Scanning

A scanned image of the original map is needed for on-screen manual digitizing and semi-automatic/automatic digitizing. A range of scanners are available for obtaining a scanned image, starting from a small-format (A4) desktop scanner with resolutions of 200–800 dpi, through to high-end flatbed and drum scanners (see Figure below) suitable for very accurate scanning of large-sized documents (A0).

A scanner illuminates a document and measures the intensity of the reflected light with a CCD array. The result is an image represented as a matrix of pixels, each of which holds an intensity value. Office scanners have a fixed maximum resolution, expressed as the highest number of pixels they can identify per inch; the unit is dots per inch (dpi). For manual on-screen digitizing of a printed map, a resolution of 200–300 dpi is usually sufficient, depending on the thickness of the thinnest lines. For manual on-screen digitizing of aerial photographs, higher resolutions are recommended—typically, at least 800 dpi.

Semi-automatic/automatic digitizing requires a resolution that results in scanned lines of several pixels wide to enable the computer to trace the centre of the lines and thus avoid displacements. For printed maps, a resolution of 300–600 dpi is usually sufficient. Automatic or semi-automatic tracing from aerial photographs can only be done in a limited number of cases. Usually the information from aerial photos is obtained through visual interpretation.

After scanning, the resulting image can be improved by various image processing techniques. It is important to understand that scanning does not result in a structured data set of classified and coded objects. Additional work is required to recognize features and to associate categories and other thematic attributes with them.