Histograms of Raster Data

Histograms are a critical tool to examine the distrubtion of pixels within a raster. A histogram shows a bar chart of the number of pixels that fall within certain ranges of values or "buckets".

- I feel this needs some examples of their use.

Histograms of an entire digital elevation model (DEM)

The image on the left is a digital elevation model (DEM) for the area around the northern end of Humboldt Bay. The chart on the right shows the distribution of pixels in this image as a histogram. Take a look at the histogram on the right and you'll see a large number of values near zero. These are all the values that are water within the bay and on the ocean. These pixels appear as black in the DEM. Then, there are a number of values up to 127. These values repreaent the topography to the east of the bay.

Note that to display an image on the web, all the values in the DEM have been changed from their original elevation values to values in the range from 0 to 255.

Raster Image	Histogram

Histograms of a portion of a digital elevation model (DEM)

The image below is the same as the DEM above but we've added a box that you can move around. On the right is a histogram of the pixels within the box. Click in the box and move it around to see how the values chagen.

Raster Image	Histogram

Histograms of a color image

On the left is a typical aerial photo of a section of the channels in Humboldt Bay. On the right is a histogram of the image. The image actually contains 3 values for each pixel, also known as a Red-Green-Blue or RGB image.

Raster Image	Histogram

Histograms of a portion of color image

On the left is the same aerial photo as above but we've added a marquee that you can drag around the image to see the impact on the histogram at the right.

Raster Image	Histogram