The vertical direction of the plot is denoted by altitude, in units of kilometers, and shows the height from which the laser signal was returned to the telescope.
The color assigned to each pixel represents the intensity of the returned signal. The color bar associated with each color-modulated plot show the colors assigned to ranges of measured signal returns in digitizer counts. The weakest signals are in black and the strongest, including signals which saturate the digitizer, are in white. The blue signals below the Earth's surface are due to detector ringing and will be removed from the final archived data.