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Buying a large format scanner for automatic raster to vector conversion (Part II)
(updated April 2011)
The two areas of large format
scanner sales most clouded in
bull and misrepresentation are
Resolution and Speed.
The two areas of large format scanner sales most clouded in bull and misrepresentation are Resolution and Speed.
Optical Resolution
Optical resolution is the resolution at which your drawings are actually scanned.
The leading high-end scanners are over-specified in terms of optical resolution, i.e. they give you more than you can practically use.
Contex's SD Series CIS scanners offer true 1200 dpi optical resolution, the same as Colortrac SmartLF Gx+ CCD scanners. To those new to large format scanning, this possibly indicates that these devices are the best in their categories.
True 1200 x 1200 dpi optical resolution is certainly as high as it gets in wide format scanning. However, 1200 dpi optical is more than can be used to practically scan an A1 / D-size, let alone an A0 / E-size, document in monochrome, let alone color. The resulting file will simply be too BIG to practically use.
200 - 400 dpi optical resolution is typically what most technical drawings are scanned at. This means that a large format scanner with 400 dpi optical resolution is arguably as practical as a 1200 dpi one! Certainly the many scanners offering 600 dpi optical resolution will cover most eventualities when scanning technical documents and maps for archiving, copying, file distribution and raster to vector conversion.
Very high optical resolution like 1200 dpi makes good advertising but provides little or no practical scanning benefits. Even those who have 600 dpi optical resolution will seldom use it. Those with 1200 dpi optical resolution will probably only use it once to prove or disprove the point about file size made here, thereafter never again.
The flipside is that scanners with very low optical resolution, like the old 200 dpi optical resolution Contex Hawk-Eye, are not good enough for raster to vector conversion because they cannot provide optical resolution more than 200 dpi when it is needed. Many of these devices are now coming onto the market as secondhand equipment. Despite what resellers have said in the past about the Hawk-Eye's suitability for raster to vector conversion, the blunt truth of it is that it is not, never has been and never will be up to the task of creating suitably sharp, well-defined scans from the many drawings that need to be scanned at a resolution of more than 200 dpi.
Interpolated Resolution
Interpolated resolution increases the scanned resolution of an image by calculating (making up!) new data to fill in between the scanned data. All claims for the benefits of interpolated resolution should be dismissed with scorn.
Some companies who should know better claim that high interpolated resolution has benefits for the user. The fact is that interpolated resolution adds no sharpness to an image. It just makes images BIGGER.
As image sharpness is the over-riding requirement for raster to vector conversion, interpolation brings absolutely no benefits to this application. In fact, it is debatable that it offers any benefits anywhere!
Océ claim that their "optional interpolated resolution of up to 9600 dpi" makes the Océ CS4300 series of CCD scanners "ideal for CAD, GIS and other professional users in need of high resolution scans for a wide variety of applications". This nonsense implies that Océ feel that their genuine 600 dpi optical resolution is not a strong enough selling point in their scanners today (it is).
This poor advice is not Océ's alone. Other scanner resellers also promote high interpolated resolution in order to sell scanners thought to be too low on optical resolution. We question the quality of the large format scanning advice which you can expect to get from any reseller who misleadingly uses technical specifications to create false impressions.
Place no value on interpolated resolution, aka enhanced and maximum resolution. As a rule, avoid large format scanner resellers who do not offer clear-cut advice on the nature of the resolution they are claiming. If they are trying to confuse you at this basic level, you should not expect objective advice elsewhere.
Scan Speed
Like resolution, scan speed is the subject of much misinformation (see our Scan Speed article on Scanners4CAD).
Claims for scan speed can largely be dismissed because raster to vector conversion is not about how fast the scanner can scan but how quickly you can produce a good quality scanned image. Contex recently introduced a new "productivity" rating which claims to rate productivity by the number of drawings scanned in one hour (700+!). We think this is ridiculous.
When scanning for raster to vector conversion, the interrogation of the scanned image by zooming into it, moving around the image, adjusting and fine-tuning of the correct scan settings to get the best possible quality scan are what are necessary and are what takes time. So, a large format scanner with an A0 / E-size scan speed of, say, 10 seconds, offers no significant benefit over one which takes 20 seconds, especially if its scanning software makes the selection of the correct scanner settings needed to create a good quality scanned image difficult and slow.
When buying a large format scanner for raster to vector conversion the supplied scanning software is a hugely important consideration.
We will look at this in Part III.
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