What are the different types of image I can download/buy?
  
   _Very brief - needs a proper entry_
   
   Raw data (typically Level 0)
          (as with other levels, annotated with appropriate metadata).
          Only useful if you're studying the RS system itself, or data
          processing systems
          
   Processed Images (typically Level 1, 2)
          Processing includes:
          
          + Radiometric correction - compensating for known
            characterisitcs of the sensor.
          + Atmospheric correction - compensating for the distortion
            (lens effect) of the atmosphere.
          + Geometric correction - referencing the image to Lat/Long on
            the Earth's surface, based on the satellite's position and
            viewing angle at the time of the acquisition. Uses either a
            spheriod model of Earth or a detailed terrain model; the
            latter enables higher precision in hills/mountains. Requires
            Ground Control Points (GCPS: points in the image which can be
            accurately located on Earth) for high precision.
            
          The various part-processed levels are suitable for a image
          processing studies. Most Remote Sensing and GIS applications
          will benefit from the highest level of processing available,
          including geocoding.
          
   Geocoded Projected Imagery (typically Level 3)
          The image is mapped to a projection of the Earth, and in some
          cases also composited (ie several images are mosaiced to show a
          larger scene).
          
   Browse Images
          Images you can download from the net are likely to be browse
          images. These are typically GIF or JPEG format, although a
          number of others exist. Whilst providing a good idea of what is
          in an image, they are not useful for serious applications. They
          have the advantage of being a manageable size - typically of
          the order of 100Kb-1Mb (compared to 100Mb for a full scene) and
          are often available free. A browse version of any image (except
          raw data) can be made.
          
   Stereopairs
          
   Multitemporal Images