It is an automatic method of digitization carried out using a scanner. The scanner senses the binary grey tone or colour values of the analogue data and outputs them as a series of pixels in parallel scan lines. Scanners may be flat bed (document mounted on flat surface) or drum (document mounted on drum surface). In both types, the actual scanning is done by a scanning head which is able to sense reflected light (for opaque documents) or transmitted light (for transparent documents) and to turn the light intensity into a pixel value.

Drum scanners occupy less space and are faster but are more expensive. Before scanning, the document must be well prepared to ensure that line widths are resolvable, line separations exceed pixel sizes and unwanted data are opaque out.

                                     

Parameters for evaluating a Scanner

  1. Scanner resolution: smallest image size sensible by scanner expressed in dpi (dots per inch).
  2. The larger dpi the fine the resolution and the slower the scan and greater the data volume and vice Most scanners have a range of resolution of 100-200 dpi.
  3. Maximum document size: modern large format scanners are
  4. Binary/grey tone/ colour capability: best scanners provide all the three
  5. Radiometric range: number of grey tone levels-the standard one is 256
  6. Geometric accuracy: how much data is distorted by the scanning process?
  7. Weight
  8. Maximum document thickness: typically 3-5mm
  9. Price

Advantages of Scanning

  1. Fast means of digitizing large or dense data formats
  2. Process is largely automatic and puts minimum strain on operator
  3. Output data can be easily integrated with satellite remotely sensed

Disadvantages of Scanning

  1. High cost of hardware/software
  2. Very intensive manuscript preparation
  3. Selective digitizing impossible