Satellite Reflectance Histograms

Option on Image analysis menu.

Satellite histograms: graph of DN versus the number of pixels with that DN.  DN is the digital number, which measures the energy sensed by the satellite if the particular band.

The reflectance histogram graphs the number of pixels (or percentage of the scene) with a particular DN in the scene. The most commonly occurring DN will mark the mode; in some cases the distribution may have multiple modes. If multiple modes exist, that means that many pixels share a common DN, few pixels have DNs in the region between the modes, and then many pixels share another DN.

The histogram represents the reflectance characteristics of the scene, which is a function of reflection characteristics and the distribution of terrain in the scene.

Because water in rivers, lakes, or the ocean has very distinctive and uniform reflectance characteristics, large water regions will usually lead to a large mode.

Note that the top diagram shows a band with a unimodal distribution, and the bottom diagram shows a band with a bimodal distribution. The bimodal distribution that the scene contains a number of points with very low reflectance (such as water), and then a number of points with varying higher reflectances.

Last revision 3/25/2008