Altimetry is a term which describes a method of measuring the altitudes of things with remote spacecraft. It is usually performed with a laser. Laser altimeters bounce beams of light from the surface and back to the spacecraft to measure the heights of mountains and depths of valleys on remote planets. This is even done for our own planet. The figure shown here shows laser altimeter results from the Mars Global Surveyor spacecraft. The red line shows the relative heights of the ground underneath the spacecraft.