Raster Pixel Values
A raster is a data type used in GIS and in your digital camera. Whenever you take a photo, your camera converts light into digital values. These values are stored in a rectangular grid of "pixels". Pixel is short for "picture element". The values stored in the pixels are then used to determine the amount of light that comes out of your computer screen when the photo is displayed.
Rasters are used in a variety of applications in GIS, especially for remotely sensed data like satellite and aerial photographs. These photographs are just like your digital photo except they were taken by a satellite or airplane with a special camera that was pointed down toward the earth. There are also raster with other types of data than just photographic, including:
- Digital elevation models: Each pixel represents the elevation at a point on the earth
- Land cover: Each pixel contains a value representing a type of land (water, forest, urban) at that pixels location on the earth
- Slope: Each pixel represents the slope of the land
Pixel values in a digital elevation model (DEM)
Pixels are always a digital number. A DEM contains one value per pixel making it look like a "black and white photograph" or a "grayscale image". Below is a DEM of the northern end of Humboldt Bay. Move your mouse cursor around the image and you'll see the values in the pixels. What are the value over water? What are the values to the west where the mountains form? Which values are larger?
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Pixel values in a Color Image.
Color photos are actually made up of three values, one each for the amount of red, green, and blue light that entered your camera for each pixel. This means we need a "3-band" image to display color. Move your mouse cursor around the image below to see the value. Note that the top value is the value for red, the middle value is for the amount of green, and the bottom value is for the amount of blue.
