The image strips acquired from different angles by the HRSC camera system were used to generate digital terrain models of the Martian surface, containing elevation information for each recorded pixel. The reference level to which the altitude readings refer is an imaginary surface that represents the same gravitational pull as that of sea level on Earth. Known as an equipotential surface, this is shaped like a biaxial ellipsoid and is called the Areoid, a name derived from Ares, the Greek word for Mars. In this image, North is to the right. The colour coding of the digital terrain model (top right) clearly indicates the elevation differences: the topographical profile of the region has an altitude of approximately 4000 metres; the difference between the highest regions of the Martian highlands in the south (beige and light brown) and a number of shallow depressions in the lowlands towards the top (azure blue) is equivalent to that of the difference in altitude between the summit of Mont Blanc and the Rhône Valley in Lyon. The mesa-like outliers in the eroded transition area between highland and lowlands tower around 2000 metres over the plain.