ArcMap > Working with rasters in ArcMap
About rasters |
|
|
Release 9.1
Last modified December 7, 2006 |
Print all topics in : "Working with rasters in ArcMap" |
Vector data—such as coverages and shapefiles—represents geographic features with lines, points, and polygons. Rasters—such as images and grids—represent geographic features by dividing the world into a regular pattern of discrete cells called pixels. Each pixel, short for picture element, represents an area, often has a geographic location, and has a value that represents the feature being observed. For example, the pixel values in an aerial photograph represent the amount of light reflecting off the earth’s surface interpreted as trees, houses, streets, and so on, while the pixel values in a DEM (Digital Elevation Model) represent elevations.
A raster can represent:
- Thematic data such as land use, or soils
- Continuous data including temperature or elevation, or spectral data such as satellite images and aerial photographs
- Pictures such as scanned maps or drawings, and building photographs
You will generally display thematic and continuous rasters as data layers along with other geographic data on your map. Picture rasters, when displayed with your geographic data, can convey additional information about map features. You may also display a data layer representing a collection of raster datasets, called a raster catalog.
Raster bands
Some rasters have a single band of data, while others have multiple bands. When there are multiple bands, then every pixel location has more than one value associated with it. For example, a satellite image commonly has multiple bands representing different wavelengths—from the ultraviolet through the visible and infrared portions of the electromagnetic spectrum. Landsat 7 imagery, for example, collects data from eight different bands of the electromagnetic spectrum. Bands 1-5, 7 and 8 represent data from the visible, near infrared and short-wave infrared regions (band 8 is also collected at a higher resolution). Band 6 collects data from the thermal infrared region.
Another example of a multi-band image would be a true color orthophoto–where there are three bands, each representing either red, green or blue light.
An example of single band data would be a DEM. Each pixel in a DEM contains only one value. You can also have a single band orthophoto, which is sometimes called a panchromatic or grayscale image.