Automatic vectorisation?

Again and again people express the hope that paper plans can be automatically converted into CAD files by using vectorisation software. One thing is certain: this is not possible.

In automatic vectorisation, the paper plan is initially scanned and a graphic image is generated. This graphic image is a “bitmap” which only recognises pixels that are switched on or off – and nothing else.

Vectorisation programs then try to interpret the clouds of dots in the scanned bitmaps and convert them into CAD objects such as lines, arcs, circles, dimensions and texts. To do this, the software analyses each individual pixel and the surrounding pixels. The result consists of interpretations of the scanned drawing. A number of problems occur:

• In spite of the integrated cleaning functions, dirt and folds on the drawings are incorrectly converted into geometrical objects.
  
• The software has great difficulty recognising hand-drawn dimensions and texts, so it converts them into line fragments.
  
• Circles and arcs are usually incorrectly converted to polygons and line fragments.
• Lines are broken down into unconnected line fragments.
• The resulting drawing is not true to dimension.
  
• The actual lines drawn do not match the dimensions.
  

To summarise: the amount of work needed to edit an automatically vectorised drawing is greater than amount of work needed to redraw it.