What is the right resolution for raster to vector conversion?

The right resolution for raster to vector conversion depends on the drawing you want to convert. If you are vectorizing small details you will need a higher resolution than if you are vectorizing large details.

For example, let's say you scanned the two drawings of circles below at the same resolution.
 

Original drawing   Original drawing
Original drawing:
Large circle

 
  Original drawing:
Small circle

 
Suitable for raster to vector conversion   Not suitable for raster to vector conversion
After scanning:
Recognizable circle

 
  After scanning:
No circle shape

 

After scanning, the large circle is still recognizable as a circle. If you were to vectorize it, it would vectorize to a circle vector.

If you were to vectorize the scanned small circle however, it would not vectorize to a circle vector. Why? Because there is no circle there for the raster to vector converter to convert.

The pixels at this resolution are too large with respect to the size of the small circle to be able to represent the circle shape. In order to be able to vectorize the small circle to a circle vector, you would need to scan it at a much higher resolution so that it was represented as a circle on the raster image.

As a general rule, a resolution of 200 to 400 dpi will be enough to properly represent the shapes on most technical drawings. For small artwork such as logos, you may need a much higher resolution - 600 dpi, 1200 dpi or even more.

After you've scanned a drawing, you should zoom into your scan, look at it, and ask yourself the following questions:

1. Do the shapes on the raster image look like the shapes you're expecting in the vector image?

Original drawing   Desired vectors
Original drawing
 
  Desired vectors
 
Suitable for raster to vector conversion   Not suitable for raster to vector conversion
Good resolution:
Circle looks like a circle

 
  Too low resolution:
Circle looks like a diamond

 

2. Are adjacent / concentric shapes separated by clear white space?

Original drawing   Desired vectors
Original drawing
 
  Desired vectors
 
Suitable for raster to vector conversion   Not suitable for raster to vector conversion
Good resolution:
Concentric circles completely separated

 
  Too low resolution:
Concentric circles touching

 

 
Original drawing   Desired vectors
Original drawing
 
  Desired vectors
 
Suitable for raster to vector conversion   Not suitable for raster to vector conversion
Good resolution:
Adjacent text characters and underlining completely separated

 
  Too low resolution:
Adjacent text characters and underlining touching

 

Note that touching adjacent or concentric entities can also be caused by scanning with an inappropriate threshold. See Explaining the threshold a vital setting for successful raster to vector conversion.

3. Do curves look like curves? (Not staircases!)

Original drawing   Desired vectors
Original drawing
 
  Desired vectors
 
Suitable for raster to vector conversion   Not suitable for raster to vector conversion
Good resolution:
Curves look like curves

 
  Too low resolution:
Curves look like staircases

 

If your answer to 1, 2 or 3 above is NO, you need to re-scan your drawing at a higher resolution.