Altimeter Shows the Difference
The picture to the left shows the measurement of the size of the bluff which separates the southern and northern hemispheres of Mars. The picture shows the surface of Mars from the north pole to the south pole, as if the planet had been cut in half. If you put your finger on the red line, you can trace the change in the height of the Martian surface.
At the left are the lower, northern plains, then the abrupt cliff which separates the northern plains from the highlands of the south, then the highlands at the right. The height of the bluff at any place along the red line can be read in both kilometers and in miles at either side of the picture.
The picture shows that the southern hemisphere is much more elevated than the northern hemisphere. The southern hemisphere is also much older than the northern hemisphere. The different sizes of the two hemispheres is suggestive that Mars has a giant southern continent.