of the temple of Atum and to determine the azimuth of its axis. This operation resulted in improved data which allowed to formulate some hypothesis about the date of the temple's foundation ceremony and its possible direct connexion with the sun rise on the date recorded in the Berliner Leather Roll during the reign of Senusret I. Prolegomene The study of the Egyptian temples' orientation is a field of researches which has already produced fruitful results 3. However, reliable an accurate data on the precise azimuth of the archaeological remains is required before taking into account the possibility that this orientation was connected with potential astronomical events. A program labelled OrTempSol in the framework of the Labex-Archimede at Montpellier, led by L. Gabolde, was thus launched in 2013 with the aim to determine precisely the orientation of some of the Egyptian temples devoted to solar deities, along the same line of work already accomplished at Karnak 4 and at Tell el-Amarna 5. The program focuses specifically on the temple of Atum at Heliopolis and the temple of Amun-Ra at Tanis 6. The present chapter provides and summarizes the preliminary results obtained at Heliopolis.