Geographic Information Systems: An Introduction, 3rd Edition

9.4 Manual Digitizing or Scanning

The advantages of manual digitizing are that it:

• Can be performed on inexpensive equipment
• Requires little training
• Does not need particularly high map quality

The disadvantages are that it:

• Is tedious
• Is time-consuming

The advantages of scanning are that it:

• Is easily performed
• Provides rapid compilation of digital map images

The disadvantages are that:

• Raster data files are large and consume memory space.
• Attribute data cannot be linked directly to line and polygon structures in raster data.
• Raster data lack “intelligence” and hence can be used only as bases for drawings.
• Improvement of data quality can be labor intensive.
• Clean maps with well-defined lines are required.
• Relatively expensive equipment is required where standards of accuracy are high.
• Expert personnel are required.

Although scanning produces large quantities of data, compressed raster data need not require more memory space than vector data of customary format (i.e., ASCII). One reason is that raster data lack the voluminous drawing instructions that are essential in vector data. During the 1990s, manual digitizing has been the technique used most frequently, but more recently scanning has taken over a lot of the market, particularly because of the opportunities for on-screen digitizing, efficient packing routines, and powerful computers. Further developments in pattern recognition will undoubtedly contribute to more widespread use of scanning.